--- 1/draft-ietf-bfd-secure-sequence-numbers-06.txt 2020-12-16 16:13:11.075229323 -0800 +++ 2/draft-ietf-bfd-secure-sequence-numbers-07.txt 2020-12-16 16:13:11.095229833 -0800 @@ -1,25 +1,25 @@ Network Working Group M. Jethanandani Internet-Draft Kloud Services Updates: 5880 (if approved) S. Agarwal Intended status: Standards Track Cisco Systems, Inc -Expires: February 6, 2021 A. Mishra +Expires: June 19, 2021 A. Mishra O3b Networks A. Saxena Ciena Corporation A. Dekok Network RADIUS SARL - August 5, 2020 + December 16, 2020 Secure BFD Sequence Numbers - draft-ietf-bfd-secure-sequence-numbers-06 + draft-ietf-bfd-secure-sequence-numbers-07 Abstract This document describes a security enhancement for the sequence number used in BFD control packets. This document updates RFC 5880. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. @@ -27,21 +27,21 @@ 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 February 6, 2021. + This Internet-Draft will expire on June 19, 2021. Copyright Notice Copyright (c) 2020 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 @@ -50,26 +50,26 @@ include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2 3. Theory of operation . . . . . . . . . . . . . . . . . . . . . 2 4. Impact of using a hash . . . . . . . . . . . . . . . . . . . 4 - 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 + 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 8.1. Normative References . . . . . . . . . . . . . . . . . . 5 - 8.2. Informative References . . . . . . . . . . . . . . . . . 6 + 8.2. Informative References . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction BFD [RFC5880] section 6.7 describes the use of monotonically incrementing 32-bit sequence numbers for use in authentication of BFD packets. While this method protects against simple replay attacks, the monotonically incrementing sequence numbers are predictable and vulnerable to more complex attack vectors. This document proposes the use of non-monotonically-incrementing sequence numbers in the BFD @@ -82,149 +82,151 @@ 2. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 3. Theory of operation Instead of inserting a monotonically, sometimes occasionally, - increasing sequence number in BFD control packets, a hash is - inserted. The hash is computed, using a shared key, on the sequence - number. That computed hash is then inserted into the sequence number - field of the packet. In case of BFD Authentication + increasing sequence number in BFD control packets, the ciphertext + result from a symmetric key algorithm operation (Symmetric-key + algorithms require both the sender and the recipient of a message to + have the same shared secret key) is inserted. The result is + computed, using a shared key, on the sequence number. That + ciphertext result is then inserted into the sequence number field of + the packet. In case of BFD Authentication [I-D.ietf-bfd-optimizing-authentication], the sequence number used in - computing an authenticated packet would be this new computed hash. - Even though the BFD Authentication + computing an authenticated packet would be this new computed + ciphertext. Even though the BFD Authentication [I-D.ietf-bfd-optimizing-authentication] sequence number is independent of this enhancement, it would benefit by using the - computed hash. + computed ciphertext. As currently defined in BFD [RFC5880], a BFD packet with authentication will undergo the following steps, where: [O]: original RFC 5880 packet with monotonically increasing sequence number [S]: pseudo random sequence number [A]: Authentication Sender Receiver [O] [S] [A] ------------- [A] [S] [O] This document proposes that for enhanced security in sequence number - encoding, the sender would identify a hash algorithm (symmetric) that - would create a 32 bit hash. The hashing key is provisioned securely - on the sender and receiver of the BFD session. The mechanism of - provisioning such a key is outside the scope of this document. - Instead of using the sequence number, the sender encodes the sequence - number with the hashing key to produce a hash. + encoding, the sender would identify a symmetric key algorithm that + would create a 32 bit ciphertext. The symmetric key is provisioned + securely on the sender and receiver of the BFD session. The + mechanism of provisioning such a key is outside the scope of this + document. This key SHOULD be different from the symmetric key used + to to authenticate the packet. Instead of sending the sequence + number, the sender encrypts the sequence number using it as input to + the symmetric algorithm to produce the ciphertext, which is then + inserted in place of the sequence number. - Upon receiving the BFD Control packet, the receiver compares the - received sequence number against the expected sequence number. The - mechanism used for comparing is an implementation detail - (implementations may pre-calculate the expected hashed sequence - number, or decrypt the received sequence number before comparing - against expected value). To tolerate dropped frames, the receiver - MUST compare the received sequence number against the current - expected sequence number (previous received sequence number + 1) and - N subsequent expected sequence numbers (where N is greater than or - equal to the detect multiplier). Note: The first sequence number can - be obtained using the same logic as used in determining Local - Discriminator value for the session or by using a random number. + Upon receiving the BFD Control packet, the receiver decrypts the + ciphertext using the same provisioned shared key to produce the + received sequence number. It compares the received sequence number + against the expected sequence number. The mechanism used for + comparing is an implementation detail (implementations may pre- + calculate the expected sequence number, or decrypt the received + sequence number before comparing against expected value). To + tolerate dropped frames, the receiver MUST compare the received + sequence number against the current expected sequence number + (previous received sequence number + 1) and N subsequent expected + sequence numbers (where N is greater than or equal to the detect + multiplier). Note: The first sequence number can be obtained using + the same logic as used in determining Local Discriminator value for + the session or by using a random number. - k: hashing key + K: symmetric key - s: sequence number + S: sequence number + + S': encrypted sequence number OR ciphertext result O: original RFC 5880 packet with monotonically increasing sequence number - R: remainder of packet - - H1: hash of s - - H2: hash of entire packet - - A: H2 + insertion in packet - - hash(s, k) = H1 - hash((H1 + R), k) = H2 - - hash'((Packet - H2), k) == H2 ? Good packet : bad packet + f(S, K) = S', where f is a symmetric encryption algorithm - hash'(H1, k) > previously received s ? Good sequence number : bad - sequence number + f(S', K) = S, where f is a symmetric decryption algorithm Sender Receiver - [O] [H1] [A] -------- [A] [H1] [O] + [O] [S'] [A] -------- [A] [S] [O] - The above diagram describes how the sender encodes and receiver - decodes the sequence number. The sender starts by taking the - monotonically increasing sequence number and hashing it. It replaces - the sequence number with the hash. It then calculates the hash for - the entire packet and appends the hash value to the end of the - packet, before transmitting it. + The above diagram describes how the sender encrypts and receiver + decrypts the sequence number. The sender starts by taking the + monotonically increasing (but non linear) sequence number and + encrypting it using a symmetric encryption algorithm. The resulting + ciphertext replaces the sequence number. As per BFD [RFC5880], it + then calculates the hash for the entire packet and appends the hash + value to the end of the packet, before transmitting it. - The receiver hashes the entire packet without H2, and compares the - hash value with the received hash (H2). If the hash values are - equal, it is a good packet, else it is a bad packet. It then - calculates the hash on the received sequence number to retreive s. - If it is greater than the previously received monotically increasing - sequence number, then the receiver knows it's a valid sequence - number. + The receiver hashes the entire packet as part of receiver + authentication. On successful authentication, it decrypts the + ciphertext with the same key used to encrypt it, in order to obtain + the original sequence number. If it is greater than the previously + received monotonically increasing sequence number, then the receiver + knows it's a valid sequence number. 4. Impact of using a hash - Under this proposal, every packet's sequence number is encoded within - a hash. Therefore there is some impact on the system and its - performance while encoding/decoding the hash. As security measures - go, this enhancement greatly increases the security of the packet - with or without authentication of the entire packet. + Under this proposal, every packet's sequence number is encoded in + ciphertext. Therefore, there is some impact on the system and its + performance while encryption/decryption. As security measures go, + this enhancement greatly increases the security of the packet with or + without authentication of the entire packet. 5. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. 6. Security Considerations - While the proposed mechanism improves overall security of BFD - mechanism, the security consderations are listed below: - - Because of the fast rate of BFD sesions and it is difficult to change - the keys (used for hashing the sequence number) during the operation - of a BFD session without affecting the stability of the BFD session. - It is, therefore, recommended to administratively disable the BFD - session before changing the keys. If the keys are not changed, an - attacker can use a replay attack. + In a symmetric key algorithm, the key is shared between the two + systems. Distribution of this key to all the systems at the same + time can be quite a cumbersome task. BFD sessions running a fast + rate will require these keys to be refreshed often, which poses a + further challenge. Therefore, it is difficult to change the keys + during the operation of a BFD session without affecting the stability + of the BFD session. Therefore, it is recommended to administratively + disable the BFD session before changing the keys. If the keys are + not changed frequently, an attacker can try to guess the key to + launch a replay attack. - Using this method allows the BFD end-points to detect a malicious - packet (the decrypted sequence number will not be in sequence) the - behavior of the session when such a packet is detected is based on + This method allows the BFD end-points to detect a malicious packet + (the decrypted sequence number will not be in sequence). The + behavior of the session, when such a packet is detected, is based on the implementation. A flood of such malicious packets may cause a - session to report BFD session to be operationally down. + BFD session to be operationally down. - The hashing algorithm and key size will determine the difficulty for - an attacker to decipher the key from the transmitted BFD frames. The - sequential nature of the payload (sequence numbers) simplifies the - decoding of the key. It is, therefore, recommended to use longer - keys or more secure hashing algorithms. + The symmetric algorithm and key size will determine the difficulty + for an attacker to decipher the key from the transmitted BFD frames. + The sequential nature of the payload (sequence numbers) simplifies + the decoding of the key. It is, therefore, recommended to use longer + keys or more secure symmetric algorithms. 7. Acknowledgements + The authors would like to thank Jeff Hass and Reshad Rahman for their + reviews of and suggestions for the document. + 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection