draft-ietf-cbor-7049bis-06.txt   draft-ietf-cbor-7049bis-07.txt 
Network Working Group C. Bormann Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track P. Hoffman Intended status: Standards Track P. Hoffman
Expires: January 3, 2020 ICANN Expires: February 26, 2020 ICANN
July 02, 2019 August 25, 2019
Concise Binary Object Representation (CBOR) Concise Binary Object Representation (CBOR)
draft-ietf-cbor-7049bis-06 draft-ietf-cbor-7049bis-07
Abstract Abstract
The Concise Binary Object Representation (CBOR) is a data format The Concise Binary Object Representation (CBOR) is a data format
whose design goals include the possibility of extremely small code whose design goals include the possibility of extremely small code
size, fairly small message size, and extensibility without the need size, fairly small message size, and extensibility without the need
for version negotiation. These design goals make it different from for version negotiation. These design goals make it different from
earlier binary serializations such as ASN.1 and MessagePack. earlier binary serializations such as ASN.1 and MessagePack.
This document obsoletes RFC 7049. This document is a revised edition of RFC 7049, with editorial
improvements, added detail, and fixed errata. This revision formally
obsoletes RFC 7049, while keeping full compatibility of the
interchange format from RFC 7049. It does not create a new version
of the format.
Contributing Contributing
This document is being worked on in the CBOR Working Group. Please This document is being worked on in the CBOR Working Group. Please
contribute on the mailing list there, or in the GitHub repository for contribute on the mailing list there, or in the GitHub repository for
this draft: https://github.com/cbor-wg/CBORbis this draft: https://github.com/cbor-wg/CBORbis
The charter for the CBOR Working Group says that the WG will update The charter for the CBOR Working Group says that the WG will update
RFC 7049 to fix verified errata. Security issues and clarifications RFC 7049 to fix verified errata. Security issues and clarifications
may be addressed, but changes to this document will ensure backward may be addressed, but changes to this document will ensure backward
skipping to change at page 1, line 49 skipping to change at page 2, line 7
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 3, 2020. This Internet-Draft will expire on February 26, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. CBOR Data Models . . . . . . . . . . . . . . . . . . . . . . 7 2. CBOR Data Models . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Extended Generic Data Models . . . . . . . . . . . . . . 8 2.1. Extended Generic Data Models . . . . . . . . . . . . . . 8
2.2. Specific Data Models . . . . . . . . . . . . . . . . . . 8 2.2. Specific Data Models . . . . . . . . . . . . . . . . . . 9
3. Specification of the CBOR Encoding . . . . . . . . . . . . . 9 3. Specification of the CBOR Encoding . . . . . . . . . . . . . 9
3.1. Major Types . . . . . . . . . . . . . . . . . . . . . . . 10 3.1. Major Types . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Indefinite Lengths for Some Major Types . . . . . . . . . 12 3.2. Indefinite Lengths for Some Major Types . . . . . . . . . 13
3.2.1. The "break" Stop Code . . . . . . . . . . . . . . . . 12 3.2.1. The "break" Stop Code . . . . . . . . . . . . . . . . 13
3.2.2. Indefinite-Length Arrays and Maps . . . . . . . . . . 12 3.2.2. Indefinite-Length Arrays and Maps . . . . . . . . . . 13
3.2.3. Indefinite-Length Byte Strings and Text Strings . . . 14 3.2.3. Indefinite-Length Byte Strings and Text Strings . . . 15
3.3. Floating-Point Numbers and Values with No Content . . . . 15 3.3. Floating-Point Numbers and Values with No Content . . . . 16
3.4. Optional Tagging of Items . . . . . . . . . . . . . . . . 17 3.4. Tagging of Items . . . . . . . . . . . . . . . . . . . . 17
3.4.1. Date and Time . . . . . . . . . . . . . . . . . . . . 19 3.4.1. Date and Time . . . . . . . . . . . . . . . . . . . . 19
3.4.2. Standard Date/Time String . . . . . . . . . . . . . . 19 3.4.2. Standard Date/Time String . . . . . . . . . . . . . . 19
3.4.3. Epoch-based Date/Time . . . . . . . . . . . . . . . . 19 3.4.3. Epoch-based Date/Time . . . . . . . . . . . . . . . . 20
3.4.4. Bignums . . . . . . . . . . . . . . . . . . . . . . . 20 3.4.4. Bignums . . . . . . . . . . . . . . . . . . . . . . . 20
3.4.5. Decimal Fractions and Bigfloats . . . . . . . . . . . 21 3.4.5. Decimal Fractions and Bigfloats . . . . . . . . . . . 21
3.4.6. Content Hints . . . . . . . . . . . . . . . . . . . . 22 3.4.6. Content Hints . . . . . . . . . . . . . . . . . . . . 23
3.4.6.1. Encoded CBOR Data Item . . . . . . . . . . . . . 22 3.4.6.1. Encoded CBOR Data Item . . . . . . . . . . . . . 23
3.4.6.2. Expected Later Encoding for CBOR-to-JSON 3.4.6.2. Expected Later Encoding for CBOR-to-JSON
Converters . . . . . . . . . . . . . . . . . . . 23 Converters . . . . . . . . . . . . . . . . . . . 23
3.4.6.3. Encoded Text . . . . . . . . . . . . . . . . . . 23 3.4.6.3. Encoded Text . . . . . . . . . . . . . . . . . . 24
3.4.7. Self-Described CBOR . . . . . . . . . . . . . . . . . 24 3.4.7. Self-Described CBOR . . . . . . . . . . . . . . . . . 25
4. Serialization Considerations . . . . . . . . . . . . . . . . 25 4. Serialization Considerations . . . . . . . . . . . . . . . . 25
4.1. Preferred Serialization . . . . . . . . . . . . . . . . . 25 4.1. Preferred Serialization . . . . . . . . . . . . . . . . . 25
4.2. Deterministically Encoded CBOR . . . . . . . . . . . . . 26 4.2. Deterministically Encoded CBOR . . . . . . . . . . . . . 26
4.2.1. Core Deterministic Encoding Requirements . . . . . . 26 4.2.1. Core Deterministic Encoding Requirements . . . . . . 26
4.2.2. Additional Deterministic Encoding Considerations . . 27 4.2.2. Additional Deterministic Encoding Considerations . . 27
4.2.3. Length-first map key ordering . . . . . . . . . . . . 28 4.2.3. Length-first map key ordering . . . . . . . . . . . . 28
5. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 29 5. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 29
5.1. CBOR in Streaming Applications . . . . . . . . . . . . . 30 5.1. CBOR in Streaming Applications . . . . . . . . . . . . . 30
5.2. Generic Encoders and Decoders . . . . . . . . . . . . . . 30 5.2. Generic Encoders and Decoders . . . . . . . . . . . . . . 31
5.3. Invalid Items . . . . . . . . . . . . . . . . . . . . . . 31 5.3. Invalid Items . . . . . . . . . . . . . . . . . . . . . . 31
5.4. Handling Unknown Simple Values and Tags . . . . . . . . . 32 5.4. Handling Unknown Simple Values and Tags . . . . . . . . . 32
5.5. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.5. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.6. Specifying Keys for Maps . . . . . . . . . . . . . . . . 33 5.6. Specifying Keys for Maps . . . . . . . . . . . . . . . . 33
5.6.1. Equivalence of Keys . . . . . . . . . . . . . . . . . 34 5.6.1. Equivalence of Keys . . . . . . . . . . . . . . . . . 34
5.7. Undefined Values . . . . . . . . . . . . . . . . . . . . 35 5.7. Undefined Values . . . . . . . . . . . . . . . . . . . . 35
5.8. Strict Decoding Mode . . . . . . . . . . . . . . . . . . 35 5.8. Strict Decoding Mode . . . . . . . . . . . . . . . . . . 35
6. Converting Data between CBOR and JSON . . . . . . . . . . . . 36 6. Converting Data between CBOR and JSON . . . . . . . . . . . . 37
6.1. Converting from CBOR to JSON . . . . . . . . . . . . . . 36 6.1. Converting from CBOR to JSON . . . . . . . . . . . . . . 37
6.2. Converting from JSON to CBOR . . . . . . . . . . . . . . 38 6.2. Converting from JSON to CBOR . . . . . . . . . . . . . . 38
7. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 39 7. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 39
7.1. Extension Points . . . . . . . . . . . . . . . . . . . . 39 7.1. Extension Points . . . . . . . . . . . . . . . . . . . . 40
7.2. Curating the Additional Information Space . . . . . . . . 40 7.2. Curating the Additional Information Space . . . . . . . . 40
8. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 40 8. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 41
8.1. Encoding Indicators . . . . . . . . . . . . . . . . . . . 41 8.1. Encoding Indicators . . . . . . . . . . . . . . . . . . . 42
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
9.1. Simple Values Registry . . . . . . . . . . . . . . . . . 42 9.1. Simple Values Registry . . . . . . . . . . . . . . . . . 43
9.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 42 9.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 43
9.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 43 9.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 43
9.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 44 9.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 44
9.5. The +cbor Structured Syntax Suffix Registration . . . . . 44 9.5. The +cbor Structured Syntax Suffix Registration . . . . . 45
10. Security Considerations . . . . . . . . . . . . . . . . . . . 45 10. Security Considerations . . . . . . . . . . . . . . . . . . . 45
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 47 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
11.1. Normative References . . . . . . . . . . . . . . . . . . 47 11.1. Normative References . . . . . . . . . . . . . . . . . . 47
11.2. Informative References . . . . . . . . . . . . . . . . . 48 11.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 50 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 50
Appendix B. Jump Table . . . . . . . . . . . . . . . . . . . . . 54 Appendix B. Jump Table . . . . . . . . . . . . . . . . . . . . . 54
Appendix C. Pseudocode . . . . . . . . . . . . . . . . . . . . . 57 Appendix C. Pseudocode . . . . . . . . . . . . . . . . . . . . . 57
Appendix D. Half-Precision . . . . . . . . . . . . . . . . . . . 59 Appendix D. Half-Precision . . . . . . . . . . . . . . . . . . . 59
Appendix E. Comparison of Other Binary Formats to CBOR's Design Appendix E. Comparison of Other Binary Formats to CBOR's Design
Objectives . . . . . . . . . . . . . . . . . . . . . 60 Objectives . . . . . . . . . . . . . . . . . . . . . 60
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to note that this is not a proposal that the grammar in RFC 8259 be to note that this is not a proposal that the grammar in RFC 8259 be
extended in general, since doing so would cause a significant extended in general, since doing so would cause a significant
backwards incompatibility with already deployed JSON documents. backwards incompatibility with already deployed JSON documents.
Instead, this document simply defines its own data model that starts Instead, this document simply defines its own data model that starts
from JSON. from JSON.
Appendix E lists some existing binary formats and discusses how well Appendix E lists some existing binary formats and discusses how well
they do or do not fit the design objectives of the Concise Binary they do or do not fit the design objectives of the Concise Binary
Object Representation (CBOR). Object Representation (CBOR).
This document obsoletes [RFC7049]. This document is a revised edition of [RFC7049], with editorial
improvements, added detail, and fixed errata. This revision formally
obsoletes RFC 7049, while keeping full compatibility of the
interchange format from RFC 7049. It does not create a new version
of the format.
1.1. Objectives 1.1. Objectives
The objectives of CBOR, roughly in decreasing order of importance, The objectives of CBOR, roughly in decreasing order of importance,
are: are:
1. The representation must be able to unambiguously encode most 1. The representation must be able to unambiguously encode most
common data formats used in Internet standards. common data formats used in Internet standards.
* It must represent a reasonable set of basic data types and * It must represent a reasonable set of basic data types and
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many applications will provide their own application-specific many applications will provide their own application-specific
encoders and/or decoders. encoders and/or decoders.
In the basic (un-extended) generic data model, a data item is one of: In the basic (un-extended) generic data model, a data item is one of:
o an integer in the range -2**64..2**64-1 inclusive o an integer in the range -2**64..2**64-1 inclusive
o a simple value, identified by a number between 0 and 255, but o a simple value, identified by a number between 0 and 255, but
distinct from that number distinct from that number
o a floating point value, distinct from an integer, out of the set o a floating-point value, distinct from an integer, out of the set
representable by IEEE 754 binary64 (including non-finites) representable by IEEE 754 binary64 (including non-finites)
[IEEE754] [IEEE754]
o a sequence of zero or more bytes ("byte string") o a sequence of zero or more bytes ("byte string")
o a sequence of zero or more Unicode code points ("text string") o a sequence of zero or more Unicode code points ("text string")
o a sequence of zero or more data items ("array") o a sequence of zero or more data items ("array")
o a mapping (mathematical function) from zero or more data items o a mapping (mathematical function) from zero or more data items
("keys") each to a data item ("values"), ("map") ("keys") each to a data item ("values"), ("map")
o a tagged data item, comprising a tag (an integer in the range o a tagged data item ("tag"), comprising a tag number (an integer in
0..2**64-1) and a value (a data item) the range 0..2**64-1) and a tagged value (a data item)
Note that integer and floating-point values are distinct in this Note that integer and floating-point values are distinct in this
model, even if they have the same numeric value. model, even if they have the same numeric value.
Also note that serialization variants, such as number of bytes of the Also note that serialization variants, such as number of bytes of the
encoded floating value, or the choice of one of the ways in which an encoded floating value, or the choice of one of the ways in which an
integer, the length of a text or byte string, the number of elements integer, the length of a text or byte string, the number of elements
in an array or pairs in a map, or a tag value, (collectively "the in an array or pairs in a map, or a tag number, (collectively "the
argument", see Section 3) can be encoded, are not visible at the argument", see Section 3) can be encoded, are not visible at the
generic data model level. generic data model level.
2.1. Extended Generic Data Models 2.1. Extended Generic Data Models
This basic generic data model comes pre-extended by the registration This basic generic data model comes pre-extended by the registration
of a number of simple values and tags right in this document, such of a number of simple values and tag numbers right in this document,
as: such as:
o "false", "true", "null", and "undefined" (simple values identified o "false", "true", "null", and "undefined" (simple values identified
by 20..23) by 20..23)
o integer and floating point values with a larger range and o integer and floating-point values with a larger range and
precision than the above (tags 2 to 5) precision than the above (tag numbers 2 to 5)
o application data types such as a point in time or an RFC 3339 o application data types such as a point in time or an RFC 3339
date/time string (tags 1, 0) date/time string (tag numbers 1, 0)
Further elements of the extended generic data model can be (and have Further elements of the extended generic data model can be (and have
been) defined via the IANA registries created for CBOR. Even if such been) defined via the IANA registries created for CBOR. Even if such
an extension is unknown to a generic encoder or decoder, data items an extension is unknown to a generic encoder or decoder, data items
using that extension can be passed to or from the application by using that extension can be passed to or from the application by
representing them at the interface to the application within the representing them at the interface to the application within the
basic generic data model, i.e., as generic values of a simple type or basic generic data model, i.e., as generic values of a simple type or
generic tagged items. generic tags.
In other words, the basic generic data model is stable as defined in In other words, the basic generic data model is stable as defined in
this document, while the extended generic data model expands by the this document, while the extended generic data model expands by the
registration of new simple values or tags, but never shrinks. registration of new simple values or tag numbers, but never shrinks.
While there is a strong expectation that generic encoders and While there is a strong expectation that generic encoders and
decoders can represent "false", "true", and "null" ("undefined" is decoders can represent "false", "true", and "null" ("undefined" is
intentionally omitted) in the form appropriate for their programming intentionally omitted) in the form appropriate for their programming
environment, implementation of the data model extensions created by environment, implementation of the data model extensions created by
tags is truly optional and a matter of implementation quality. tags is truly optional and a matter of implementation quality.
2.2. Specific Data Models 2.2. Specific Data Models
The specific data model for a CBOR-based protocol usually subsets the The specific data model for a CBOR-based protocol usually subsets the
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of data items, it is preferred to identify the types by the names of data items, it is preferred to identify the types by the names
they have in the generic data model ("negative integer", "array") they have in the generic data model ("negative integer", "array")
instead of by referring to aspects of their CBOR representation instead of by referring to aspects of their CBOR representation
("major type 1", "major type 4"). ("major type 1", "major type 4").
Specific data models can also specify what values (including values Specific data models can also specify what values (including values
of different types) are equivalent for the purposes of map keys and of different types) are equivalent for the purposes of map keys and
encoder freedom. For example, in the generic data model, a valid map encoder freedom. For example, in the generic data model, a valid map
MAY have both "0" and "0.0" as keys, and an encoder MUST NOT encode MAY have both "0" and "0.0" as keys, and an encoder MUST NOT encode
"0.0" as an integer (major type 0, Section 3.1). However, if a "0.0" as an integer (major type 0, Section 3.1). However, if a
specific data model declares that floating point and integer specific data model declares that floating-point and integer
representations of integral values are equivalent, using both map representations of integral values are equivalent, using both map
keys "0" and "0.0" in a single map would be considered duplicates and keys "0" and "0.0" in a single map would be considered duplicates and
so invalid, and an encoder could encode integral-valued floats as so invalid, and an encoder could encode integral-valued floats as
integers or vice versa, perhaps to save encoded bytes. integers or vice versa, perhaps to save encoded bytes.
3. Specification of the CBOR Encoding 3. Specification of the CBOR Encoding
A CBOR data item (Section 2) is encoded to or decoded from a byte A CBOR data item (Section 2) is encoded to or decoded from a byte
string carrying a well-formed encoded data item as described in this string carrying a well-formed encoded data item as described in this
section. The encoding is summarized in Table 5. An encoder MUST section. The encoding is summarized in Table 6. An encoder MUST
produce only well-formed encoded data items. A decoder MUST NOT produce only well-formed encoded data items. A decoder MUST NOT
return a decoded data item when it encounters input that is not a return a decoded data item when it encounters input that is not a
well-formed encoded CBOR data item (this does not detract from the well-formed encoded CBOR data item (this does not detract from the
usefulness of diagnostic and recovery tools that might make available usefulness of diagnostic and recovery tools that might make available
some information from a damaged encoded CBOR data item). some information from a damaged encoded CBOR data item).
The initial byte of each encoded data item contains both information The initial byte of each encoded data item contains both information
about the major type (the high-order 3 bits, described in about the major type (the high-order 3 bits, described in
Section 3.1) and additional information (the low-order 5 bits). With Section 3.1) and additional information (the low-order 5 bits). With
a few exceptions, the additional information's value describes how to a few exceptions, the additional information's value describes how to
load an unsigned integer "argument": load an unsigned integer "argument":
Less than 24: The argument's value is the value of the additional Less than 24: The argument's value is the value of the additional
information. information.
24, 25, 26, or 27: The argument's value is held in the following 1, 24, 25, 26, or 27: The argument's value is held in the following 1,
2, 4, or 8 bytes, respectively, in network byte order. For major 2, 4, or 8 bytes, respectively, in network byte order. For major
type 7 and additional information value 25, 26, 27, these bytes type 7 and additional information value 25, 26, 27, these bytes
are not used as an integer argument, but as a floating point value are not used as an integer argument, but as a floating-point value
(see Section 3.3). (see Section 3.3).
28, 29, 30: These values are reserved for future additions to the 28, 29, 30: These values are reserved for future additions to the
CBOR format. In the present version of CBOR, the encoded item is CBOR format. In the present version of CBOR, the encoded item is
not well-formed. not well-formed.
31: No argument value is derived. If the major type is 0, 1, or 6, 31: No argument value is derived. If the major type is 0, 1, or 6,
the encoded item is not well-formed. For major types 2 to 5, the the encoded item is not well-formed. For major types 2 to 5, the
item's length is indefinite, and for major type 7, the byte does item's length is indefinite, and for major type 7, the byte does
not consitute a data item at all but terminates an indefinite not consitute a data item at all but terminates an indefinite
length item; both are described in Section 3.2. length item; both are described in Section 3.2.
The initial byte and any additional bytes consumed to construct the
argument are collectively referred to as the "head" of the data item.
The meaning of this argument depends on the major type. For example, The meaning of this argument depends on the major type. For example,
in major type 0, the argument is the value of the data item itself in major type 0, the argument is the value of the data item itself
(and in major type 1 the value of the data item is computed from the (and in major type 1 the value of the data item is computed from the
argument); in major type 2 and 3 it gives the length of the string argument); in major type 2 and 3 it gives the length of the string
data in bytes that follows; and in major types 4 and 5 it is used to data in bytes that follows; and in major types 4 and 5 it is used to
determine the number of data items enclosed. determine the number of data items enclosed.
If the encoded sequence of bytes ends before the end of a data item, If the encoded sequence of bytes ends before the end of a data item,
that item is not well-formed. If the encoded sequence of bytes still that item is not well-formed. If the encoded sequence of bytes still
has bytes remaining after the outermost encoded item is decoded, that has bytes remaining after the outermost encoded item is decoded, that
encoding is not a single well-formed CBOR item; depending on the encoding is not a single well-formed CBOR item; depending on the
application, the decoder may either treat the encoding as not well- application, the decoder may either treat the encoding as not well-
formed or just identify the start of the remaining bytes to the formed or just identify the start of the remaining bytes to the
application. application.
A CBOR decoder implementation can be based on a jump table with all A CBOR decoder implementation can be based on a jump table with all
256 defined values for the initial byte (Table 5). A decoder in a 256 defined values for the initial byte (Table 6). A decoder in a
constrained implementation can instead use the structure of the constrained implementation can instead use the structure of the
initial byte and following bytes for more compact code (see initial byte and following bytes for more compact code (see
Appendix C for a rough impression of how this could look). Appendix C for a rough impression of how this could look).
3.1. Major Types 3.1. Major Types
The following lists the major types and the additional information The following lists the major types and the additional information
and other bytes associated with the type. and other bytes associated with the type.
Major type 0: an integer in the range 0..2**64-1 inclusive. The Major type 0: an integer in the range 0..2**64-1 inclusive. The
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Major type 5: a map of pairs of data items. Maps are also called Major type 5: a map of pairs of data items. Maps are also called
tables, dictionaries, hashes, or objects (in JSON). A map is tables, dictionaries, hashes, or objects (in JSON). A map is
comprised of pairs of data items, each pair consisting of a key comprised of pairs of data items, each pair consisting of a key
that is immediately followed by a value. The argument is the that is immediately followed by a value. The argument is the
number of _pairs_ of data items in the map. For example, a map number of _pairs_ of data items in the map. For example, a map
that contains 9 pairs would have an initial byte of 0b101_01001 that contains 9 pairs would have an initial byte of 0b101_01001
(major type of 5, additional information of 9 for the number of (major type of 5, additional information of 9 for the number of
pairs) followed by the 18 remaining items. The first item is the pairs) followed by the 18 remaining items. The first item is the
first key, the second item is the first value, the third item is first key, the second item is the first value, the third item is
the second key, and so on. A map that has duplicate keys may be the second key, and so on. Because items in a map come in pairs,
their total number is always even: A map that contains an odd
number of items (no value data present after the last key data
item) is not well-formed. A map that has duplicate keys may be
well-formed, but it is not valid, and thus it causes indeterminate well-formed, but it is not valid, and thus it causes indeterminate
decoding; see also Section 5.6. decoding; see also Section 5.6.
Major type 6: a tagged data item whose tag is the argument and whose Major type 6: a tagged data item ("tag") whose tag number is the
value is the single following encoded item. See Section 3.4. argument and whose enclosed data item is the single encoded data
item that follows the head. See Section 3.4.
Major type 7: floating-point numbers and simple values, as well as Major type 7: floating-point numbers and simple values, as well as
the "break" stop code. See Section 3.3. the "break" stop code. See Section 3.3.
These eight major types lead to a simple table showing which of the These eight major types lead to a simple table showing which of the
256 possible values for the initial byte of a data item are used 256 possible values for the initial byte of a data item are used
(Table 5). (Table 6).
In major types 6 and 7, many of the possible values are reserved for In major types 6 and 7, many of the possible values are reserved for
future specification. See Section 9 for more information on these future specification. See Section 9 for more information on these
values. values.
Table 1 summarizes the major types defined by CBOR, ignoring the next
section for now. The number N in this table stands for the argument,
mt for the major type.
+----+-----------------------+---------------------------------+
| mt | Meaning | Content |
+----+-----------------------+---------------------------------+
| 0 | unsigned integer N | - |
| | | |
| 1 | negative integer -1-N | - |
| | | |
| 2 | byte string | N bytes |
| | | |
| 3 | text string | N bytes (UTF-8 text) |
| | | |
| 4 | array | N data items (elements) |
| | | |
| 5 | map | 2N data items (key/value pairs) |
| | | |
| 6 | tag of number N | 1 data item |
| | | |
| 7 | simple/float | - |
+----+-----------------------+---------------------------------+
Table 1: Overview over CBOR major types (definite length encoded)
3.2. Indefinite Lengths for Some Major Types 3.2. Indefinite Lengths for Some Major Types
Four CBOR items (arrays, maps, byte strings, and text strings) can be Four CBOR items (arrays, maps, byte strings, and text strings) can be
encoded with an indefinite length using additional information value encoded with an indefinite length using additional information value
31. This is useful if the encoding of the item needs to begin before 31. This is useful if the encoding of the item needs to begin before
the number of items inside the array or map, or the total length of the number of items inside the array or map, or the total length of
the string, is known. (The application of this is often referred to the string, is known. (The application of this is often referred to
as "streaming" within a data item.) as "streaming" within a data item.)
Indefinite-length arrays and maps are dealt with differently than Indefinite-length arrays and maps are dealt with differently than
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If the "break" stop code appears anywhere where a data item is If the "break" stop code appears anywhere where a data item is
expected, other than directly inside an indefinite-length string, expected, other than directly inside an indefinite-length string,
array, or map -- for example directly inside a definite-length array array, or map -- for example directly inside a definite-length array
or map -- the enclosing item is not well-formed. or map -- the enclosing item is not well-formed.
3.2.2. Indefinite-Length Arrays and Maps 3.2.2. Indefinite-Length Arrays and Maps
Indefinite-length arrays and maps are represented using their major Indefinite-length arrays and maps are represented using their major
type with the additional information value of 31, followed by an type with the additional information value of 31, followed by an
arbitrary-length sequence of items for an array or key/value pairs arbitrary-length sequence of zero or more items for an array or key/
for a map, followed by the "break" stop code (Section 3.2.1). In value pairs for a map, followed by the "break" stop code
other words, indefinite-length arrays and maps look identical to (Section 3.2.1). In other words, indefinite-length arrays and maps
other arrays and maps except for beginning with the additional look identical to other arrays and maps except for beginning with the
information value of 31 and ending with the "break" stop code. additional information value of 31 and ending with the "break" stop
code.
If the break stop code appears after a key in a map, in place of that If the break stop code appears after a key in a map, in place of that
key's value, the map is not well-formed. key's value, the map is not well-formed.
There is no restriction against nesting indefinite-length array or There is no restriction against nesting indefinite-length array or
map items. A "break" only terminates a single item, so nested map items. A "break" only terminates a single item, so nested
indefinite-length items need exactly as many "break" stop codes as indefinite-length items need exactly as many "break" stop codes as
there are type bytes starting an indefinite-length item. there are type bytes starting an indefinite-length item.
For example, assume an encoder wants to represent the abstract array For example, assume an encoder wants to represent the abstract array
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F5 -- First value, true F5 -- First value, true
63 -- Second key, UTF-8 string length 3 63 -- Second key, UTF-8 string length 3
416d74 -- "Amt" 416d74 -- "Amt"
21 -- Second value, -2 21 -- Second value, -2
FF -- "break" FF -- "break"
3.2.3. Indefinite-Length Byte Strings and Text Strings 3.2.3. Indefinite-Length Byte Strings and Text Strings
Indefinite-length strings are represented by a byte containing the Indefinite-length strings are represented by a byte containing the
major type and additional information value of 31, followed by a major type and additional information value of 31, followed by a
series of byte or text strings ("chunks") that have definite lengths, series of zero or more byte or text strings ("chunks") that have
followed by the "break" stop code (Section 3.2.1). The data item definite lengths, followed by the "break" stop code (Section 3.2.1).
represented by the indefinite-length string is the concatenation of The data item represented by the indefinite-length string is the
the chunks. concatenation of the chunks (i.e., the empty byte or text string,
respectively, if no chunk is present).
If any item between the indefinite-length string indicator If any item between the indefinite-length string indicator
(0b010_11111 or 0b011_11111) and the "break" stop code is not a (0b010_11111 or 0b011_11111) and the "break" stop code is not a
definite-length string item of the same major type, the string is not definite-length string item of the same major type, the string is not
well-formed. well-formed.
If any definite-length text string inside an indefinite-length text If any definite-length text string inside an indefinite-length text
string is invalid, the indefinite-length text string is invalid. string is invalid, the indefinite-length text string is invalid.
Note that this implies that the bytes of a single UTF-8 character Note that this implies that the bytes of a single UTF-8 character
cannot be spread between chunks: a new chunk can only be started at a cannot be spread between chunks: a new chunk can only be started at a
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FF -- "break" FF -- "break"
After decoding, this results in a single byte string with seven After decoding, this results in a single byte string with seven
bytes: 0xaabbccddeeff99. bytes: 0xaabbccddeeff99.
3.3. Floating-Point Numbers and Values with No Content 3.3. Floating-Point Numbers and Values with No Content
Major type 7 is for two types of data: floating-point numbers and Major type 7 is for two types of data: floating-point numbers and
"simple values" that do not need any content. Each value of the "simple values" that do not need any content. Each value of the
5-bit additional information in the initial byte has its own separate 5-bit additional information in the initial byte has its own separate
meaning, as defined in Table 1. Like the major types for integers, meaning, as defined in Table 2. Like the major types for integers,
items of this major type do not carry content data; all the items of this major type do not carry content data; all the
information is in the initial bytes. information is in the initial bytes.
+------------+------------------------------------------------------+ +------------+------------------------------------------------------+
| 5-Bit | Semantics | | 5-Bit | Semantics |
| Value | | | Value | |
+------------+------------------------------------------------------+ +------------+------------------------------------------------------+
| 0..23 | Simple value (value 0..23) | | 0..23 | Simple value (value 0..23) |
| | | | | |
| 24 | Simple value (value 32..255 in following byte) | | 24 | Simple value (value 32..255 in following byte) |
| | | | | |
| 25 | IEEE 754 Half-Precision Float (16 bits follow) | | 25 | IEEE 754 Half-Precision Float (16 bits follow) |
| | | | | |
| 26 | IEEE 754 Single-Precision Float (32 bits follow) | | 26 | IEEE 754 Single-Precision Float (32 bits follow) |
| | | | | |
| 27 | IEEE 754 Double-Precision Float (64 bits follow) | | 27 | IEEE 754 Double-Precision Float (64 bits follow) |
| | | | | |
| 28-30 | Unassigned, not well-formed in the present document | | 28-30 | Reserved, not well-formed in the present document |
| | | | | |
| 31 | "break" stop code for indefinite-length items | | 31 | "break" stop code for indefinite-length items |
| | (Section 3.2.1) | | | (Section 3.2.1) |
+------------+------------------------------------------------------+ +------------+------------------------------------------------------+
Table 1: Values for Additional Information in Major Type 7 Table 2: Values for Additional Information in Major Type 7
As with all other major types, the 5-bit value 24 signifies a single- As with all other major types, the 5-bit value 24 signifies a single-
byte extension: it is followed by an additional byte to represent the byte extension: it is followed by an additional byte to represent the
simple value. (To minimize confusion, only the values 32 to 255 are simple value. (To minimize confusion, only the values 32 to 255 are
used.) This maintains the structure of the initial bytes: as for the used.) This maintains the structure of the initial bytes: as for the
other major types, the length of these always depends on the other major types, the length of these always depends on the
additional information in the first byte. Table 2 lists the values additional information in the first byte. Table 3 lists the values
assigned and available for simple types. assigned and available for simple types.
+---------+-----------------+ +---------+-----------------+
| Value | Semantics | | Value | Semantics |
+---------+-----------------+ +---------+-----------------+
| 0..19 | (Unassigned) | | 0..19 | (Unassigned) |
| | | | | |
| 20 | False | | 20 | False |
| | | | | |
| 21 | True | | 21 | True |
| | | | | |
| 22 | Null | | 22 | Null |
| | | | | |
| 23 | Undefined value | | 23 | Undefined value |
| | | | | |
| 24..31 | (Reserved) | | 24..31 | (Reserved) |
| | | | | |
| 32..255 | (Unassigned) | | 32..255 | (Unassigned) |
+---------+-----------------+ +---------+-----------------+
Table 2: Simple Values Table 3: Simple Values
An encoder MUST NOT issue two-byte sequences that start with 0xf8 An encoder MUST NOT issue two-byte sequences that start with 0xf8
(major type = 7, additional information = 24) and continue with a (major type = 7, additional information = 24) and continue with a
byte less than 0x20 (32 decimal). Such sequences are not well- byte less than 0x20 (32 decimal). Such sequences are not well-
formed. (This implies that an encoder cannot encode false, true, formed. (This implies that an encoder cannot encode false, true,
null, or undefined in two-byte sequences, only the one-byte variants null, or undefined in two-byte sequences, only the one-byte variants
of these are well-formed.) of these are well-formed.)
The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit
IEEE 754 binary floating-point values [IEEE754]. These floating- IEEE 754 binary floating-point values [IEEE754]. These floating-
point values are encoded in the additional bytes of the appropriate point values are encoded in the additional bytes of the appropriate
size. (See Appendix D for some information about 16-bit floating size. (See Appendix D for some information about 16-bit floating
point.) point.)
3.4. Optional Tagging of Items 3.4. Tagging of Items
In CBOR, a data item can optionally be preceded by a tag to give it In CBOR, a data item can be enclosed by a tag to give it additional
additional semantics while retaining its structure. The tag is major semantics while retaining its structure. The tag is major type 6,
type 6, and represents an unsigned integer as indicated by the tag's and represents an unsigned integer as indicated by the tag's argument
argument (Section 3); the (sole) data item is carried as content (Section 3); the (sole) enclosed data item is carried as content
data. If a tag requires structured data, this structure is encoded data. If a tag requires structured data, this structure is encoded
into the nested data item. The definition of a tag usually restricts into the nested data item. The definition of a tag number usually
what kinds of nested data item or items are valid for this tag. restricts what kinds of nested data item or items are valid for tags
using this tag number.
For example, assume that a byte string of length 12 is marked with a For example, assume that a byte string of length 12 is marked with a
tag to indicate it is a positive bignum (Section 3.4.4). This would tag of number 2 to indicate it is a positive bignum (Section 3.4.4).
be marked as 0b110_00010 (major type 6, additional information 2 for This would be marked as 0b110_00010 (major type 6, additional
the tag) followed by 0b010_01100 (major type 2, additional information 2 for the tag number) followed by 0b010_01100 (major type
information of 12 for the length) followed by the 12 bytes of the 2, additional information of 12 for the length) followed by the 12
bignum. bytes of the bignum.
Decoders do not need to understand tags, and thus tags may be of Decoders do not need to understand tags of every tag number, and tags
little value in applications where the implementation creating a may be of little value in applications where the implementation
particular CBOR data item and the implementation decoding that stream creating a particular CBOR data item and the implementation decoding
know the semantic meaning of each item in the data flow. Their that stream know the semantic meaning of each item in the data flow.
primary purpose in this specification is to define common data types Their primary purpose in this specification is to define common data
such as dates. A secondary purpose is to allow optional tagging when types such as dates. A secondary purpose is to allow optional
the decoder is a generic CBOR decoder that might be able to benefit tagging when the decoder is a generic CBOR decoder that might be able
from hints about the content of items. Understanding the semantic to benefit from hints about the content of items. Understanding the
tags is optional for a decoder; it can just jump over the initial semantic tags is optional for a decoder; it can just jump over the
bytes of the tag and interpret the tagged data item itself. initial bytes of the tag and interpret the tagged data item itself.
A tag always applies to the item that directly follows it. Thus, if A tag applies semantics to the data item it encloses. Thus, if tag A
tag A is followed by tag B, which is followed by data item C, tag A encloses tag B, which encloses data item C, tag A applies to the
applies to the result of applying tag B on data item C. That is, a result of applying tag B on data item C. That is, a tagged item is a
tagged item is a data item consisting of a tag and a value. The data item consisting of a tag number and an enclosed value. The
content of the tagged item is the data item (the value) that is being content of the tagged item (the enclosed data item) is the data item
tagged. (the value) that is being tagged.
IANA maintains a registry of tag values as described in Section 9.2. IANA maintains a registry of tag numbers as described in Section 9.2.
Table 3 provides a list of values that were defined in [RFC7049], Table 4 provides a list of tag numbers that were defined in
with definitions in the rest of this section. Note that many other [RFC7049], with definitions in the rest of this section. Note that
tags have been defined since the publication of [RFC7049]; see the many other tag numbers have been defined since the publication of
registry described at Section 9.2 for the complete list. [RFC7049]; see the registry described at Section 9.2 for the complete
list.
+-------+-----------+-----------------------------------------------+ +----------+----------+---------------------------------------------+
| Tag | Data Item | Semantics | | Tag | Data | Semantics |
+-------+-----------+-----------------------------------------------+ | Number | Item | |
| 0 | UTF-8 | Standard date/time string; see Section 3.4.2 | +----------+----------+---------------------------------------------+
| | string | | | 0 | text | Standard date/time string; see |
| | | | | | string | Section 3.4.2 |
| 1 | multiple | Epoch-based date/time; see Section 3.4.3 | | | | |
| | | | | 1 | multiple | Epoch-based date/time; see Section 3.4.3 |
| 2 | byte | Positive bignum; see Section 3.4.4 | | | | |
| | string | | | 2 | byte | Positive bignum; see Section 3.4.4 |
| | | | | | string | |
| 3 | byte | Negative bignum; see Section 3.4.4 | | | | |
| | string | | | 3 | byte | Negative bignum; see Section 3.4.4 |
| | | | | | string | |
| 4 | array | Decimal fraction; see Section 3.4.5 | | | | |
| | | | | 4 | array | Decimal fraction; see Section 3.4.5 |
| 5 | array | Bigfloat; see Section 3.4.5 | | | | |
| | | | | 5 | array | Bigfloat; see Section 3.4.5 |
| 21 | multiple | Expected conversion to base64url encoding; | | | | |
| | | see Section 3.4.6.2 | | 21 | multiple | Expected conversion to base64url encoding; |
| | | | | | | see Section 3.4.6.2 |
| 22 | multiple | Expected conversion to base64 encoding; see | | | | |
| | | Section 3.4.6.2 | | 22 | multiple | Expected conversion to base64 encoding; see |
| | | | | | | Section 3.4.6.2 |
| 23 | multiple | Expected conversion to base16 encoding; see | | | | |
| | | Section 3.4.6.2 | | 23 | multiple | Expected conversion to base16 encoding; see |
| | | | | | | Section 3.4.6.2 |
| 24 | byte | Encoded CBOR data item; see Section 3.4.6.1 | | | | |
| | string | | | 24 | byte | Encoded CBOR data item; see Section 3.4.6.1 |
| | | | | | string | |
| 32 | UTF-8 | URI; see Section 3.4.6.3 | | | | |
| | string | | | 32 | text | URI; see Section 3.4.6.3 |
| | | | | | string | |
| 33 | UTF-8 | base64url; see Section 3.4.6.3 | | | | |
| | string | | | 33 | text | base64url; see Section 3.4.6.3 |
| | | | | | string | |
| 34 | UTF-8 | base64; see Section 3.4.6.3 | | | | |
| | string | | | 34 | text | base64; see Section 3.4.6.3 |
| | | | | | string | |
| 35 | UTF-8 | Regular expression; see Section 3.4.6.3 | | | | |
| | string | | | 35 | text | Regular expression; see Section 3.4.6.3 |
| | | | | | string | |
| 36 | UTF-8 | MIME message; see Section 3.4.6.3 | | | | |
| | string | | | 36 | text | MIME message; see Section 3.4.6.3 |
| | | | | | string | |
| 55799 | multiple | Self-described CBOR; see Section 3.4.7 | | | | |
+-------+-----------+-----------------------------------------------+ | 55799 | multiple | Self-described CBOR; see Section 3.4.7 |
+----------+----------+---------------------------------------------+
Table 3: Values for Tags Table 4: Tag numbers defined in RFC 7049
3.4.1. Date and Time 3.4.1. Date and Time
Protocols using tag values 0 and 1 extend the generic data model Protocols using tag numbers 0 and 1 extend the generic data model
(Section 2) with data items representing points in time. (Section 2) with data items representing points in time.
3.4.2. Standard Date/Time String 3.4.2. Standard Date/Time String
Tag value 0 contains a text string in the standard format described Tag number 0 contains a text string in the standard format described
by the "date-time" production in [RFC3339], as refined by Section 3.3 by the "date-time" production in [RFC3339], as refined by Section 3.3
of [RFC4287], representing the point in time described there. A of [RFC4287], representing the point in time described there. A
nested item of another type or that doesn't match the [RFC4287] nested item of another type or that doesn't match the [RFC4287]
format is invalid. format is invalid.
3.4.3. Epoch-based Date/Time 3.4.3. Epoch-based Date/Time
Tag value 1 contains a numerical value counting the number of seconds Tag number 1 contains a numerical value counting the number of
from 1970-01-01T00:00Z in UTC time to the represented point in civil seconds from 1970-01-01T00:00Z in UTC time to the represented point
time. in civil time.
The tagged item MUST be an unsigned or negative integer (major types The enclosed item MUST be an unsigned or negative integer (major
0 and 1), or a floating-point number (major type 7 with additional types 0 and 1), or a floating-point number (major type 7 with
information 25, 26, or 27). Other contained types are invalid. additional information 25, 26, or 27). Other contained types are
invalid.
Non-negative values (major type 0 and non-negative floating-point Non-negative values (major type 0 and non-negative floating-point
numbers) stand for time values on or after 1970-01-01T00:00Z UTC and numbers) stand for time values on or after 1970-01-01T00:00Z UTC and
are interpreted according to POSIX [TIME_T]. (POSIX time is also are interpreted according to POSIX [TIME_T]. (POSIX time is also
known as UNIX Epoch time. Note that leap seconds are handled known as UNIX Epoch time. Note that leap seconds are handled
specially by POSIX time and this results in a 1 second discontinuity specially by POSIX time and this results in a 1 second discontinuity
several times per decade.) Note that applications that require the several times per decade.) Note that applications that require the
expression of times beyond early 2106 cannot leave out support of expression of times beyond early 2106 cannot leave out support of
64-bit integers for the tagged value. 64-bit integers for the enclosed value.
Negative values (major type 1 and negative floating-point numbers) Negative values (major type 1 and negative floating-point numbers)
are interpreted as determined by the application requirements as are interpreted as determined by the application requirements as
there is no universal standard for UTC count-of-seconds time before there is no universal standard for UTC count-of-seconds time before
1970-01-01T00:00Z (this is particularly true for points in time that 1970-01-01T00:00Z (this is particularly true for points in time that
precede discontinuities in national calendars). The same applies to precede discontinuities in national calendars). The same applies to
non-finite values. non-finite values.
To indicate fractional seconds, floating point values can be used To indicate fractional seconds, floating-point values can be used
within Tag 1 instead of integer values. Note that this generally within Tag number 1 instead of integer values. Note that this
requires binary64 support, as binary16 and binary32 provide non-zero generally requires binary64 support, as binary16 and binary32 provide
fractions of seconds only for a short period of time around early non-zero fractions of seconds only for a short period of time around
1970. An application that requires Tag 1 support may restrict the early 1970. An application that requires Tag number 1 support may
tagged value to be an integer (or a floating-point value) only. restrict the enclosed value to be an integer (or a floating-point
value) only.
3.4.4. Bignums 3.4.4. Bignums
Protocols using tag values 2 and 3 extend the generic data model Protocols using tag numbers 2 and 3 extend the generic data model
(Section 2) with "bignums" representing arbitrarily sized integers. (Section 2) with "bignums" representing arbitrarily sized integers.
In the generic data model, bignum values are not equal to integers In the generic data model, bignum values are not equal to integers
from the basic data model, but specific data models can define that from the basic data model, but specific data models can define that
equivalence, and preferred encoding never makes use of bignums that equivalence, and preferred encoding never makes use of bignums that
also can be expressed as basic integers (see below). also can be expressed as basic integers (see below).
Bignums are encoded as a byte string data item, which is interpreted Bignums are encoded as a byte string data item, which is interpreted
as an unsigned integer n in network byte order. Contained items of as an unsigned integer n in network byte order. Contained items of
other types are invalid. For tag value 2, the value of the bignum is other types are invalid. For tag number 2, the value of the bignum
n. For tag value 3, the value of the bignum is -1 - n. The is n. For tag number 3, the value of the bignum is -1 - n. The
preferred encoding of the byte string is to leave out any leading preferred encoding of the byte string is to leave out any leading
zeroes (note that this means the preferred encoding for n = 0 is the zeroes (note that this means the preferred encoding for n = 0 is the
empty byte string, but see below). Decoders that understand these empty byte string, but see below). Decoders that understand these
tags MUST be able to decode bignums that do have leading zeroes. The tags MUST be able to decode bignums that do have leading zeroes. The
preferred encoding of an integer that can be represented using major preferred encoding of an integer that can be represented using major
type 0 or 1 is to encode it this way instead of as a bignum (which type 0 or 1 is to encode it this way instead of as a bignum (which
means that the empty string never occurs in a bignum when using means that the empty string never occurs in a bignum when using
preferred encoding). Note that this means the non-preferred choice preferred encoding). Note that this means the non-preferred choice
of a bignum representation instead of a basic integer for encoding a of a bignum representation instead of a basic integer for encoding a
number is not intended to have application semantics (just as the number is not intended to have application semantics (just as the
choice of a longer basic integer representation than needed, such as choice of a longer basic integer representation than needed, such as
0x1800 for 0x00 does not). 0x1800 for 0x00 does not).
For example, the number 18446744073709551616 (2**64) is represented For example, the number 18446744073709551616 (2**64) is represented
as 0b110_00010 (major type 6, tag 2), followed by 0b010_01001 (major as 0b110_00010 (major type 6, tag number 2), followed by 0b010_01001
type 2, length 9), followed by 0x010000000000000000 (one byte 0x01 (major type 2, length 9), followed by 0x010000000000000000 (one byte
and eight bytes 0x00). In hexadecimal: 0x01 and eight bytes 0x00). In hexadecimal:
C2 -- Tag 2 C2 -- Tag 2
49 -- Byte string of length 9 49 -- Byte string of length 9
010000000000000000 -- Bytes content 010000000000000000 -- Bytes content
3.4.5. Decimal Fractions and Bigfloats 3.4.5. Decimal Fractions and Bigfloats
Protocols using tag value 4 extend the generic data model with data Protocols using tag number 4 extend the generic data model with data
items representing arbitrary-length decimal fractions m*(10*e). items representing arbitrary-length decimal fractions m*(10*e).
Protocols using tag value 5 extend the generic data model with data Protocols using tag number 5 extend the generic data model with data
items representing arbitrary-length binary fractions m*(2*e). As items representing arbitrary-length binary fractions m*(2*e). As
with bignums, values of different types are not equal in the generic with bignums, values of different types are not equal in the generic
data model. data model.
Decimal fractions combine an integer mantissa with a base-10 scaling Decimal fractions combine an integer mantissa with a base-10 scaling
factor. They are most useful if an application needs the exact factor. They are most useful if an application needs the exact
representation of a decimal fraction such as 1.1 because there is no representation of a decimal fraction such as 1.1 because there is no
exact representation for many decimal fractions in binary floating exact representation for many decimal fractions in binary floating
point. point.
Bigfloats combine an integer mantissa with a base-2 scaling factor. Bigfloats combine an integer mantissa with a base-2 scaling factor.
They are binary floating-point values that can exceed the range or They are binary floating-point values that can exceed the range or
the precision of the three IEEE 754 formats supported by CBOR the precision of the three IEEE 754 formats supported by CBOR
(Section 3.3). Bigfloats may also be used by constrained (Section 3.3). Bigfloats may also be used by constrained
applications that need some basic binary floating-point capability applications that need some basic binary floating-point capability
without the need for supporting IEEE 754. without the need for supporting IEEE 754.
A decimal fraction or a bigfloat is represented as a tagged array A decimal fraction or a bigfloat is represented as a tagged array
that contains exactly two integer numbers: an exponent e and a that contains exactly two integer numbers: an exponent e and a
mantissa m. Decimal fractions (tag 4) use base-10 exponents; the mantissa m. Decimal fractions (tag number 4) use base-10 exponents;
value of a decimal fraction data item is m*(10**e). Bigfloats (tag the value of a decimal fraction data item is m*(10**e). Bigfloats
5) use base-2 exponents; the value of a bigfloat data item is (tag number 5) use base-2 exponents; the value of a bigfloat data
m*(2**e). The exponent e MUST be represented in an integer of major item is m*(2**e). The exponent e MUST be represented in an integer
type 0 or 1, while the mantissa also can be a bignum (Section 3.4.4). of major type 0 or 1, while the mantissa also can be a bignum
Contained items with other structures are invalid. (Section 3.4.4). Contained items with other structures are invalid.
An example of a decimal fraction is that the number 273.15 could be An example of a decimal fraction is that the number 273.15 could be
represented as 0b110_00100 (major type of 6 for the tag, additional represented as 0b110_00100 (major type of 6 for the tag, additional
information of 4 for the type of tag), followed by 0b100_00010 (major information of 4 for the number of tag), followed by 0b100_00010
type of 4 for the array, additional information of 2 for the length (major type of 4 for the array, additional information of 2 for the
of the array), followed by 0b001_00001 (major type of 1 for the first length of the array), followed by 0b001_00001 (major type of 1 for
integer, additional information of 1 for the value of -2), followed the first integer, additional information of 1 for the value of -2),
by 0b000_11001 (major type of 0 for the second integer, additional followed by 0b000_11001 (major type of 0 for the second integer,
information of 25 for a two-byte value), followed by additional information of 25 for a two-byte value), followed by
0b0110101010110011 (27315 in two bytes). In hexadecimal: 0b0110101010110011 (27315 in two bytes). In hexadecimal:
C4 -- Tag 4 C4 -- Tag 4
82 -- Array of length 2 82 -- Array of length 2
21 -- -2 21 -- -2
19 6ab3 -- 27315 19 6ab3 -- 27315
An example of a bigfloat is that the number 1.5 could be represented An example of a bigfloat is that the number 1.5 could be represented
as 0b110_00101 (major type of 6 for the tag, additional information as 0b110_00101 (major type of 6 for the tag, additional information
of 5 for the type of tag), followed by 0b100_00010 (major type of 4 of 5 for the number of tag), followed by 0b100_00010 (major type of 4
for the array, additional information of 2 for the length of the for the array, additional information of 2 for the length of the
array), followed by 0b001_00000 (major type of 1 for the first array), followed by 0b001_00000 (major type of 1 for the first
integer, additional information of 0 for the value of -1), followed integer, additional information of 0 for the value of -1), followed
by 0b000_00011 (major type of 0 for the second integer, additional by 0b000_00011 (major type of 0 for the second integer, additional
information of 3 for the value of 3). In hexadecimal: information of 3 for the value of 3). In hexadecimal:
C5 -- Tag 5 C5 -- Tag 5
82 -- Array of length 2 82 -- Array of length 2
20 -- -1 20 -- -1
03 -- 3 03 -- 3
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3.4.6. Content Hints 3.4.6. Content Hints
The tags in this section are for content hints that might be used by The tags in this section are for content hints that might be used by
generic CBOR processors. These content hints do not extend the generic CBOR processors. These content hints do not extend the
generic data model. generic data model.
3.4.6.1. Encoded CBOR Data Item 3.4.6.1. Encoded CBOR Data Item
Sometimes it is beneficial to carry an embedded CBOR data item that Sometimes it is beneficial to carry an embedded CBOR data item that
is not meant to be decoded immediately at the time the enclosing data is not meant to be decoded immediately at the time the enclosing data
item is being decoded. Tag 24 (CBOR data item) can be used to tag item is being decoded. Tag number 24 (CBOR data item) can be used to
the embedded byte string as a data item encoded in CBOR format. tag the embedded byte string as a data item encoded in CBOR format.
Contained items that aren't byte strings are invalid. Any contained Contained items that aren't byte strings are invalid. Any contained
byte string is valid, even if it encodes an invalid or ill-formed byte string is valid, even if it encodes an invalid or ill-formed
CBOR item. CBOR item.
3.4.6.2. Expected Later Encoding for CBOR-to-JSON Converters 3.4.6.2. Expected Later Encoding for CBOR-to-JSON Converters
Tags 21 to 23 indicate that a byte string might require a specific Tags number 21 to 23 indicate that a byte string might require a
encoding when interoperating with a text-based representation. These specific encoding when interoperating with a text-based
tags are useful when an encoder knows that the byte string data it is representation. These tags are useful when an encoder knows that the
writing is likely to be later converted to a particular JSON-based byte string data it is writing is likely to be later converted to a
usage. That usage specifies that some strings are encoded as base64, particular JSON-based usage. That usage specifies that some strings
base64url, and so on. The encoder uses byte strings instead of doing are encoded as base64, base64url, and so on. The encoder uses byte
the encoding itself to reduce the message size, to reduce the code strings instead of doing the encoding itself to reduce the message
size of the encoder, or both. The encoder does not know whether or size, to reduce the code size of the encoder, or both. The encoder
not the converter will be generic, and therefore wants to say what it does not know whether or not the converter will be generic, and
believes is the proper way to convert binary strings to JSON. therefore wants to say what it believes is the proper way to convert
binary strings to JSON.
The data item tagged can be a byte string or any other data item. In The data item tagged can be a byte string or any other data item. In
the latter case, the tag applies to all of the byte string data items the latter case, the tag applies to all of the byte string data items
contained in the data item, except for those contained in a nested contained in the data item, except for those contained in a nested
data item tagged with an expected conversion. data item tagged with an expected conversion.
These three tag types suggest conversions to three of the base data These three tag numbers suggest conversions to three of the base data
encodings defined in [RFC4648]. For base64url encoding (tag 21), encodings defined in [RFC4648]. For base64url encoding (tag number
padding is not used (see Section 3.2 of RFC 4648); that is, all 21), padding is not used (see Section 3.2 of RFC 4648); that is, all
trailing equals signs ("=") are removed from the encoded string. For trailing equals signs ("=") are removed from the encoded string. For
base64 encoding (tag 22), padding is used as defined in RFC 4648. base64 encoding (tag number 22), padding is used as defined in RFC
For both base64url and base64, padding bits are set to zero (see 4648. For both base64url and base64, padding bits are set to zero
Section 3.5 of RFC 4648), and encoding is performed without the (see Section 3.5 of RFC 4648), and encoding is performed without the
inclusion of any line breaks, whitespace, or other additional inclusion of any line breaks, whitespace, or other additional
characters. Note that, for all three tags, the encoding of the empty characters. Note that, for all three tag numbers, the encoding of
byte string is the empty text string. the empty byte string is the empty text string.
3.4.6.3. Encoded Text 3.4.6.3. Encoded Text
Some text strings hold data that have formats widely used on the Some text strings hold data that have formats widely used on the
Internet, and sometimes those formats can be validated and presented Internet, and sometimes those formats can be validated and presented
to the application in appropriate form by the decoder. There are to the application in appropriate form by the decoder. There are
tags for some of these formats. As with tags 21 to 23, if these tags tags for some of these formats. As with tag numbers 21 to 23, if
are applied to an item other than a text string, they apply to all these tags are applied to an item other than a text string, they
text string data items it contains. apply to all text string data items it contains.
o Tag 32 is for URIs, as defined in [RFC3986]. If the text string o Tag number 32 is for URIs, as defined in [RFC3986]. If the text
doesn't match the "URI-reference" production, the string is string doesn't match the "URI-reference" production, the string is
invalid. invalid.
o Tags 33 and 34 are for base64url- and base64-encoded text strings, o Tag numbers 33 and 34 are for base64url- and base64-encoded text
as defined in [RFC4648]. If any of: strings, as defined in [RFC4648]. If any of:
* the encoded text string contains non-alphabet characters or * the encoded text string contains non-alphabet characters or
only 1 character in the last block of 4, or only 1 character in the last block of 4, or
* the padding bits in a 2- or 3-character block are not 0, or * the padding bits in a 2- or 3-character block are not 0, or
* the base64 encoding has the wrong number of padding characters, * the base64 encoding has the wrong number of padding characters,
or or
* the base64url encoding has padding characters, * the base64url encoding has padding characters,
the string is invalid. the string is invalid.
o Tag 35 is for regular expressions that are roughly in Perl o Tag number 35 is for regular expressions that are roughly in Perl
Compatible Regular Expressions (PCRE/PCRE2) form [PCRE] or a Compatible Regular Expressions (PCRE/PCRE2) form [PCRE] or a
version of the JavaScript regular expression syntax [ECMA262]. version of the JavaScript regular expression syntax [ECMA262].
(Note that more specific identification may be necessary if the (Note that more specific identification may be necessary if the
actual version of the specification underlying the regular actual version of the specification underlying the regular
expression, or more than just the text of the regular expression expression, or more than just the text of the regular expression
itself, need to be conveyed.) Any contained string value is itself, need to be conveyed.) Any contained string value is
valid. valid.
o Tag 36 is for MIME messages (including all headers), as defined in o Tag number 36 is for MIME messages (including all headers), as
[RFC2045]. A text string that isn't a valid MIME message is defined in [RFC2045]. A text string that isn't a valid MIME
invalid. message is invalid.
Note that tags 33 and 34 differ from 21 and 22 in that the data is Note that tag numbers 33 and 34 differ from 21 and 22 in that the
transported in base-encoded form for the former and in raw byte data is transported in base-encoded form for the former and in raw
string form for the latter. byte string form for the latter.
3.4.7. Self-Described CBOR 3.4.7. Self-Described CBOR
In many applications, it will be clear from the context that CBOR is In many applications, it will be clear from the context that CBOR is
being employed for encoding a data item. For instance, a specific being employed for encoding a data item. For instance, a specific
protocol might specify the use of CBOR, or a media type is indicated protocol might specify the use of CBOR, or a media type is indicated
that specifies its use. However, there may be applications where that specifies its use. However, there may be applications where
such context information is not available, such as when CBOR data is such context information is not available, such as when CBOR data is
stored in a file that does not have disambiguating metadata. Here, stored in a file that does not have disambiguating metadata. Here,
it may help to have some distinguishing characteristics for the data it may help to have some distinguishing characteristics for the data
itself. itself.
Tag 55799 is defined for this purpose. It does not impart any Tag number 55799 is defined for this purpose. It does not impart any
special semantics on the data item that follows; that is, the special semantics on the data item that it encloses; that is, the
semantics of a data item tagged with tag 55799 is exactly identical semantics of a data item enclosed in tag number 55799 is exactly
to the semantics of the data item itself. identical to the semantics of the data item itself.
The serialization of this tag is 0xd9d9f7, which does not appear to The serialization of this tag's head is 0xd9d9f7, which does not
be in use as a distinguishing mark for any frequently used file appear to be in use as a distinguishing mark for any frequently used
types. In particular, 0xd9d9f7 is not a valid start of a Unicode file types. In particular, 0xd9d9f7 is not a valid start of a
text in any Unicode encoding if it is followed by a valid CBOR data Unicode text in any Unicode encoding if it is followed by a valid
item. CBOR data item.
For instance, a decoder might be able to decode both CBOR and JSON. For instance, a decoder might be able to decode both CBOR and JSON.
Such a decoder would need to mechanically distinguish the two Such a decoder would need to mechanically distinguish the two
formats. An easy way for an encoder to help the decoder would be to formats. An easy way for an encoder to help the decoder would be to
tag the entire CBOR item with tag 55799, the serialization of which tag the entire CBOR item with tag number 55799, the serialization of
will never be found at the beginning of a JSON text. which will never be found at the beginning of a JSON text.
4. Serialization Considerations 4. Serialization Considerations
4.1. Preferred Serialization 4.1. Preferred Serialization
For some values at the data model level, CBOR provides multiple For some values at the data model level, CBOR provides multiple
serializations. For many applications, it is desirable that an serializations. For many applications, it is desirable that an
encoder always chooses a preferred serialization; however, the encoder always chooses a preferred serialization; however, the
present specification does not put the burden of enforcing this present specification does not put the burden of enforcing this
preference on either encoder or decoder. preference on either encoder or decoder.
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on only ever receiving preferred serializations ("variation-tolerant on only ever receiving preferred serializations ("variation-tolerant
decoder") can there be said to be more universally interoperable (it decoder") can there be said to be more universally interoperable (it
might very well optimize for the case of receiving preferred might very well optimize for the case of receiving preferred
serializations, though). Full implementations of CBOR decoders are serializations, though). Full implementations of CBOR decoders are
by definition variation-tolerant; the distinction is only relevant if by definition variation-tolerant; the distinction is only relevant if
a constrained implementation of a CBOR decoder meets a variant a constrained implementation of a CBOR decoder meets a variant
encoder. encoder.
The preferred serialization always uses the shortest form of The preferred serialization always uses the shortest form of
representing the argument (Section 3)); it also uses the shortest representing the argument (Section 3)); it also uses the shortest
floating point encoding that preserves the value being encoded (see floating-point encoding that preserves the value being encoded (see
Section 5.5). Definite length encoding is preferred whenever the Section 5.5). Definite length encoding is preferred whenever the
length is known at the time the serialization of the item starts. length is known at the time the serialization of the item starts.
4.2. Deterministically Encoded CBOR 4.2. Deterministically Encoded CBOR
Some protocols may want encoders to only emit CBOR in a particular Some protocols may want encoders to only emit CBOR in a particular
deterministic format; those protocols might also have the decoders deterministic format; those protocols might also have the decoders
check that their input is in that deterministic format. Those check that their input is in that deterministic format. Those
protocols are free to define what they mean by a "deterministic protocols are free to define what they mean by a "deterministic
format" and what encoders and decoders are expected to do. This format" and what encoders and decoders are expected to do. This
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definite-length items instead. definite-length items instead.
4.2.2. Additional Deterministic Encoding Considerations 4.2.2. Additional Deterministic Encoding Considerations
If a protocol allows for IEEE floats, then additional deterministic If a protocol allows for IEEE floats, then additional deterministic
encoding rules might need to be added. One example rule might be to encoding rules might need to be added. One example rule might be to
have all floats start as a 64-bit float, then do a test conversion to have all floats start as a 64-bit float, then do a test conversion to
a 32-bit float; if the result is the same numeric value, use the a 32-bit float; if the result is the same numeric value, use the
shorter value and repeat the process with a test conversion to a shorter value and repeat the process with a test conversion to a
16-bit float. (This rule selects 16-bit float for positive and 16-bit float. (This rule selects 16-bit float for positive and
negative Infinity as well.) Also, there are many representations for negative Infinity as well.) Although IEEE floats can represent both
NaN. If NaN is an allowed value, it must always be represented as positive and negative zero as distinct values, the application might
0xf97e00. not distinguish these and might decide to represent all zero values
with a positive sign, disallowing negative zero. Also, there are
many representations for NaN. If NaN is an allowed value, it must
always be represented as 0xf97e00.
CBOR tags present additional considerations for deterministic CBOR tags present additional considerations for deterministic
encoding. The absence or presence of tags in a deterministic format encoding. The absence or presence of tags in a deterministic format
is determined by the optionality of the tags in the protocol. In a is determined by the optionality of the tags in the protocol. In a
CBOR-based protocol that allows optional tagging anywhere, the CBOR-based protocol that allows optional tagging anywhere, the
deterministic format must not allow them. In a protocol that deterministic format must not allow them. In a protocol that
requires tags in certain places, the tag needs to appear in the requires tags in certain places, the tag needs to appear in the
deterministic format. A CBOR-based protocol that uses deterministic deterministic format. A CBOR-based protocol that uses deterministic
encoding might instead say that all tags that appear in a message encoding might instead say that all tags that appear in a message
must be retained regardless of whether they are optional. must be retained regardless of whether they are optional.
Protocols that include floating, big integer, or other complex values Protocols that include floating, big integer, or other complex values
need to define extra requirements on their deterministic encodings. need to define extra requirements on their deterministic encodings.
For example: For example:
o If a protocol includes a field that can express floating values o If a protocol includes a field that can express floating-point
(Section 3.3), the protocol's deterministic encoding needs to values (Section 3.3), the protocol's deterministic encoding needs
specify whether the integer 1.0 is encoded as 0x01, 0xf93c00, to specify whether the integer 1.0 is encoded as 0x01, 0xf93c00,
0xfa3f800000, or 0xfb3ff0000000000000. Three sensible rules for 0xfa3f800000, or 0xfb3ff0000000000000. Three sensible rules for
this are: this are:
1. Encode integral values that fit in 64 bits as values from 1. Encode integral values that fit in 64 bits as values from
major types 0 and 1, and other values as the smallest of 16-, major types 0 and 1, and other values as the smallest of 16-,
32-, or 64-bit floating point that accurately represents the 32-, or 64-bit floating point that accurately represents the
value, value,
2. Encode all values as the smallest of 16-, 32-, or 64-bit 2. Encode all values as the smallest of 16-, 32-, or 64-bit
floating point that accurately represents the value, even for floating point that accurately represents the value, even for
integral values, or integral values, or
3. Encode all values as 64-bit floating point. 3. Encode all values as 64-bit floating point.
If NaN is an allowed value, the protocol needs to pick a single If NaN is an allowed value, the protocol needs to pick a single
representation, for example 0xf97e00. representation, for example 0xf97e00.
o If a protocol includes a field that can express integers larger o If a protocol includes a field that can express integers with an
than 2^64 using tag 2 (Section 3.4.4), the protocol's absolute value of 2^64 or larger using tag numbers 2 or 3
deterministic encoding needs to specify whether small integers are (Section 3.4.4), the protocol's deterministic encoding needs to
expressed using the tag or major types 0 and 1. specify whether small integers are expressed using the tag or
major types 0 and 1.
o A protocol might give encoders the choice of representing a URL as o A protocol might give encoders the choice of representing a URL as
either a text string or, using Section 3.4.6.3, tag 32 containing either a text string or, using Section 3.4.6.3, tag number 32
a text string. This protocol's deterministic encoding needs to containing a text string. This protocol's deterministic encoding
either require that the tag is present or require that it's needs to either require that the tag is present or require that
absent, not allow either one. it's absent, not allow either one.
4.2.3. Length-first map key ordering 4.2.3. Length-first map key ordering
The core deterministic encoding requirements sort map keys in a The core deterministic encoding requirements sort map keys in a
different order from the one suggested by Section 3.9 of [RFC7049] different order from the one suggested by Section 3.9 of [RFC7049]
(called "Canonical CBOR" there). Protocols that need to be (called "Canonical CBOR" there). Protocols that need to be
compatible with [RFC7049]'s order can instead be specified in terms compatible with [RFC7049]'s order can instead be specified in terms
of this specification's "length-first core deterministic encoding of this specification's "length-first core deterministic encoding
requirements": requirements":
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5.2. Generic Encoders and Decoders 5.2. Generic Encoders and Decoders
A generic CBOR decoder can decode all well-formed CBOR data and A generic CBOR decoder can decode all well-formed CBOR data and
present them to an application. See Appendix C. present them to an application. See Appendix C.
Even though CBOR attempts to minimize these cases, not all well- Even though CBOR attempts to minimize these cases, not all well-
formed CBOR data is valid: for example, the encoded text string formed CBOR data is valid: for example, the encoded text string
"0x62c0ae" does not contain valid UTF-8 and so is not a valid CBOR "0x62c0ae" does not contain valid UTF-8 and so is not a valid CBOR
item. Also, specific tags may make semantic constraints that may be item. Also, specific tags may make semantic constraints that may be
violated, such as a bignum tag containing another tag, or an instance violated, such as a bignum tag enclosing another tag, or an instance
of tag 0 containing a byte string or a text string with contents that of tag number 0 containing a byte string or a text string with
do not match [RFC3339]'s "date-time" production. There is no contents that do not match [RFC3339]'s "date-time" production. There
requirement that generic encoders and decoders make unnatural choices is no requirement that generic encoders and decoders make unnatural
for their application interface to enable the processing of invalid choices for their application interface to enable the processing of
data. Generic encoders and decoders are expected to forward simple invalid data. Generic encoders and decoders are expected to forward
values and tags even if their specific codepoints are not registered simple values and tags even if their specific codepoints are not
at the time the encoder/decoder is written (Section 5.4). registered at the time the encoder/decoder is written (Section 5.4).
Generic decoders provide ways to present well-formed CBOR values, Generic decoders provide ways to present well-formed CBOR values,
both valid and invalid, to an application. The diagnostic notation both valid and invalid, to an application. The diagnostic notation
(Section 8) may be used to present well-formed CBOR values to humans. (Section 8) may be used to present well-formed CBOR values to humans.
Generic encoders provide an application interface that allows the Generic encoders provide an application interface that allows the
application to specify any well-formed value, including simple values application to specify any well-formed value, including simple values
and tags unknown to the encoder. and tags unknown to the encoder.
5.3. Invalid Items 5.3. Invalid Items
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Duplicate keys in a map: Generic decoders (Section 5.2) make data Duplicate keys in a map: Generic decoders (Section 5.2) make data
available to applications using the native CBOR data model. That available to applications using the native CBOR data model. That
data model includes maps (key-value mappings with unique keys), data model includes maps (key-value mappings with unique keys),
not multimaps (key-value mappings where multiple entries can have not multimaps (key-value mappings where multiple entries can have
the same key). Thus, a generic decoder that gets a CBOR map item the same key). Thus, a generic decoder that gets a CBOR map item
that has duplicate keys will decode to a map with only one that has duplicate keys will decode to a map with only one
instance of that key, or it might stop processing altogether. On instance of that key, or it might stop processing altogether. On
the other hand, a "streaming decoder" may not even be able to the other hand, a "streaming decoder" may not even be able to
notice (Section 5.6). notice (Section 5.6).
Inadmissible type on the value following a tag: Tags (Section 3.4) Inadmissible type on the value enclosed by a tag: Tags (Section 3.4)
specify what type of data item is supposed to follow the tag; for specify what type of data item is supposed to be enclosed by the
example, the tags for positive or negative bignums are supposed to tag; for example, the tags for positive or negative bignums are
be put on byte strings. A decoder that decodes the tagged data supposed to be put on byte strings. A decoder that decodes the
item into a native representation (a native big integer in this tagged data item into a native representation (a native big
example) is expected to check the type of the data item being integer in this example) is expected to check the type of the data
tagged. Even decoders that don't have such native representations item being tagged. Even decoders that don't have such native
available in their environment may perform the check on those tags representations available in their environment may perform the
known to them and react appropriately. check on those tags known to them and react appropriately.
Invalid UTF-8 string: A decoder might or might not want to verify Invalid UTF-8 string: A decoder might or might not want to verify
that the sequence of bytes in a UTF-8 string (major type 3) is that the sequence of bytes in a UTF-8 string (major type 3) is
actually valid UTF-8 and react appropriately. actually valid UTF-8 and react appropriately.
5.4. Handling Unknown Simple Values and Tags 5.4. Handling Unknown Simple Values and Tags
A decoder that comes across a simple value (Section 3.3) that it does A decoder that comes across a simple value (Section 3.3) that it does
not recognize, such as a value that was added to the IANA registry not recognize, such as a value that was added to the IANA registry
after the decoder was deployed or a value that the decoder chose not after the decoder was deployed or a value that the decoder chose not
to implement, might issue a warning, might stop processing to implement, might issue a warning, might stop processing
altogether, might handle the error by making the unknown value altogether, might handle the error by making the unknown value
available to the application as such (as is expected of generic available to the application as such (as is expected of generic
decoders), or take some other type of action. decoders), or take some other type of action.
A decoder that comes across a tag (Section 3.4) that it does not A decoder that comes across a tag number (Section 3.4) that it does
recognize, such as a tag that was added to the IANA registry after not recognize, such as a tag number that was added to the IANA
the decoder was deployed or a tag that the decoder chose not to registry after the decoder was deployed or a tag number that the
implement, might issue a warning, might stop processing altogether, decoder chose not to implement, might issue a warning, might stop
might handle the error and present the unknown tag value together processing altogether, might handle the error and present the unknown
with the contained data item to the application (as is expected of tag number together with the enclosed data item to the application
generic decoders), might ignore the tag and simply present the (as is expected of generic decoders), might ignore the tag and simply
contained data item only to the application, or take some other type present the contained data item only to the application, or take some
of action. other type of action.
5.5. Numbers 5.5. Numbers
CBOR-based protocols should take into account that different language CBOR-based protocols should take into account that different language
environments pose different restrictions on the range and precision environments pose different restrictions on the range and precision
of numbers that are representable. For example, the JavaScript of numbers that are representable. For example, the JavaScript
number system treats all numbers as floating point, which may result number system treats all numbers as floating point, which may result
in silent loss of precision in decoding integers with more than 53 in silent loss of precision in decoding integers with more than 53
significant bits. A protocol that uses numbers should define its significant bits. A protocol that uses numbers should define its
expectations on the handling of non-trivial numbers in decoders and expectations on the handling of non-trivial numbers in decoders and
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A CBOR-based protocol designed for compactness may want to exclude A CBOR-based protocol designed for compactness may want to exclude
specific integer encodings that are longer than necessary for the specific integer encodings that are longer than necessary for the
application, such as to save the need to implement 64-bit integers. application, such as to save the need to implement 64-bit integers.
There is an expectation that encoders will use the most compact There is an expectation that encoders will use the most compact
integer representation that can represent a given value. However, a integer representation that can represent a given value. However, a
compact application should accept values that use a longer-than- compact application should accept values that use a longer-than-
needed encoding (such as encoding "0" as 0b000_11001 followed by two needed encoding (such as encoding "0" as 0b000_11001 followed by two
bytes of 0x00) as long as the application can decode an integer of bytes of 0x00) as long as the application can decode an integer of
the given size. the given size.
The preferred encoding for a floating point value is the shortest The preferred encoding for a floating-point value is the shortest
floating point encoding that preserves its value, e.g., 0xf94580 for floating-point encoding that preserves its value, e.g., 0xf94580 for
the number 5.5, and 0xfa45ad9c00 for the number 5555.5, unless the the number 5.5, and 0xfa45ad9c00 for the number 5555.5, unless the
CBOR-based protocol specifically excludes the use of the shorter CBOR-based protocol specifically excludes the use of the shorter
floating point encodings. For NaN values, a shorter encoding is floating-point encodings. For NaN values, a shorter encoding is
preferred if zero-padding the shorter significand towards the right preferred if zero-padding the shorter significand towards the right
reconstitutes the original NaN value (for many applications, the reconstitutes the original NaN value (for many applications, the
single NaN encoding 0xf97e00 will suffice). single NaN encoding 0xf97e00 will suffice).
5.6. Specifying Keys for Maps 5.6. Specifying Keys for Maps
The encoding and decoding applications need to agree on what types of The encoding and decoding applications need to agree on what types of
keys are going to be used in maps. In applications that need to keys are going to be used in maps. In applications that need to
interwork with JSON-based applications, keys probably should be interwork with JSON-based applications, keys probably should be
limited to UTF-8 strings only; otherwise, there has to be a specified limited to UTF-8 strings only; otherwise, there has to be a specified
mapping from the other CBOR types to Unicode characters, and this mapping from the other CBOR types to Unicode characters, and this
often leads to implementation errors. In applications where keys are often leads to implementation errors. In applications where keys are
numeric in nature and numeric ordering of keys is important to the numeric in nature and numeric ordering of keys is important to the
application, directly using the numbers for the keys is useful. application, directly using the numbers for the keys is useful.
If multiple types of keys are to be used, consideration should be If multiple types of keys are to be used, consideration should be
given to how these types would be represented in the specific given to how these types would be represented in the specific
programming environments that are to be used. For example, in programming environments that are to be used. For example, in
JavaScript Maps [ECMA262], a key of integer 1 cannot be distinguished JavaScript Maps [ECMA262], a key of integer 1 cannot be distinguished
from a key of floating point 1.0. This means that, if integer keys from a key of floating-point 1.0. This means that, if integer keys
are used, the protocol needs to avoid use of floating-point keys the are used, the protocol needs to avoid use of floating-point keys the
values of which happen to be integer numbers in the same map. values of which happen to be integer numbers in the same map.
Decoders that deliver data items nested within a CBOR data item Decoders that deliver data items nested within a CBOR data item
immediately on decoding them ("streaming decoders") often do not keep immediately on decoding them ("streaming decoders") often do not keep
the state that is necessary to ascertain uniqueness of a key in a the state that is necessary to ascertain uniqueness of a key in a
map. Similarly, an encoder that can start encoding data items before map. Similarly, an encoder that can start encoding data items before
the enclosing data item is completely available ("streaming encoder") the enclosing data item is completely available ("streaming encoder")
may want to reduce its overhead significantly by relying on its data may want to reduce its overhead significantly by relying on its data
source to maintain uniqueness. source to maintain uniqueness.
skipping to change at page 34, line 6 skipping to change at page 34, line 35
except that it might have a rule that having identical keys in a map except that it might have a rule that having identical keys in a map
indicates a malformed map and that the decoder has to stop with an indicates a malformed map and that the decoder has to stop with an
error. Duplicate keys are also prohibited by CBOR decoders that are error. Duplicate keys are also prohibited by CBOR decoders that are
using strict mode (Section 5.8). using strict mode (Section 5.8).
The CBOR data model for maps does not allow ascribing semantics to The CBOR data model for maps does not allow ascribing semantics to
the order of the key/value pairs in the map representation. Thus, a the order of the key/value pairs in the map representation. Thus, a
CBOR-based protocol MUST NOT specify that changing the key/value pair CBOR-based protocol MUST NOT specify that changing the key/value pair
order in a map would change the semantics, except to specify that order in a map would change the semantics, except to specify that
some, orders are disallowed, for example where they would not meet some, orders are disallowed, for example where they would not meet
the requirements of a deterministic encoding (Section 4.2. (Any the requirements of a deterministic encoding (Section 4.2). (Any
secondary effects of map ordering such as on timing, cache usage, and secondary effects of map ordering such as on timing, cache usage, and
other potential side channels are not considered part of the other potential side channels are not considered part of the
semantics but may be enough reason on its own for a protocol to semantics but may be enough reason on its own for a protocol to
require a deterministic encoding format.) require a deterministic encoding format.)
Applications for constrained devices that have maps with 24 or fewer Applications for constrained devices that have maps with 24 or fewer
frequently used keys should consider using small integers (and those frequently used keys should consider using small integers (and those
with up to 48 frequently used keys should consider also using small with up to 48 frequently used keys should consider also using small
negative integers) because the keys can then be encoded in a single negative integers) because the keys can then be encoded in a single
byte. byte.
5.6.1. Equivalence of Keys 5.6.1. Equivalence of Keys
The specific data model applying to a CBOR data item is used to The specific data model applying to a CBOR data item is used to
determine whether keys occurring in maps are duplicates or distinct. determine whether keys occurring in maps are duplicates or distinct.
At the generic data model level, numerically equivalent integer and At the generic data model level, numerically equivalent integer and
floating point values are distinct from each other, as they are from floating-point values are distinct from each other, as they are from
the various big numbers (Tags 2 to 5). Similarly, text strings are the various big numbers (Tags 2 to 5). Similarly, text strings are
distinct from byte strings, even if composed of the same bytes. A distinct from byte strings, even if composed of the same bytes. A
tagged value is distinct from an untagged value or from a value tagged value is distinct from an untagged value or from a value
tagged with a different tag. tagged with a different tag.
Within each of these groups, numeric values are distinct unless they Within each of these groups, numeric values are distinct unless they
are numerically equal (specifically, -0.0 is equal to 0.0); for the are numerically equal (specifically, -0.0 is equal to 0.0); for the
purpose of map key equivalence, NaN (not a number) values are purpose of map key equivalence, NaN (not a number) values are
equivalent if they have the same significand after zero-extending equivalent if they have the same significand after zero-extending
both significands at the right to 64 bits. both significands at the right to 64 bits.
(Byte and text) strings are compared byte by byte, arrays element by (Byte and text) strings are compared byte by byte, arrays element by
element, and are equal if they have the same number of bytes/elements element, and are equal if they have the same number of bytes/elements
and the same values at the same positions. Two maps are equal if and the same values at the same positions. Two maps are equal if
they have the same set of pairs regardless of their order; pairs are they have the same set of pairs regardless of their order; pairs are
equal if both the key and value are equal. equal if both the key and value are equal.
Tagged values are equal if both the tag and the value are equal. Tagged values are equal if both the tag number and the enclosed item
Simple values are equal if they simply have the same value. Nothing are equal. Simple values are equal if they simply have the same
else is equal in the generic data model, a simple value 2 is not value. Nothing else is equal in the generic data model, a simple
equivalent to an integer 2 and an array is never equivalent to a map. value 2 is not equivalent to an integer 2 and an array is never
equivalent to a map.
As discussed in Section 2.2, specific data models can make values As discussed in Section 2.2, specific data models can make values
equivalent for the purpose of comparing map keys that are distinct in equivalent for the purpose of comparing map keys that are distinct in
the generic data model. Note that this implies that a generic the generic data model. Note that this implies that a generic
decoder may deliver a decoded map to an application that needs to be decoder may deliver a decoded map to an application that needs to be
checked for duplicate map keys by that application (alternatively, checked for duplicate map keys by that application (alternatively,
the decoder may provide a programming interface to perform this the decoder may provide a programming interface to perform this
service for the application). Specific data models cannot service for the application). Specific data models cannot
distinguish values for map keys that are equal for this purpose at distinguish values for map keys that are equal for this purpose at
the generic data model level. the generic data model level.
skipping to change at page 35, line 50 skipping to change at page 36, line 32
(and does not return data) for a CBOR data item that contains any of (and does not return data) for a CBOR data item that contains any of
the following: the following:
o a map (major type 5) that has more than one entry with the same o a map (major type 5) that has more than one entry with the same
key key
o a tag that is used on a data item of the incorrect type o a tag that is used on a data item of the incorrect type
o a data item that is incorrectly formatted for the type given to o a data item that is incorrectly formatted for the type given to
it, such as invalid UTF-8 or data that cannot be interpreted with it, such as invalid UTF-8 or data that cannot be interpreted with
the specific tag that it has been tagged with the specific tag number that it has been tagged with
A decoder in strict mode can do one of two things when it encounters A decoder in strict mode can do one of two things when it encounters
a tag or simple value that it does not recognize: a tag number or simple value that it does not recognize:
o It can report an error (and not return data). o It can report an error (and not return data).
o It can emit the unknown item (type, value, and, for tags, the o It can emit the unknown item (type, value, and, for tags, the
decoded tagged data item) to the application calling the decoder decoded tagged data item) to the application calling the decoder
with an indication that the decoder did not recognize that tag or with an indication that the decoder did not recognize that tag
simple value. number or simple value.
The latter approach, which is also appropriate for non-strict The latter approach, which is also appropriate for non-strict
decoders, supports forward compatibility with newly registered tags decoders, supports forward compatibility with newly registered tags
and simple values without the requirement to update the encoder at and simple values without the requirement to update the encoder at
the same time as the calling application. (For this, the API for the the same time as the calling application. (For this, the API for the
decoder needs to have a way to mark unknown items so that the calling decoder needs to have a way to mark unknown items so that the calling
application can handle them in a manner appropriate for the program.) application can handle them in a manner appropriate for the program.)
Since some of this processing may have an appreciable cost (in Since some of this processing may have an appreciable cost (in
particular with duplicate detection for maps), support of strict mode particular with duplicate detection for maps), support of strict mode
is not a requirement placed on all CBOR decoders. is not a requirement placed on all CBOR decoders.
Some encoders will rely on their applications to provide input data Some encoders will rely on their applications to provide input data
in such a way that unambiguously decodable CBOR results. A generic in such a way that unambiguously decodable CBOR results. A generic
encoder also may want to provide a strict mode where it reliably encoder also may want to provide a strict mode where it reliably
limits its output to unambiguously decodable CBOR, independent of limits its output to unambiguously decodable CBOR, independent of
whether or not its application is providing API-conformant data. whether or not its application is providing API-conformant data.
skipping to change at page 37, line 44 skipping to change at page 38, line 25
o A floating-point value (major type 7, additional information 25 o A floating-point value (major type 7, additional information 25
through 27) becomes a JSON number if it is finite (that is, it can through 27) becomes a JSON number if it is finite (that is, it can
be represented in a JSON number); if the value is non-finite (NaN, be represented in a JSON number); if the value is non-finite (NaN,
or positive or negative Infinity), it is represented by the or positive or negative Infinity), it is represented by the
substitute value. substitute value.
o Any other simple value (major type 7, any additional information o Any other simple value (major type 7, any additional information
value not yet discussed) is represented by the substitute value. value not yet discussed) is represented by the substitute value.
o A bignum (major type 6, tag value 2 or 3) is represented by o A bignum (major type 6, tag number 2 or 3) is represented by
encoding its byte string in base64url without padding and becomes encoding its byte string in base64url without padding and becomes
a JSON string. For tag value 3 (negative bignum), a "~" (ASCII a JSON string. For tag number 3 (negative bignum), a "~" (ASCII
tilde) is inserted before the base-encoded value. (The conversion tilde) is inserted before the base-encoded value. (The conversion
to a binary blob instead of a number is to prevent a likely to a binary blob instead of a number is to prevent a likely
numeric overflow for the JSON decoder.) numeric overflow for the JSON decoder.)
o A byte string with an encoding hint (major type 6, tag value 21 o A byte string with an encoding hint (major type 6, tag number 21
through 23) is encoded as described and becomes a JSON string. through 23) is encoded as described and becomes a JSON string.
o For all other tags (major type 6, any other tag value), the o For all other tags (major type 6, any other tag number), the
embedded CBOR item is represented as a JSON value; the tag value enclosed CBOR item is represented as a JSON value; the tag number
is ignored. is ignored.
o Indefinite-length items are made definite before conversion. o Indefinite-length items are made definite before conversion.
6.2. Converting from JSON to CBOR 6.2. Converting from JSON to CBOR
All JSON values, once decoded, directly map into one or more CBOR All JSON values, once decoded, directly map into one or more CBOR
values. As with any kind of CBOR generation, decisions have to be values. As with any kind of CBOR generation, decisions have to be
made with respect to number representation. In a suggested made with respect to number representation. In a suggested
conversion: conversion:
o JSON numbers without fractional parts (integer numbers) are o JSON numbers without fractional parts (integer numbers) are
represented as integers (major types 0 and 1, possibly major type represented as integers (major types 0 and 1, possibly major type
6 tag value 2 and 3), choosing the shortest form; integers longer 6 tag number 2 and 3), choosing the shortest form; integers longer
than an implementation-defined threshold (which is usually either than an implementation-defined threshold (which is usually either
32 or 64 bits) may instead be represented as floating-point 32 or 64 bits) may instead be represented as floating-point
values. (If the JSON was generated from a JavaScript values. (If the JSON was generated from a JavaScript
implementation, its precision is already limited to 53 bits implementation, its precision is already limited to 53 bits
maximum.) maximum.)
o Numbers with fractional parts are represented as floating-point o Numbers with fractional parts are represented as floating-point
values. Preferably, the shortest exact floating-point values. Preferably, the shortest exact floating-point
representation is used; for instance, 1.5 is represented in a representation is used; for instance, 1.5 is represented in a
16-bit floating-point value (not all implementations will be 16-bit floating-point value (not all implementations will be
skipping to change at page 40, line 5 skipping to change at page 40, line 33
(and 224 slightly less efficient) values, only a small number have (and 224 slightly less efficient) values, only a small number have
been allocated. Implementations receiving an unknown simple data been allocated. Implementations receiving an unknown simple data
item may be able to process it as such, given that the structure item may be able to process it as such, given that the structure
of the value is indeed simple. The IANA registry in Section 9.1 of the value is indeed simple. The IANA registry in Section 9.1
is the appropriate way to address the extensibility of this is the appropriate way to address the extensibility of this
codepoint space. codepoint space.
o the "tag" space (values in major type 6). Again, only a small o the "tag" space (values in major type 6). Again, only a small
part of the codepoint space has been allocated, and the space is part of the codepoint space has been allocated, and the space is
abundant (although the early numbers are more efficient than the abundant (although the early numbers are more efficient than the
later ones). Implementations receiving an unknown tag can choose later ones). Implementations receiving an unknown tag number can
to simply ignore it or to process it as an unknown tag wrapping choose to simply ignore it or to process it as an unknown tag
the following data item. The IANA registry in Section 9.2 is the number wrapping the enclosed data item. The IANA registry in
appropriate way to address the extensibility of this codepoint Section 9.2 is the appropriate way to address the extensibility of
space. this codepoint space.
o the "additional information" space. An implementation receiving o the "additional information" space. An implementation receiving
an unknown additional information value has no way to continue an unknown additional information value has no way to continue
decoding, so allocating codepoints to this space is a major step. decoding, so allocating codepoints to this space is a major step.
There are also very few codepoints left. There are also very few codepoints left.
7.2. Curating the Additional Information Space 7.2. Curating the Additional Information Space
The human mind is sometimes drawn to filling in little perceived gaps The human mind is sometimes drawn to filling in little perceived gaps
to make something neat. We expect the remaining gaps in the to make something neat. We expect the remaining gaps in the
skipping to change at page 41, line 14 skipping to change at page 41, line 44
The notation borrows the JSON syntax for numbers (integer and The notation borrows the JSON syntax for numbers (integer and
floating point), True (>true<), False (>false<), Null (>null<), UTF-8 floating point), True (>true<), False (>false<), Null (>null<), UTF-8
strings, arrays, and maps (maps are called objects in JSON; the strings, arrays, and maps (maps are called objects in JSON; the
diagnostic notation extends JSON here by allowing any data item in diagnostic notation extends JSON here by allowing any data item in
the key position). Undefined is written >undefined< as in the key position). Undefined is written >undefined< as in
JavaScript. The non-finite floating-point numbers Infinity, JavaScript. The non-finite floating-point numbers Infinity,
-Infinity, and NaN are written exactly as in this sentence (this is -Infinity, and NaN are written exactly as in this sentence (this is
also a way they can be written in JavaScript, although JSON does not also a way they can be written in JavaScript, although JSON does not
allow them). A tagged item is written as an integer number for the allow them). A tagged item is written as an integer number for the
tag followed by the item in parentheses; for instance, an RFC 3339 tag, followed by the item in parentheses; for instance, an RFC 3339
(ISO 8601) date could be notated as: (ISO 8601) date could be notated as:
0("2013-03-21T20:04:00Z") 0("2013-03-21T20:04:00Z")
or the equivalent relative time as or the equivalent relative time as
1(1363896240) 1(1363896240)
Byte strings are notated in one of the base encodings, without Byte strings are notated in one of the base encodings, without
padding, enclosed in single quotes, prefixed by >h< for base16, >b32< padding, enclosed in single quotes, prefixed by >h< for base16, >b32<
skipping to change at page 42, line 29 skipping to change at page 43, line 11
IANA has created two registries for new CBOR values. The registries IANA has created two registries for new CBOR values. The registries
are separate, that is, not under an umbrella registry, and follow the are separate, that is, not under an umbrella registry, and follow the
rules in [RFC8126]. IANA has also assigned a new MIME media type and rules in [RFC8126]. IANA has also assigned a new MIME media type and
an associated Constrained Application Protocol (CoAP) Content-Format an associated Constrained Application Protocol (CoAP) Content-Format
entry. entry.
9.1. Simple Values Registry 9.1. Simple Values Registry
IANA has created the "Concise Binary Object Representation (CBOR) IANA has created the "Concise Binary Object Representation (CBOR)
Simple Values" registry at [IANA.cbor-simple-values]. The initial Simple Values" registry at [IANA.cbor-simple-values]. The initial
values are shown in Table 2. values are shown in Table 3.
New entries in the range 0 to 19 are assigned by Standards Action. New entries in the range 0 to 19 are assigned by Standards Action.
It is suggested that these Standards Actions allocate values starting It is suggested that these Standards Actions allocate values starting
with the number 16 in order to reserve the lower numbers for with the number 16 in order to reserve the lower numbers for
contiguous blocks (if any). contiguous blocks (if any).
New entries in the range 32 to 255 are assigned by Specification New entries in the range 32 to 255 are assigned by Specification
Required. Required.
9.2. Tags Registry 9.2. Tags Registry
skipping to change at page 45, line 47 skipping to change at page 46, line 12
remotely crash a node, or even remotely execute arbitrary code on it. remotely crash a node, or even remotely execute arbitrary code on it.
CBOR attempts to narrow the opportunities for introducing such CBOR attempts to narrow the opportunities for introducing such
vulnerabilities by reducing parser complexity, by giving the entire vulnerabilities by reducing parser complexity, by giving the entire
range of encodable values a meaning where possible. range of encodable values a meaning where possible.
Because CBOR decoders are often used as a first step in processing Because CBOR decoders are often used as a first step in processing
unvalidated input, they need to be fully prepared for all types of unvalidated input, they need to be fully prepared for all types of
hostile input that may be designed to corrupt, overrun, or achieve hostile input that may be designed to corrupt, overrun, or achieve
control of the system decoding the CBOR data item. A CBOR decoder control of the system decoding the CBOR data item. A CBOR decoder
needs to assume that all input may be hostile even if it has been needs to assume that all input may be hostile even if it has been
checked by a firewall, has come over a TLS-secured channel, is checked by a firewall, has come over a secure channel such as TLS, is
encrypted or signed, or has come from some other source that is encrypted or signed, or has come from some other source that is
presumed trusted. presumed trusted.
Hostile input may be constructed to overrun buffers, overflow or Hostile input may be constructed to overrun buffers, overflow or
underflow integer arithmetic, or cause other decoding disruption. underflow integer arithmetic, or cause other decoding disruption.
CBOR data items might have lengths or sizes that are intentionally CBOR data items might have lengths or sizes that are intentionally
extremely large or too short. Resource exhaustion attacks might extremely large or too short. Resource exhaustion attacks might
attempt to lure a decoder into allocating very big data items attempt to lure a decoder into allocating very big data items
(strings, arrays, maps) or exhaust the stack depth by setting up (strings, arrays, maps, or even arbitrary precision numbers) or
deeply nested items. Decoders need to have appropriate resource exhaust the stack depth by setting up deeply nested items. Decoders
management to mitigate these attacks. (Items for which very large need to have appropriate resource management to mitigate these
sizes are given can also attempt to exploit integer overflow attacks. (Items for which very large sizes are given can also
vulnerabilities.) attempt to exploit integer overflow vulnerabilities.)
A CBOR decoder, by definition, only accepts well-formed CBOR; this is A CBOR decoder, by definition, only accepts well-formed CBOR; this is
the first step to its robustness. Input that is not well-formed CBOR the first step to its robustness. Input that is not well-formed CBOR
causes no further processing from the point where the lack of well- causes no further processing from the point where the lack of well-
formedness was detected. If possible, any data decoded up to this formedness was detected. If possible, any data decoded up to this
point should have no impact on the application using the CBOR point should have no impact on the application using the CBOR
decoder. decoder.
In addition to ascertaining well-formedness, a CBOR decoder might In addition to ascertaining well-formedness, a CBOR decoder might
also perform validity checks on the CBOR data. Alternatively, it can also perform validity checks on the CBOR data. Alternatively, it can
leave those checks to the application using the decoder. This choice leave those checks to the application using the decoder. This choice
needs to be clearly documented in the decoder. Beyond the validity needs to be clearly documented in the decoder. Beyond the validity
at the CBOR level, an application also needs to ascertain that the at the CBOR level, an application also needs to ascertain that the
input is in alignment with the application protocol that is input is in alignment with the application protocol that is
serialized in CBOR. serialized in CBOR.
The input check itself may consume resources. This is usually linear
in the size of the input, which means that an attacker has to spend
resources that are commensurate to the resources spent by the
defender on input validation. Processing for arbitrary-precision
numbers may exceed linear effort. Also, some hash-table
implementations that are used by decoders to build in-memory
representations of maps can be attacked to spend quadratic effort,
unless a secret key is employed (see Section 7 of [SIPHASH]). Such
superlinear efforts can be employed by an attacker to exhaust
resources at or before the input validator; they therefore need to be
avoided in a CBOR decoder implementation. Note that Tag number
definitions and their implementations can add security considerations
of this kind; this should then be discussed in the security
considerations of the Tag number definition.
CBOR encoders do not receive input directly from the network and are CBOR encoders do not receive input directly from the network and are
thus not directly attackable in the same way as CBOR decoders. thus not directly attackable in the same way as CBOR decoders.
However, CBOR encoders often have an API that takes input from However, CBOR encoders often have an API that takes input from
another level in the implementation and can be attacked through that another level in the implementation and can be attacked through that
API. The design and implementation of that API should assume the API. The design and implementation of that API should assume the
behavior of its caller may be based on hostile input or on coding behavior of its caller may be based on hostile input or on coding
mistakes. It should check inputs for buffer overruns, overflow and mistakes. It should check inputs for buffer overruns, overflow and
underflow of integer arithmetic, and other such errors that are aimed underflow of integer arithmetic, and other such errors that are aimed
to disrupt the encoder. to disrupt the encoder.
skipping to change at page 49, line 15 skipping to change at page 49, line 43
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014, DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>. <https://www.rfc-editor.org/info/rfc7228>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259, Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017, DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>. <https://www.rfc-editor.org/info/rfc8259>.
[SIPHASH] Aumasson, J. and D. Bernstein, "SipHash: A Fast Short-
Input PRF", Lecture Notes in Computer Science pp. 489-508,
DOI 10.1007/978-3-642-34931-7_28, 2012.
[YAML] Ben-Kiki, O., Evans, C., and I. Net, "YAML Ain't Markup [YAML] Ben-Kiki, O., Evans, C., and I. Net, "YAML Ain't Markup
Language (YAML[TM]) Version 1.2", 3rd Edition, October Language (YAML[TM]) Version 1.2", 3rd Edition, October
2009, <http://www.yaml.org/spec/1.2/spec.html>. 2009, <http://www.yaml.org/spec/1.2/spec.html>.
Appendix A. Examples Appendix A. Examples
The following table provides some CBOR-encoded values in hexadecimal The following table provides some CBOR-encoded values in hexadecimal
(right column), together with diagnostic notation for these values (right column), together with diagnostic notation for these values
(left column). Note that the string "\u00fc" is one form of (left column). Note that the string "\u00fc" is one form of
diagnostic notation for a UTF-8 string containing the single Unicode diagnostic notation for a UTF-8 string containing the single Unicode
skipping to change at page 54, line 6 skipping to change at page 54, line 6
| 16, 17, 18, 19, 20, 21, 22, | | | 16, 17, 18, 19, 20, 21, 22, | |
| 23, 24, 25] | | | 23, 24, 25] | |
| | | | | |
| {_ "a": 1, "b": [_ 2, 3]} | 0xbf61610161629f0203ffff | | {_ "a": 1, "b": [_ 2, 3]} | 0xbf61610161629f0203ffff |
| | | | | |
| ["a", {_ "b": "c"}] | 0x826161bf61626163ff | | ["a", {_ "b": "c"}] | 0x826161bf61626163ff |
| | | | | |
| {_ "Fun": true, "Amt": -2} | 0xbf6346756ef563416d7421ff | | {_ "Fun": true, "Amt": -2} | 0xbf6346756ef563416d7421ff |
+------------------------------+------------------------------------+ +------------------------------+------------------------------------+
Table 4: Examples of Encoded CBOR Data Items Table 5: Examples of Encoded CBOR Data Items
Appendix B. Jump Table Appendix B. Jump Table
For brevity, this jump table does not show initial bytes that are For brevity, this jump table does not show initial bytes that are
reserved for future extension. It also only shows a selection of the reserved for future extension. It also only shows a selection of the
initial bytes that can be used for optional features. (All unsigned initial bytes that can be used for optional features. (All unsigned
integers are in network byte order.) integers are in network byte order.)
+------------+------------------------------------------------------+ +------------+------------------------------------------------------+
| Byte | Structure/Semantics | | Byte | Structure/Semantics |
skipping to change at page 57, line 10 skipping to change at page 57, line 10
| | | | | |
| 0xf9 | Half-Precision Float (two-byte IEEE 754) | | 0xf9 | Half-Precision Float (two-byte IEEE 754) |
| | | | | |
| 0xfa | Single-Precision Float (four-byte IEEE 754) | | 0xfa | Single-Precision Float (four-byte IEEE 754) |
| | | | | |
| 0xfb | Double-Precision Float (eight-byte IEEE 754) | | 0xfb | Double-Precision Float (eight-byte IEEE 754) |
| | | | | |
| 0xff | "break" stop code | | 0xff | "break" stop code |
+------------+------------------------------------------------------+ +------------+------------------------------------------------------+
Table 5: Jump Table for Initial Byte Table 6: Jump Table for Initial Byte
Appendix C. Pseudocode Appendix C. Pseudocode
The well-formedness of a CBOR item can be checked by the pseudocode The well-formedness of a CBOR item can be checked by the pseudocode
in Figure 1. The data is well-formed if and only if: in Figure 1. The data is well-formed if and only if:
o the pseudocode does not "fail"; o the pseudocode does not "fail";
o after execution of the pseudocode, no bytes are left in the input o after execution of the pseudocode, no bytes are left in the input
(except in streaming applications) (except in streaming applications)
skipping to change at page 58, line 5 skipping to change at page 57, line 34
o take(n) reads n bytes from the input data and returns them as a o take(n) reads n bytes from the input data and returns them as a
byte string. If n bytes are no longer available, take(n) fails. byte string. If n bytes are no longer available, take(n) fails.
o uint() converts a byte string into an unsigned integer by o uint() converts a byte string into an unsigned integer by
interpreting the byte string in network byte order. interpreting the byte string in network byte order.
o Arithmetic works as in C. o Arithmetic works as in C.
o All variables are unsigned integers of sufficient range. o All variables are unsigned integers of sufficient range.
Note that "well_formed" returns the major type for well-formed
definite length items, but 0 for an indefinite length item (or -1 for
a break stop code, only if "breakable" is set). This is used in
"well_formed_indefinite" to ascertain that indefinite length strings
only contain definite length strings as chunks.
well_formed (breakable = false) { well_formed (breakable = false) {
// process initial bytes // process initial bytes
ib = uint(take(1)); ib = uint(take(1));
mt = ib >> 5; mt = ib >> 5;
val = ai = ib & 0x1f; val = ai = ib & 0x1f;
switch (ai) { switch (ai) {
case 24: val = uint(take(1)); break; case 24: val = uint(take(1)); break;
case 25: val = uint(take(2)); break; case 25: val = uint(take(2)); break;
case 26: val = uint(take(4)); break; case 26: val = uint(take(4)); break;
case 27: val = uint(take(8)); break; case 27: val = uint(take(8)); break;
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case 6: well_formed(); break; // 1 embedded data item case 6: well_formed(); break; // 1 embedded data item
case 7: if (ai == 24 && val < 32) fail(); // bad simple case 7: if (ai == 24 && val < 32) fail(); // bad simple
} }
return mt; // finite data item return mt; // finite data item
} }
well_formed_indefinite(mt, breakable) { well_formed_indefinite(mt, breakable) {
switch (mt) { switch (mt) {
case 2: case 3: case 2: case 3:
while ((it = well_formed(true)) != -1) while ((it = well_formed(true)) != -1)
if (it != mt) // need finite embedded if (it != mt) // need finite-length chunk
fail(); // of same type fail(); // of same type
break; break;
case 4: while (well_formed(true) != -1); break; case 4: while (well_formed(true) != -1); break;
case 5: while (well_formed(true) != -1) well_formed(); break; case 5: while (well_formed(true) != -1) well_formed(); break;
case 7: case 7:
if (breakable) if (breakable)
return -1; // signal break out return -1; // signal break out
else fail(); // no enclosing indefinite else fail(); // no enclosing indefinite
default: fail(); // wrong mt default: fail(); // wrong mt
} }
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the years from the MessagePack user community to separate out binary the years from the MessagePack user community to separate out binary
and text strings in the encoding recently have led to an extension and text strings in the encoding recently have led to an extension
proposal that would leave MessagePack's "raw" data ambiguous between proposal that would leave MessagePack's "raw" data ambiguous between
its usages for binary and text data. The extension mechanism for its usages for binary and text data. The extension mechanism for
MessagePack remains unclear. MessagePack remains unclear.
E.3. BSON E.3. BSON
[BSON] is a data format that was developed for the storage of JSON- [BSON] is a data format that was developed for the storage of JSON-
like maps (JSON objects) in the MongoDB database. Its major like maps (JSON objects) in the MongoDB database. Its major
distinguishing feature is the capability for in-place update, distinguishing feature is the capability for in-place update, which
foregoing a compact representation. BSON uses a counted prevents a compact representation. BSON uses a counted
representation except for map keys, which are null-byte terminated. representation except for map keys, which are null-byte terminated.
While BSON can be used for the representation of JSON-like objects on While BSON can be used for the representation of JSON-like objects on
the wire, its specification is dominated by the requirements of the the wire, its specification is dominated by the requirements of the
database application and has become somewhat baroque. The status of database application and has become somewhat baroque. The status of
how BSON extensions will be implemented remains unclear. how BSON extensions will be implemented remains unclear.
E.4. MSDTP: RFC 713 E.4. MSDTP: RFC 713
Message Services Data Transmission (MSDTP) is a very early example of Message Services Data Transmission (MSDTP) is a very early example of
a compact message format; it is described in [RFC0713], written in a compact message format; it is described in [RFC0713], written in
1976. It is included here for its historical value, not because it 1976. It is included here for its historical value, not because it
was ever widely used. was ever widely used.
E.5. Conciseness on the Wire E.5. Conciseness on the Wire
While CBOR's design objective of code compactness for encoders and While CBOR's design objective of code compactness for encoders and
decoders is a higher priority than its objective of conciseness on decoders is a higher priority than its objective of conciseness on
the wire, many people focus on the wire size. Table 6 shows some the wire, many people focus on the wire size. Table 7 shows some
encoding examples for the simple nested array [1, [2, 3]]; where some encoding examples for the simple nested array [1, [2, 3]]; where some
form of indefinite-length encoding is supported by the encoding, form of indefinite-length encoding is supported by the encoding,
[_ 1, [2, 3]] (indefinite length on the outer array) is also shown. [_ 1, [2, 3]] (indefinite length on the outer array) is also shown.
+-------------+--------------------------+--------------------------+ +-------------+--------------------------+--------------------------+
| Format | [1, [2, 3]] | [_ 1, [2, 3]] | | Format | [1, [2, 3]] | [_ 1, [2, 3]] |
+-------------+--------------------------+--------------------------+ +-------------+--------------------------+--------------------------+
| RFC 713 | c2 05 81 c2 02 82 83 | | | RFC 713 | c2 05 81 c2 02 82 83 | |
| | | | | | | |
| ASN.1 BER | 30 0b 02 01 01 30 06 02 | 30 80 02 01 01 30 06 02 | | ASN.1 BER | 30 0b 02 01 01 30 06 02 | 30 80 02 01 01 30 06 02 |
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| | | | | | | |
| BSON | 22 00 00 00 10 30 00 01 | | | BSON | 22 00 00 00 10 30 00 01 | |
| | 00 00 00 04 31 00 13 00 | | | | 00 00 00 04 31 00 13 00 | |
| | 00 00 10 30 00 02 00 00 | | | | 00 00 10 30 00 02 00 00 | |
| | 00 10 31 00 03 00 00 00 | | | | 00 10 31 00 03 00 00 00 | |
| | 00 00 | | | | 00 00 | |
| | | | | | | |
| CBOR | 82 01 82 02 03 | 9f 01 82 02 03 ff | | CBOR | 82 01 82 02 03 | 9f 01 82 02 03 ff |
+-------------+--------------------------+--------------------------+ +-------------+--------------------------+--------------------------+
Table 6: Examples for Different Levels of Conciseness Table 7: Examples for Different Levels of Conciseness
Appendix F. Changes from RFC 7049 Appendix F. Changes from RFC 7049
The following is a list of known changes from RFC 7049. This list is The following is a list of known changes from RFC 7049. This list is
non-authoritative. It is meant to help reviewers see the significant non-authoritative. It is meant to help reviewers see the significant
differences. differences.
o Updated reference for [RFC4627] to [RFC8259] in many places o Updated reference for [RFC4627] to [RFC8259] in many places
o Updated reference for [CNN-TERMS] to [RFC7228] o Updated reference for [CNN-TERMS] to [RFC7228]
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for his msgpack-js and msgpack-js-browser projects. Many people have for his msgpack-js and msgpack-js-browser projects. Many people have
contributed to the discussion about extending MessagePack to separate contributed to the discussion about extending MessagePack to separate
text string representation from byte string representation. text string representation from byte string representation.
The encoding of the additional information in CBOR was inspired by The encoding of the additional information in CBOR was inspired by
the encoding of length information designed by Klaus Hartke for CoAP. the encoding of length information designed by Klaus Hartke for CoAP.
This document also incorporates suggestions made by many people, This document also incorporates suggestions made by many people,
notably Dan Frost, James Manger, Jeffrey Yaskin, Joe Hildebrand, notably Dan Frost, James Manger, Jeffrey Yaskin, Joe Hildebrand,
Keith Moore, Laurence Lundblade, Matthew Lepinski, Michael Keith Moore, Laurence Lundblade, Matthew Lepinski, Michael
Richardson, Nico Williams, Phillip Hallam-Baker, Ray Polk, Tim Bray, Richardson, Nico Williams, Peter Occil, Phillip Hallam-Baker, Ray
Tony Finch, Tony Hansen, and Yaron Sheffer. Polk, Tim Bray, Tony Finch, Tony Hansen, and Yaron Sheffer.
Authors' Addresses Authors' Addresses
Carsten Bormann Carsten Bormann
Universitaet Bremen TZI Universitaet Bremen TZI
Postfach 330440 Postfach 330440
D-28359 Bremen D-28359 Bremen
Germany Germany
Phone: +49-421-218-63921 Phone: +49-421-218-63921
 End of changes. 119 change blocks. 
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