Internet Draft R. Atkinson <draft-li-isis-hmac-shs-00.txt> Extreme Networks Category: Informational T. Li Expires 16 Aug 2007 cisco Systems M. Fanto Ford Motor Company 16 February 2007 IS-IS Authentication with HMAC-SHS <draft-li-isis-hmac-shs-00.txt> Status of this Memo Distribution of this memo is unlimited. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document describes how the NIST Secure Hash Standard family of algorithms are used for IS-IS authentication. 1. INTRODUCTION This document provides an update to IS-IS Cryptographic Authentication, which is specified in RFC-3567. This document does not deprecate RFC-3567. IS-IS as deployed in the Internet is defined by RFC-1195 and ISO 10589. This document adds support for all algorithms defined in the US NIST Secure Hash Standard (SHS) as defined in NIST FIPS 180-2. [FIPS-180-2] includes SHA-1, SHA-256, SHA-384, and SHA-512. The HMAC authentication mode as defined in NIST FIPS 198 is used. [FIPS-198] This creation of this addition to IS-IS was driven by operator requests that they be able to use the NIST SHS family of algorithms, instead of being forced to use the MD5 algorithm. While there are no openly published attacks on the HMAC-MD5 mechanism, some reports [Dobb96a, Dobb96b] create concern about the ultimate strength of the MD5 cryptographic hash function. 2. AUTHENTICATION PROCEDURES The authentication type used for any of the HMAC-SHS algorithms is 54 (0x36), which is the same authentication type used for the existing HMAC-MD5 algorithm. The length of the Authentication Value varies with the specific hashing algorithm used. The length field in the TLV is the sum of the length of the Authentication Value field and the number 1. For example, when SHA-256 is in use, the length of the Authentication Value is 32 and the length field in the TLV is 33. 2.1 Authentication Process. When calculating the HMAC-SHS result for Sequence Number PDUs, Level 1 Sequence Number PDUs SHALL use the Area Authentication string as in Level 1 Link State PDUs. Level 2 Sequence Number PDUs shall use the domain authentication string as in Level 2 Link State PDUs. IS-IS HELLO PDUs SHALL use the Link Level Authentication String, which MAY be different from that of Link State PDUs. The HMAC-SHS result for the IS-IS HELLO PDUs SHALL be calculated after the Packet is padded to the MTU size, if padding is not disabled. Implementations that support the optional checksum for the Sequence Number PDUs and IS-IS HELLO PDUs MUST NOT include the Checksum TLV. 2.2 Cryptographic Aspects In the algorithm description below, the following nomenclature, which is consistent with [FIPS-198], is used: H is the specific hashing algorithm (e.g. SHA-256). K is the password for the PDU type as per ISO 10589. Ko is the cryptographic key used with the hash algorithm. B is the block size of H, measured in octets rather than bits. Note that B is the internal block size, not the hash size. For SHA-1 and SHA-256: B == 64 For SHA-384 and SHA-512: B == 128 L is the length of the hash, measured in octets rather than bits. XOR is the exclusive-or operation. Opad is the hexadecimal value 0x5c repeated B times. Ipad is the hexadecimal value 0x36 repeated B times. Apad is the hexadecimal value 0x878FE1F3 repeated (L/4) times. (1) PREPARATION OF KEY In this application, Ko is always L octets long. If the Authentication Key (K) is L octets long, then Ko is equal to K. If the Authentication Key (K) is more than L octets long, then Ko is set to H(K). If the Authentication Key (K) is less than L octets long, then Ko is set to the Authentication Key (K) with zeros appended to the end of the Authentication Key (K) such that Ko is L octets long. (2) FIRST HASH First, the IS-IS packet's Authentication Data field is filled with the value Apad and the Authentication Type field is set to 54. Then, a first hash, also known as the inner hash, is computed as follows: First-Hash = H(Ko XOR Ipad || (IS-IS Packet)) (3) SECOND HASH Then a second hash, also known as the outer hash, is computed as follows: Second-Hash = H(Ko XOR Opad || First-Hash) (4) RESULT The result Second-Hash becomes the Authentication Data that is sent in the Authentication Data field of the IS-IS packet. The length of the Authentication Data field is always identical to the message digest size of the specific hash function H that is being used. This also means that the use of hash functions with larger output sizes will also increase the size of the IS-IS packet as transmitted on the wire. 2.3 Accepting Authenticated IS-IS Packets To authenticate an incoming PDU, a system should save the value of the Authentication Value field, the Checksum, and the Remaining Lifetime fields. Then set the Authentication Value equal to Apad and zero the Checksum and Remaining Lifetime fields. The Authentication Value is then computed as described above, then the values of these three fields is restored. An implementation that implements HMAC-SHS authentication and receives HMAC-SHS Authentication Information MUST discard the PDU if the Authentication Value is incorrect. An implementation MAY have a transition mode where it includes HMAC-SHS Authentication Information in PDUs but does not verify the HMAC-SHS authentication information. This is a transition aid for networks in the process of deploying authentication. An implementation MAY check a set of passwords when verifying the Authentication Value. This provides a mechanism for incrementally changing passwords in a network. An implementation that does not implement HMAC-SHS authentication MAY accept a PDU that contains the HMAC-SHS Authentication Type. ISes (routers) that implement HMAC-SHS authentication and initiate LSP purges MUST remove the body of the LSP and add the authentication TLV. ISes implementing HMAC-SHS authentication MUST NOT accept unauthenticated purges. ISes MUST NOT accept purges that contain TLVs other than the authentication TLV. These restrictions are necessary to prevent a hostile system from receiving an LSP, setting the Remaining Lifetime field to zero, and flooding it, thereby initiating a purge without knowing the authentication password. 2.4 Implementation Considerations There is an implementation issue just after password rollover on an IS-IS router that might benefit from additional commentary. Immediately after password rollover on the router, the router or IS-IS process may restart. If this happens, this causes the LSP Sequence Number restarts from the value 1 using the new password. However, neighbors will reject those new LSPs because the Sequence Number is smaller. The router can not increase its own LSP Sequence Number because it fails to authenticate its own old LSP that neighbors keep sending to it. So the router can not update its LSP Sequence Number to its neighbors until all the neighbors time out all of the original LSPs. One possible solution to this problem is for the IS-IS process to detect if any inbound LSP with an authentication failure has the local System ID and also has a higher Sequence Number than the IS-IS process has. In this event, the IS-IS process SHOULD increase its own LSP Sequence Number accordingly and re-flood the LSPs. However, as this scenario could also be triggered by an active attack by an adversary, it is recommended that a counter also be kept on this case to mitigate the risk from such an active attack. 3. SECURITY CONSIDERATIONS This document enhances the security of the IS-IS routing protocol by adding support for additional cryptographic hash functions. Because a routing protocol contains information that need not be kept secret, privacy is not a requirement. However, authentication of the messages within the protocol is of interest, to reduce the risk of an adversary compromising the routing system by deliberately injecting false information into the routing system. The technology in this document provides an authentication mechanism for IS-IS. The mechanism described here is not perfect and does not need to be perfect. Instead, this mechanism represents a significant increase in the work function of an adversary attacking the IS-IS protocol, while not causing undue implementation, deployment, or operational complexity. This mechanism does not prevent replay attacks, however, in most cases, such attacks would trigger existing mechanisms in the IS-IS protocol that would effectively reject old information. Denial of service attacks are not generally preventable in a useful networking protocol [VK83]. Because of implementation considerations, including the need for backwards compatibility, this specification uses the same mechanism as specified in RFC-3567 and limits itself to adding support for additional cryptographic hash functions. Network operators have indicated they strongly prefer to retain the basic mechanism defined in RFC-3567. If a stronger authentication were believed to be required, then the use of a full digital signature [RFC-2154] would be an approach that should be seriously considered. It was rejected for this purpose at this time because the computational burden of full digital signatures is believed to be much higher than is reasonable given the current threat environment in operational commercial networks. 4. IANA CONSIDERATIONS There are no IANA considerations for this document. 5. ACKNOWLEDGEMENTS TBD 6. REFERENCES 6.1 Normative References [ISO-10589] International Standards Organisation, "Intermediate System to Intermediate System Routing Information Exchange Protocol for use in Conjunction with the Protocol for Providing the Connectionless-mode Network Service (ISO 8473)", ISO/IEC 10589:2002, Second Edition, 2002. [FIPS-180-2] US National Institute of Standards & Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-2, August 2002. [FIPS-198] US National Institute of Standards & Technology, "The Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB 198, March 2002. 6.2 Informative References [Dobb96a] Dobbertin, H, "Cryptanalysis of MD5 Compress", Technical Report, 2 May 1996. (Presented at the Rump Session of EuroCrypt 1996.) [Dobb96b] Dobbertin, H, "The Status of MD5 After a Recent Attack", CryptoBytes, Vol. 2, No. 2, Summer 1996. [RFC-1195] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments", RFC 1195, December 1990. [RFC-2154] Murphy, S., Badger, M. and B. Wellington, "OSPF with Digital Signatures", RFC 2154, June 1997. [VK83] Voydock, V. and S. Kent, "Security Mechanisms in High-level Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983. AUTHORS Randall J. Atkinson Extreme Networks 3585 Monroe Street Santa Clara, CA 95051 USA Phone: +1 (408) 579-2800 EMail: rja@extremenetworks.com Tony Li Cisco Systems San Jose, CA USA EMail: tli@cisco.com Matt Fanto Ford Motor Company Michigan USA EMail: tbd Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST, AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.