--- 1/draft-ietf-core-echo-request-tag-03.txt 2019-03-23 18:13:14.617134186 -0700 +++ 2/draft-ietf-core-echo-request-tag-04.txt 2019-03-23 18:13:14.669135454 -0700 @@ -1,51 +1,53 @@ CoRE Working Group C. Amsuess Internet-Draft Updates: 7252 (if approved) J. Mattsson Intended status: Standards Track G. Selander -Expires: April 25, 2019 Ericsson AB - October 22, 2018 +Expires: September 25, 2019 Ericsson AB + March 24, 2019 - Echo and Request-Tag - draft-ietf-core-echo-request-tag-03 + CoAP: Echo, Request-Tag, and Token Processing + draft-ietf-core-echo-request-tag-04 Abstract - This document specifies security enhancements to the Constrained - Application Protocol (CoAP). Two optional extensions are defined: - the Echo option and the Request-Tag option. Each of these options - provide additional features to CoAP and protects against certain - attacks. The document also updates the processing requirements on - the Token of RFC 7252. The updated Token processing ensures secure - binding of responses to requests. + This document specifies enhancements to the Constrained Application + Protocol (CoAP) that mitigate security issues in particular use + cases. The Echo option enables a CoAP server to verify the freshness + of a request or to force a client to demonstrate reachability at its + claimed network address. The Request-Tag option allows the CoAP + server to match Block-Wise message fragments belonging to the same + request. The updated Token processing requirements for clients + ensure secure binding of responses to requests when CoAP is used with + security. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on April 25, 2019. + This Internet-Draft will expire on September 25, 2019. Copyright Notice - Copyright (c) 2018 IETF Trust and the persons identified as the + Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as @@ -54,101 +56,106 @@ Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Request Freshness . . . . . . . . . . . . . . . . . . . . 3 1.2. Fragmented Message Body Integrity . . . . . . . . . . . . 4 1.3. Request-Response Binding . . . . . . . . . . . . . . . . 4 1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 2. The Echo Option . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 6 2.2. Echo Processing . . . . . . . . . . . . . . . . . . . . . 7 - 2.3. Applications . . . . . . . . . . . . . . . . . . . . . . 9 + 2.3. Applications . . . . . . . . . . . . . . . . . . . . . . 10 3. The Request-Tag Option . . . . . . . . . . . . . . . . . . . 11 3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Request-Tag Processing by Servers . . . . . . . . . . . . 12 3.3. Setting the Request-Tag . . . . . . . . . . . . . . . . . 13 - 3.4. Applications . . . . . . . . . . . . . . . . . . . . . . 13 - 3.4.1. Body Integrity Based on Payload Integrity . . . . . . 13 - 3.4.2. Multiple Concurrent Blockwise Operations . . . . . . 14 + 3.4. Applications . . . . . . . . . . . . . . . . . . . . . . 14 + 3.4.1. Body Integrity Based on Payload Integrity . . . . . . 14 + 3.4.2. Multiple Concurrent Blockwise Operations . . . . . . 15 3.4.3. Simplified Block-Wise Handling for Constrained - Proxies . . . . . . . . . . . . . . . . . . . . . . . 15 - 3.5. Rationale for the Option Properties . . . . . . . . . . . 15 + Proxies . . . . . . . . . . . . . . . . . . . . . . . 16 + 3.5. Rationale for the Option Properties . . . . . . . . . . . 16 3.6. Rationale for Introducing the Option . . . . . . . . . . 16 - 4. Block2 / ETag Processing . . . . . . . . . . . . . . . . . . 16 - 5. Token Processing . . . . . . . . . . . . . . . . . . . . . . 16 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 - 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 - 9.1. Normative References . . . . . . . . . . . . . . . . . . 18 - 9.2. Informative References . . . . . . . . . . . . . . . . . 18 + 4. Block2 / ETag Processing . . . . . . . . . . . . . . . . . . 17 + 5. Token Processing . . . . . . . . . . . . . . . . . . . . . . 17 + 6. Security Considerations . . . . . . . . . . . . . . . . . . . 17 + 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 + 9.1. Normative References . . . . . . . . . . . . . . . . . . 19 + 9.2. Informative References . . . . . . . . . . . . . . . . . 19 Appendix A. Methods for Generating Echo Option Values . . . . . 20 - Appendix B. Request-Tag Message Size Impact . . . . . . . . . . 21 - Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 21 - Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 + Appendix B. Request-Tag Message Size Impact . . . . . . . . . . 22 + Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 22 + Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 1. Introduction The initial Constrained Application Protocol (CoAP) suite of specifications ([RFC7252], [RFC7641], and [RFC7959]) was designed with the assumption that security could be provided on a separate layer, in particular by using DTLS ([RFC6347]). However, for some use cases, additional functionality or extra processing is needed to support secure CoAP operations. This document specifies security enhancements to the Constrained Application Protocol (CoAP). - This document specifies two server-oriented CoAP options, the Echo - option and the Request-Tag option: The Echo option enables a CoAP - server to verify the freshness of a request, synchronize state, or - force a client to demonstrate reachability at its apparent network - address. The Request-Tag option allows the CoAP server to match - message fragments belonging to the same request, fragmented using the - CoAP Block-Wise Transfer mechanism, which mitigates attacks and - enables concurrent blockwise operations. These options in themselves - do not replace the need for a security protocol; they specify the - format and processing of data which, when integrity protected using - e.g. DTLS ([RFC6347]), TLS ([RFC8446]), or OSCORE - ([I-D.ietf-core-object-security]), provide the additional security - features. + This document specifies two CoAP options, the Echo option and the + Request-Tag option: The Echo option enables a CoAP server to verify + the freshness of a request, synchronize state, or force a client to + demonstrate reachability at its claimed network address. The + Request-Tag option allows the CoAP server to match message fragments + belonging to the same request, fragmented using the CoAP Block-Wise + Transfer mechanism, which mitigates attacks and enables concurrent + blockwise operations. These options in themselves do not replace the + need for a security protocol; they specify the format and processing + of data which, when integrity protected using e.g. DTLS ([RFC6347]), + TLS ([RFC8446]), or OSCORE ([I-D.ietf-core-object-security]), provide + the additional security features. - The document also updates the processing requirements on the Token. - The updated processing ensures secure binding of responses to - requests, thus mitigating error cases and attacks where the client + The document also updates the Token processing requirements for + clients specified in [RFC7252]. The updated processing ensures + secure binding of responses to requests when CoAP is used with + security, thus mitigating error cases and attacks where the client may erroneously associate the wrong response to a request. 1.1. Request Freshness A CoAP server receiving a request is in general not able to verify when the request was sent by the CoAP client. This remains true even if the request was protected with a security protocol, such as DTLS. This makes CoAP requests vulnerable to certain delay attacks which - are particularly incriminating in the case of actuators - ([I-D.mattsson-core-coap-actuators]). Some attacks are possible to - mitigate by establishing fresh session keys, e.g. performing a DTLS - handshake for each actuation, but in general this is not a solution - suitable for constrained environments, for example, due to increased - message overhead and latency. Additionally, if there are proxies, - fresh DTLS session keys between server and proxy does not say - anything about when the client made the request. In a general hop- - by-hop setting, freshness may need to be verified in each hop. + are particularly perilous in the case of actuators + ([I-D.mattsson-core-coap-actuators]). Some attacks can be mitigated + by establishing fresh session keys, e.g. performing a DTLS handshake + for each request, but in general this is not a solution suitable for + constrained environments, for example, due to increased message + overhead and latency. Additionally, if there are proxies, fresh DTLS + session keys between server and proxy does not say anything about + when the client made the request. In a general hop-by-hop setting, + freshness may need to be verified in each hop. A straightforward mitigation of potential delayed requests is that the CoAP server rejects a request the first time it appears and asks the CoAP client to prove that it intended to make the request at this point in time. The Echo option, defined in this document, specifies such a mechanism which thereby enables a CoAP server to verify the freshness of a request. This mechanism is not only important in the case of actuators, or other use cases where the CoAP operations require freshness of requests, but also in general for synchronizing - state between CoAP client and server and to verify aliveness of the - client. + state between CoAP client and server, verify aliveness of the client, + or force a client to demonstrate reachability at its claimed network + address. The same functionality can be provided by echoing freshness + tokens in CoAP payloads, but this only works for methods and response + codes defined to have a payload. The Echo option provides a + convention to transfer freshness tokens that works for all methods + and response codes. 1.2. Fragmented Message Body Integrity CoAP was designed to work over unreliable transports, such as UDP, and include a lightweight reliability feature to handle messages which are lost or arrive out of order. In order for a security protocol to support CoAP operations over unreliable transports, it must allow out-of-order delivery of messages using e.g. a sliding replay window such as described in Section 4.1.2.6 of DTLS ([RFC6347]). @@ -164,60 +171,63 @@ protocol such as DTLS or OSCORE, within the replay window ([I-D.mattsson-core-coap-actuators]), which is a vulnerability of CoAP when using RFC7959. A straightforward mitigation of mixing up blocks from different messages is to use unique identifiers for different message bodies, which would provide equivalent protection to the case where the complete body fits into a single payload. The ETag option [RFC7252], set by the CoAP server, identifies a response body fragmented using the Block2 option. This document defines the Request-Tag option for - identifying the request body fragmented using the Block1 option, - similar to ETag, but ephemeral and set by the CoAP client. + identifying request bodies, similar to ETag, but ephemeral and set by + the CoAP client. The Request-Tag option is only used in requests + that carry the Block1 option, and in Block2 requests following these. 1.3. Request-Response Binding A fundamental requirement of secure REST operations is that the client can bind a response to a particular request. If this is not - valid a client may erroneously associate the wrong response to a + ensured, a client may erroneously associate the wrong response to a request. The wrong response may be an old response for the same resource or for a completely different resource (see e.g. - Section 2.3 of [I-D.mattsson-core-coap-actuators]). For example a + Section 2.3 of [I-D.mattsson-core-coap-actuators]). For example, a request for the alarm status "GET /status" may be associated to a prior response "on", instead of the correct response "off". - In HTTPS, binding is assured by the ordered and reliable delivery as - well as mandating that the server sends responses in the same order - that the requests were received. The same is not true for CoAP where - the server (or an attacker) can return responses in any order. - Concurrent requests are instead differentiated by their Token. Note - that the CoAP Message ID cannot be used for this purpose since those - are typically different for REST request and corresponding response - in case of "separate response", see Section 2.2 of [RFC7252]. + In HTTPS, this type of binding is always assured by the ordered and + reliable delivery as well as mandating that the server sends + responses in the same order that the requests were received. The + same is not true for CoAP where the server (or an attacker) can + return responses in any order and where there can be any number of + responses to a request (see e.g. [RFC7641]). In CoAP, concurrent + requests are differentiated by their Token. Note that the CoAP + Message ID cannot be used for this purpose since those are typically + different for REST request and corresponding response in case of + "separate response", see Section 2.2 of [RFC7252]. - Unfortunately, CoAP [RFC7252] does not treat Token as a - cryptographically important value and does not give stricter - guidelines than that the tokens currently "in use" SHOULD (not SHALL) - be unique. If used with security protocol not providing bindings - between requests and responses (e.g. DTLS and TLS) token reuse may - result in situations where a client matches a response to the wrong - request. Note that mismatches can also happen for other reasons than - a malicious attacker, e.g. delayed delivery or a server sending - notifications to an uninterested client. + CoAP [RFC7252] does not treat Token as a cryptographically important + value and does not give stricter guidelines than that the tokens + currently "in use" SHOULD (not SHALL) be unique. If used with a + security protocol not providing bindings between requests and + responses (e.g. DTLS and TLS) token reuse may result in situations + where a client matches a response to the wrong request. Note that + mismatches can also happen for other reasons than a malicious + attacker, e.g. delayed delivery or a server sending notifications to + an uninterested client. - A straightforward mitigation is to mandate clients to never reuse - tokens until the AEAD keys have been replaced. As there may be any - number of responses to a request (see e.g. [RFC7641]), the easiest - way to accomplish this is to implement the token as a counter and - never reuse any tokens at all. This document updates the Token - processing in [RFC7252] to always assure a cryptographically secure - binding of responses to requests. + A straightforward mitigation is to mandate clients to not reuse + tokens until the traffic keys have been replaced. The easiest way to + accomplish this is to implement the token as a counter starting at + zero for each new or rekeyed secure connection. This document + updates the Token processing in [RFC7252] to always assure a + cryptographically secure binding of responses to requests for secure + REST operations like "coaps". 1.4. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Unless otherwise specified, the terms "client" and "server" refers to @@ -245,29 +255,31 @@ block of the second request is exchanged even though the client still intends to exchange further blocks in the first operation. (Concurrent blockwise request operations are impossible with the options of [RFC7959] because the second operation's block overwrites any state of the first exchange.). The Echo and Request-Tag options are defined in this document. 2. The Echo Option - The Echo option is a lightweight server-driven challenge-response - mechanism for CoAP, motivated by the need for a server to verify - freshness of a request as described in Section 1.1. With request - freshness we mean that the server can determine that the client (or - in the case of hop-by-hop security the proxy) sent the request - recently. The time threshold for being fresh is application - specific. The Echo option value is a challenge from the server to + A fresh request is one whose age has not yet exceeded the freshness + requirements set by the server. The freshness requirements are + application specific and may vary based on resource, method, and + parameters outside of coap such as policies. The Echo option is a + lightweight challenge-response mechanism for CoAP, motivated by a + need for a server to verify freshness of a request as described in + Section 1.1. The Echo option value is a challenge from the server to the client included in a CoAP response and echoed back to the server - in one or more CoAP requests. + in one or more CoAP requests. The Echo option provides a convention + to transfer freshness tokens that works for all CoAP methods and + response codes. 2.1. Option Format The Echo Option is elective, safe-to-forward, not part of the cache- key, and not repeatable, see Figure 1, which extends Table 4 of [RFC7252]). +-----+---+---+---+---+-------------+--------+------+---------+---+---+ | No. | C | U | N | R | Name | Format | Len. | Default | E | U | +-----+---+---+---+---+-------------+--------+------+---------+---+---+ @@ -277,129 +289,128 @@ C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable, E = Encrypt and Integrity Protect (when using OSCORE) Figure 1: Echo Option Summary [ Note to RFC editor: If this document is released before core- object-security, then the following paragraph and the "E"/"U" columns above need to move into core-object-security, as they are defined in that draft. ] - The Echo option MAY be an Inner or Outer option - [I-D.ietf-core-object-security], and the Inner and Outer values are - independent. The Inner option is encrypted and integrity protected - between the endpoints, whereas the Outer option is not protected by - OSCORE and visible between the endpoints to the extent it is not - protected by some other security protocol. E.g. in the case of DTLS - hop-by-hop between the endpoints, the Outer option is visible to - proxies along the path. - The Echo option value is generated by a server, and its content and structure are implementation specific. Different methods for generating Echo option values are outlined in Appendix A. Clients and intermediaries MUST treat an Echo option value as opaque and make no assumptions about its content or structure. When receiving an Echo option in a request, the server MUST be able - to verify that the Echo option value was generated by the server as - well as the point in time when the Echo option value was generated. + to verify when the Echo option value was generated. This implies + that the server MUST be able to verify that the Echo option value was + generated by the server or some other party that the server trusts. + Depending on the freshness requirements the server may verify exactly + when the Echo option value was generated (time-based freshness) or + verify that the Echo option was generated after a specific event + (event-based freshness). As the request is bound to the Echo option + value, the server can determine that the request is not older that + the Echo option value. + + When the Echo option is used with OSCORE + [I-D.ietf-core-object-security] it MAY be an Inner or Outer option, + and the Inner and Outer values are independent. The Inner option is + encrypted and integrity protected between the endpoints, whereas the + Outer option is not protected by OSCORE and visible between the + endpoints to the extent it is not protected by some other security + protocol. E.g. in the case of DTLS hop-by-hop between the endpoints, + the Outer option is visible to proxies along the path. 2.2. Echo Processing The Echo option MAY be included in any request or response (see Section 2.3 for different applications), but the Echo option MUST NOT - be used with empty CoAP requests (i.e. Code=0.00). - - If a server receives a request which has freshness requirements, the - request does not contain a fresh Echo option value, and the server - cannot verify the freshness of the request in some other way, the - server MUST NOT process the request further and SHOULD send a 4.01 - Unauthorized response with an Echo option. The server MAY include - the same Echo option value in several different responses and to - different clients. + be used with empty CoAP requests (i.e., Code=0.00). The application decides under what conditions a CoAP request to a resource is required to be fresh. These conditions can for example include what resource is requested, the request method and other data in the request, and conditions in the environment such as the state of the server or the time of the day. + If a certain request is required to be fresh, the request does not + contain a fresh Echo option value, and the server cannot verify the + freshness of the request in some other way, the server MUST NOT + process the request further and SHOULD send a 4.01 Unauthorized + response with an Echo option. The server MAY include the same Echo + option value in several different responses and to different clients. + The server may use request freshness provided by the Echo option to verify the aliveness of a client or to synchronize state. The server may also include the Echo option in a response to force a client to - demonstrate reachability at their apparent network address. + demonstrate reachability at its claimed network address. Upon receiving a 4.01 Unauthorized response with the Echo option, the client SHOULD resend the original request with the addition of an Echo option with the received Echo option value. The client MAY send a different request compared to the original request. Upon receiving any other response with the Echo option, the client SHOULD echo the Echo option value in the next request to the server. The client MAY include the same Echo option value in several different requests to the server. Upon receiving a request with the Echo option, the server determines - if the request has freshness requirements. If the request does not - have freshness requirements, the Echo option MAY be ignored. If the - request has freshness requirements and the server cannot verify the - freshness of the request in some other way, the server MUST verify - that the Echo option value was generated by the server; otherwise the - request is not processed further. The server MUST then calculate the - round-trip time RTT = (t1 - t0), where t1 is the request receive time - and t0 is the time when the Echo option value was generated. The - server MUST only accept requests with a round-trip time below a - certain threshold T, i.e. RTT < T. If the server cannot verify that - the Echo option value was generated by the server or the round-trip - time is not below the threshold the request is not processed further, - and an error message MAY be sent. The error message SHOULD include a - new Echo option. The threshold T is application specific, its value - depends e.g. on the freshness requirements of the request. An - example message flow is illustrated in Figure 2. + if the request is required to be fresh. If not, the Echo option MAY + be ignored. If the request is required to be fresh and the server + cannot verify the freshness of the request in some other way, the + server MUST use the Echo option to verify that the request is fresh + enough. If the server cannot verify that the request is fresh + enough, the request is not processed further, and an error message + MAY be sent. The error message SHOULD include a new Echo option. + + One way for the server to verify freshness is that to bind the Echo + value to a specific point in time and verify that the request is not + older than a certain threshold T. The server can verify this by + checking that (t1 - t0) < T, where t1 is the request receive time and + t0 is the time when the Echo option value was generated. An example + message flow is illustrated in Figure 2. Client Server | | +------>| Code: 0.03 (PUT) | PUT | Token: 0x41 | | Uri-Path: lock | | Payload: 0 (Unlock) | | - |<------+ t0 Code: 4.01 (Unauthorized) + |<------+ Code: 4.01 (Unauthorized) | 4.01 | Token: 0x41 - | | Echo: 0x437468756c687521 + | | Echo: 0x437468756c687521 (t0) | | +------>| t1 Code: 0.03 (PUT) | PUT | Token: 0x42 | | Uri-Path: lock - | | Echo: 0x437468756c687521 + | | Echo: 0x437468756c687521 (t0) | | Payload: 0 (Unlock) | | |<------+ Code: 2.04 (Changed) | 2.04 | Token: 0x42 | | Figure 2: Example Echo Option Message Flow - Note that the server does not have to synchronize the time used for - the Echo timestamps with any other party. However, if the server - loses time continuity, e.g. due to reboot, it MUST reject all Echo - values that was created before time continuity was lost. - When used to serve freshness requirements (including client aliveness and state synchronizing), CoAP messages containing the Echo option MUST be integrity protected between the intended endpoints, e.g. using DTLS, TLS, or an OSCORE Inner option ([I-D.ietf-core-object-security]). When used to demonstrate - reachability at their apparent network address, the Echo option MAY - be unprotected. + reachability at a claimed network address, the Echo option SHOULD + contain the client's network address, but MAY be unprotected. A CoAP-to-CoAP proxy MAY respond to requests with 4.01 with an Echo - option to ensure the client's reachability at its apparent address, + option to ensure the client's reachability at its claimed address, and MUST remove the Echo option it recognizes as one generated by itself on follow-up requests. However, it MUST relay the Echo option of responses unmodified, and MUST relay the Echo option of requests it does not recognize as generated by itself unmodified. The CoAP server side of CoAP-to-HTTP proxies MAY request freshness, especially if they have reason to assume that access may require it (e.g. because it is a PUT or POST); how this is determined is out of scope for this document. The CoAP client side of HTTP-to-CoAP proxies SHOULD respond to Echo challenges themselves if they know @@ -460,23 +471,23 @@ address in the source address of a CoAP request). For this purpose, a server MAY ask a client to Echo its request to verify its source address. This needs to be done only once per peer and limits the range of potential victims from the general Internet to endpoints that have been previously in contact with the server. For this application, the Echo option can be used in messages that are not integrity protected, for example during discovery. * In the presence of a proxy, a server will not be able to - distiguish different origin client endpoints. Following from + distinguish different origin client endpoints. Following from the recommendation above, a proxy that sends large responses - to unauthenticatied peers SHOULD mitigate amplification + to unauthenticated peers SHOULD mitigate amplification attacks. The proxy MAY use Echo to verify origin reachability as described in Section 2.2. The proxy MAY forward idempotent requests immediately to have a cached result available when the client's Echoed request arrives. 4. A server may want to use the request freshness provided by the Echo to verify the aliveness of a client. Note that in a deployment with hop-by-hop security and proxies, the server can only verify aliveness of the closest proxy. @@ -596,20 +607,27 @@ When Block1 and Block2 are combined in an operation, the Request-Tag of the Block1 phase is set in the Block2 phase as well for otherwise the request would have a different set of options and would not be recognized any more. Clients are encouraged to generate compact messages. This means sending messages without Request-Tag options whenever possible, and using short values when the absent option can not be recycled. + The Request-Tag options MAY be present in request messages that carry + a Block2 option even if those messages are not part of a blockwise + request operation (this is to allow the operation described in + Section 3.4.3). The Request-Tag option MUST NOT be present in + response messages, and MUST NOT be present if neither the Block1 nor + the Block2 option is present. + 3.4. Applications 3.4.1. Body Integrity Based on Payload Integrity When a client fragments a request body into multiple message payloads, even if the individual messages are integrity protected, it is still possible for a man-in-the-middle to maliciously replace a later operation's blocks with an earlier operation's blocks (see Section 2.5 of [I-D.mattsson-core-coap-actuators]). Therefore, the integrity protection of each block does not extend to the operation's @@ -699,26 +717,26 @@ The Request-Tag option can be elective, because to servers unaware of the Request-Tag option, operations with differing request tags will not be matchable. The Request-Tag option can be safe to forward but part of the cache key, because to proxies unaware of the Request-Tag option will consider operations with differing request tags unmatchable but can still forward them. The Request-Tag option is repeatable because this easily allows - stateless proxies to "chain" their origin address. Were it a single - option, they would need to employ some length/value scheme to avoid - confusing requests without a Request-Tag option with requests that - carry a zero-length request tag. + stateless proxies to "chain" their origin address. They can perform + the steps of Section 3.4.3 without the need to create an option value + that is the concatenation of the received option and their own value, + and can simply add a new Request-Tag option unconditionally. - In earlier versions of this draft, the Request-Tag option used to be + In draft versions of this document, the Request-Tag option used to be critical and unsafe to forward. That design was based on an erroneous understanding of which blocks could be composed according to [RFC7959]. 3.6. Rationale for Introducing the Option An alternative that was considered to the Request-Tag option for coping with the problem of fragmented message body integrity (Section 3.4.1) was to update [RFC7959] to say that blocks could only be assembled if their fragments' order corresponded to the sequence @@ -742,66 +760,74 @@ To gain equivalent protection to Section 3.4.1, a server MUST use the Block2 option in conjunction with the ETag option ([RFC7252], Section 5.10.6), and MUST NOT use the same ETag value for different representations of a resource. 5. Token Processing As described in Section 1.3, the client must be able to verify that a response corresponds to a particular request. This section updates - the Token processing in Section 5.3.1 of [RFC7252] by adding the - following text: + the CoAP Token processing requirements for clients. The Token + processing for servers is not updated. Token processing in + Section 5.3.1 of [RFC7252] is updated by adding the following text: When CoAP is used with a security protocol not providing bindings - between requests and responses, the client MUST NOT reuse tokens - until the traffic keys have been replaced. The easiest way to - accomplish this is to implement the Token as a counter, this approach - SHOULD be followed. + between requests and responses, the tokens have cryptographic + importance. The client MUST make sure that tokens are not used in a + way so that responses risk being associated with the wrong request. + The easiest way to accomplish this is to implement the Token (or part + of the Token) as a sequence number starting at zero for each new or + rekeyed secure connection, this approach SHOULD be followed. To + avoid collisions the sequence number can be encoded with a fixed + length or with some length-value encoding. 6. Security Considerations The availability of a secure pseudorandom number generator and truly random seeds are essential for the security of the Echo option. If no true random number generator is available, a truly random seed must be provided from an external source. - An Echo value with 64 (pseudo-)random bits gives the same theoretical - security level against forgeries as a 64-bit MAC (as used in e.g. - AES_128_CCM_8). In practice, forgery of an Echo option value is much - harder as an attacker must also forge the MAC in the security - protocol. The Echo option value MUST contain 32 (pseudo-)random bits - that are not predictable for any other party than the server, and - SHOULD contain 64 (pseudo-)random bits. A server MAY use different - security levels for different uses cases (client aliveness, request - freshness, state synchronization, network address reachability, - etc.). + A single active Echo value with 64 (pseudo-)random bits gives the + same theoretical security level against forgeries as a 64-bit MAC (as + used in e.g. AES_128_CCM_8). In practice, forgery of an Echo option + value is much harder as an attacker must also forge the MAC in the + security protocol. The Echo option value MUST contain 32 + (pseudo-)random bits that are not predictable for any other party + than the server, and SHOULD contain 64 (pseudo-)random bits. A + server MAY use different security levels for different uses cases + (client aliveness, request freshness, state synchronization, network + address reachability, etc.). The security provided by the Echo and Request-Tag options depends on the security protocol used. CoAP and HTTP proxies require (D)TLS to be terminated at the proxies. The proxies are therefore able to manipulate, inject, delete, or reorder options or packets. The security claims in such architectures only hold under the assumption that all intermediaries are fully trusted and have not been compromised. - Servers MUST use a monotonic clock to generate timestamps and compute - round-trip times. Use of non-monotonic clocks is not secure as the - server will accept expired Echo option values if the clock is moved - backward. The server will also reject fresh Echo option values if - the clock is moved forward. + Servers SHOULD use a monotonic clock to generate timestamps and + compute round-trip times. Use of non-monotonic clocks is not secure + as the server will accept expired Echo option values if the clock is + moved backward. The server will also reject fresh Echo option values + if the clock is moved forward. Non-monotonic clocks MAY be used as + long as they have deviations that are acceptable given the freshness + requirements. If the deviations from a monotonic clock are known, it + may be possible to adjust the threshold accordingly. - Servers are not allowed to use wall clock time for timestamps, as - wall clock time is not monotonic. Furthermore, an attacker may be - able to affect the server's wall clock time in various ways such as - setting up a fake NTP server or broadcasting false time signals to - radio-controlled clocks. + Servers SHOULD NOT use wall clock time for timestamps, as wall clock + time have large deviations from a monotonic clock. Furthermore, an + attacker may be able to affect the server's wall clock time in + various ways such as setting up a fake NTP server or broadcasting + false time signals to radio-controlled clocks. Servers MAY use the time since reboot measured in some unit of time. Servers MAY reset the timer at certain times and MAY generate a random offset applied to all timestamps. When resetting the timer, the server MUST reject all Echo values that was created before the reset. Servers that use the List of Cached Random Values and Timestamps method described in Appendix A may be vulnerable to resource exhaustion attacks. One way to minimize state is to use the @@ -854,33 +880,33 @@ [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 9.2. Informative References [I-D.hartke-core-stateless] Hartke, K., "Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP)", draft-hartke- - core-stateless-01 (work in progress), September 2018. + core-stateless-02 (work in progress), October 2018. [I-D.ietf-core-object-security] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments - (OSCORE)", draft-ietf-core-object-security-15 (work in - progress), August 2018. + (OSCORE)", draft-ietf-core-object-security-16 (work in + progress), March 2019. [I-D.ietf-core-oscore-groupcomm] Tiloca, M., Selander, G., Palombini, F., and J. Park, "Group OSCORE - Secure Group Communication for CoAP", - draft-ietf-core-oscore-groupcomm-03 (work in progress), - October 2018. + draft-ietf-core-oscore-groupcomm-04 (work in progress), + March 2019. [I-D.mattsson-core-coap-actuators] Mattsson, J., Fornehed, J., Selander, G., Palombini, F., and C. Amsuess, "Controlling Actuators with CoAP", draft- mattsson-core-coap-actuators-06 (work in progress), September 2018. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, . @@ -916,52 +942,56 @@ Different mechanisms have different tradeoffs between the size of the Echo option value, the amount of server state, the amount of computation, and the security properties offered. A server MAY use different methods and security levels for different uses cases (client aliveness, request freshness, state synchronization, network address reachability, etc.). 1. List of Cached Random Values and Timestamps. The Echo option value is a (pseudo-)random byte string. The server caches a list containing the random byte strings and their transmission times. - Assuming 64-bit random values and 32-bit timestamps, the size of the - Echo option value is 8 bytes and the amount of server state is 12n - bytes, where n is the number of active Echo Option values. If the - server loses time continuity, e.g. due to reboot, the entries in the - old list MUST be deleted. + Assuming 72-bit random values and 32-bit timestamps, the size of the + Echo option value is 9 bytes and the amount of server state is 13n + bytes, where n is the number of active Echo Option values. The + security against forged echo values is given by s = bit length of r - + log2(n). The length of r and the maximum allowed n should be set so + that the security level is harmonized with other parts of the + deployment, e.g., s >= 64. If the server loses time continuity, e.g. + due to reboot, the entries in the old list MUST be deleted. Echo option value: random value r Server State: random value r, timestamp t0 2. Integrity Protected Timestamp. The Echo option value is an integrity protected timestamp. The timestamp can have different resolution and range. A 32-bit timestamp can e.g. give a resolution of 1 second with a range of 136 years. The (pseudo-)random secret key is generated by the server and not shared with any other party. The use of truncated HMAC-SHA-256 is RECOMMENDED. With a 32-bit timestamp and a 64-bit MAC, the size of the Echo option value is 12 - bytes and the Server state is small and constant. If the server + bytes and the Server state is small and constant. The security + against forged echo values is given by the MAC length. If the server loses time continuity, e.g. due to reboot, the old key MUST be deleted and replaced by a new random secret key. Note that the privacy considerations in Section 7 may apply to the timestamp. A server MAY want to encrypt its timestamps, and, depending on the choice of encryption algorithms, this may require a nonce to be included in the Echo option value. Echo option value: timestamp t0, MAC(k, t0) Server State: secret key k Other mechanisms complying with the security and privacy considerations may be used. The use of encrypted timestamps in the Echo option typically requires an IV to be included in the Echo option value, which adds overhead and makes the specification of such - a mechanims slightly more complicated than the two mechanisms + a mechanism slightly more complicated than the two mechanisms specified here. Appendix B. Request-Tag Message Size Impact In absence of concurrent operations, the Request-Tag mechanism for body integrity (Section 3.4.1) incurs no overhead if no messages are lost (more precisely: in OSCORE, if no operations are aborted due to repeated transmission failure; in DTLS, if no packages are lost), or when blockwise request operations happen rarely (in OSCORE, if there is always only one request blockwise operation in the replay window). @@ -983,20 +1013,57 @@ o In OSCORE, the sequence number can be artificially increased so that all lost messages are outside of the replay window by the time the first request of the new operation gets processed, and all earlier operations can therefore be regarded as concluded. Appendix C. Change Log [ The editor is asked to remove this section before publication. ] + o Changes since draft-ietf-core-echo-request-tag-03: + + * Mention token processing changes in title + + * Abstract reworded + + * Clarify updates to token processing + + * Describe security levels from Echo length + + * Allow non-monotonic clocks under certain conditions for + freshness + + * Simplify freshness expressions + + * Describe when a Request-Tag can be set + + * Add note on application-level freshness mechanisms + + * Minor editorial changes + + o Changes since draft-ietf-core-echo-request-tag-02: + + * Define "freshness" + + * Note limitations of "aliveness" + + * Clarify proxy and OSCORE handling in presence of "echo" + + * Clarify when Echo values may be reused + + * Update security considerations + + * Various minor clarifications + + * Minor editorial changes + o Major changes since draft-ietf-core-echo-request-tag-01: * Follow-up changes after the "relying on blockwise" change in -01: + Simplify the description of Request-Tag and matchability + Do not update RFC7959 any more * Make Request-Tag repeatable. @@ -1031,27 +1098,28 @@ * The interaction between the new option and (cross) proxies is now covered. * Two messages being "Request-Tag matchable" was introduced to replace the older concept of having a request tag value with its slightly awkward equivalence definition. Acknowledgments - The authors want to thank Jim Schaad for providing valuable input to - the draft. + The authors want to thank Jim Schaad and Carsten Bormann for + providing valuable input to the draft. Authors' Addresses Christian Amsuess Email: christian@amsuess.com + John Mattsson Ericsson AB Email: john.mattsson@ericsson.com Goeran Selander Ericsson AB Email: goran.selander@ericsson.com