





Network Working Group                                              T. Li
Request for Comments: 5304                        Redback Networks, Inc.
Obsoletes: 3567                                              R. Atkinson
Updates: 1195                                     Extreme Networks, Inc.
Category: Standards Track                                   October 2008


                   IS-IS Cryptographic Authentication

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes the authentication of Intermediate System to
   Intermediate System (IS-IS) Protocol Data Units (PDUs) using the
   Hashed Message Authentication Codes - Message Digest 5 (HMAC-MD5)
   algorithm as found in RFC 2104.  IS-IS is specified in International
   Standards Organization (ISO) 10589, with extensions to support
   Internet Protocol version 4 (IPv4) described in RFC 1195.  The base
   specification includes an authentication mechanism that allows for
   multiple authentication algorithms.  The base specification only
   specifies the algorithm for cleartext passwords.  This document
   replaces RFC 3567.

   This document proposes an extension to that specification that allows
   the use of the HMAC-MD5 authentication algorithm to be used in
   conjunction with the existing authentication mechanisms.


















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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Authentication Procedures . . . . . . . . . . . . . . . . . . . 3
     2.1.  Implementation Considerations . . . . . . . . . . . . . . . 5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
     3.1.  Security Limitations  . . . . . . . . . . . . . . . . . . . 5
     3.2.  Assurance . . . . . . . . . . . . . . . . . . . . . . . . . 6
     3.3.  Key Configuration . . . . . . . . . . . . . . . . . . . . . 6
     3.4.  Other Considerations  . . . . . . . . . . . . . . . . . . . 7
     3.5.  Future Directions . . . . . . . . . . . . . . . . . . . . . 7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 9



































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1.  Introduction

   The IS-IS protocol, as specified in [ISO-10589], provides for the
   authentication of Link State Protocol Data Units (LSPs) through the
   inclusion of authentication information as part of the LSP.  This
   authentication information is encoded as a Type-Length-Value (TLV)
   tuple.  The use of IS-IS for IPv4 networks is described in [RFC1195].

   The type of the TLV is specified as 10.  The length of the TLV is
   variable.  The value of the TLV depends on the authentication
   algorithm and related secrets being used.  The first octet of the
   value is used to specify the authentication type.  Type 0 is
   reserved, type 1 indicates a cleartext password, and type 255 is used
   for routing domain private authentication methods.  The remainder of
   the TLV value is known as the Authentication Value.

   This document extends the above situation by allocating a new
   authentication type for HMAC-MD5 and specifying the algorithms for
   the computation of the Authentication Value.  This document also
   describes modifications to the base protocol to ensure that the
   authentication mechanisms described in this document are effective.

   This document is a publication of the IS-IS Working Group within the
   IETF.  This document replaces [RFC3567], which is an Informational
   RFC.  This document is on the Standards Track.  This document has
   revised Section 3, with the significant addition of a discussion of
   recent attacks on MD5 in Section 3.2.  This document has also added a
   substantive "IANA Considerations" section to create a missing
   codepoint registry.

   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 [RFC2119].

2.  Authentication Procedures

   The authentication type used for HMAC-MD5 is 54 (0x36).  The length
   of the Authentication Value for HMAC-MD5 is 16, and the length field
   in the TLV is 17.

   The HMAC-MD5 algorithm requires a key K and text T as input
   [RFC2104].  The key K is the password for the PDU type, as specified
   in ISO 10589.  The text T is the IS-IS PDU to be authenticated with
   the Authentication Value field (inside of the Authentication
   Information TLV) set to zero.  Note that the Authentication Type is
   set to 54 and the length of the TLV is set to 17 before
   authentication is computed.  When LSPs are authenticated, the




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   Checksum and Remaining Lifetime fields are set to zero (0) before
   authentication is computed.  The result of the algorithm is placed in
   the Authentication Value field.

   When calculating the HMAC-MD5 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-MD5 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.

   To authenticate an incoming PDU, a system should save the values of
   the Authentication Value field, the Checksum field, and the Remaining
   Lifetime field, set these fields to zero, compute authentication, and
   then restore the values of these fields.

   An implementation that implements HMAC-MD5 authentication and
   receives HMAC-MD5 Authentication Information MUST discard the PDU if
   the Authentication Value is incorrect.

   An implementation MAY have a transition mode where it includes HMAC-
   MD5 Authentication Information in PDUs but does not verify the HMAC-
   MD5 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-MD5 authentication MAY
   accept a PDU that contains the HMAC-MD5 Authentication Type.  ISes
   (routers) that implement HMAC-MD5 authentication and initiate LSP
   purges MUST remove the body of the LSP and add the authentication
   TLV.  ISes implementing HMAC-MD5 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.








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2.1.  Implementation Considerations

   There is an implementation issue that occurs just after password
   rollover on an IS-IS router and 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 to restart from the value 1 using the new
   password.  However, neighbors will reject those new LSPs because the
   Sequence Number is smaller.  The router cannot increase its own LSP
   Sequence Number because it fails to authenticate its own old LSP that
   neighbors keep sending to it.  So the router cannot 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 be kept on
   this case to mitigate the risk from such an attack.

3.  Security Considerations

   This document enhances the security of the IS-IS routing protocol.
   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 in order to reduce the
   risk of an adversary compromising the routing system by deliberately
   injecting false information into that system.

3.1.  Security Limitations

   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.  It provides improved security against
   passive attacks, as defined in [RFC1704], when compared to cleartext
   password authentication.

   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 [DoS].





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   The mechanisms in this document do not provide protection against
   compromised, malfunctioning, or misconfigured routers.  Such routers
   can, either accidentally or deliberately, cause malfunctions that
   affect the whole routing domain.  The reader is encouraged to consult
   [RFC4593] for a more comprehensive description of threats to routing
   protocols.

3.2.  Assurance

   Users need to understand that the quality of the security provided by
   this mechanism depends completely on the strength of the implemented
   authentication algorithms, the strength of the key being used, and
   the correct implementation of the security mechanism in all
   communicating IS-IS implementations.  This mechanism also depends on
   the IS-IS Authentication Key being kept confidential by all parties.
   If any of these are incorrect or insufficiently secure, then no real
   security will be provided to the users of this mechanism.

   Since Dobbertin's attacks on MD5 [Dobb96a] [Dobb96b] [Dobb98] were
   first published a dozen years ago, there have been growing concerns
   about the effectiveness of the compression function within MD5.  More
   recent work by Wang and Yu [WY05] accentuates these concerns.
   However, despite these research results, there are no published
   attacks at present on either Keyed-MD5 or HMAC-MD5.  A recent paper
   by Bellare [Bell06a] [Bell06b] provides new proofs for the security
   of HMAC that require fewer assumptions than previous published proofs
   for HMAC.  Those proofs indicate that the published issues with MD5
   (and separately with SHA-1) do not create an attack on HMAC-MD5 (or
   HMAC SHA-1).  Most recently, Fouque and others [FLN07] have published
   new attacks on NMAC-MD4, HMAC-MD4, and NMAC-MD5.  However, their
   attacks are non-trivial computationally, and they have not found an
   equivalent attack on HMAC-MD5.  So, despite the published issues with
   the MD5 algorithm, there is currently no published attack that
   applies to HMAC-MD5 as used in this IS-IS specification.  As with any
   cryptographic technique, there is the possibility of the discovery of
   future attacks against this mechanism.

3.3.  Key Configuration

   It should be noted that the key configuration mechanism of routers
   may restrict the possible keys that may be used between peers.  It is
   strongly recommended that an implementation be able to support, at
   minimum, a key composed of a string of printable ASCII of 80 bytes or
   less, as this is current practice.







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3.4.  Other Considerations

   Changes to the authentication mechanism described here (primarily: to
   add a Key-ID field such as that of OSPFv2 and RIPv2) were considered
   at some length, but ultimately were rejected.  The mechanism here was
   already widely implemented in 1999.  As of this writing, this
   mechanism is fairly widely deployed within the users interested in
   cryptographic authentication of IS-IS.  The improvement provided by
   the proposed revised mechanism was not large enough to justify the
   change, given the installed base and lack of operator interest in
   deploying a revised mechanism.

   If and when a key management protocol appears that is both widely
   implemented and easily deployed to secure routing protocols such as
   IS-IS, a different authentication mechanism that is designed for use
   with that key management schema could be added if desired.

3.5.  Future Directions

   If a stronger authentication were believed to be required, then the
   use of a full digital signature [RFC2154] 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.

   If and when additional authentication mechanisms are defined (for
   example, to provide a cryptographically stronger hash function), it
   will also be necessary to define mechanisms that allow graceful
   transition from the existing mechanisms (as defined in this document)
   to any future mechanism.

4.  IANA Considerations

   IANA has created a new codepoint registry to administer the
   Authentication Type codepoints for TLV 10.  This registry is part of
   the existing IS-IS codepoints registry as established by [RFC3563]
   and [RFC3359].  This registry is managed using the Designated Expert
   policy as described in [RFC5226] and is called "IS-IS Authentication
   Type Codes for TLV 10".

   The values in the "IS-IS Authentication Type Codes for TLV 10"
   registry should be recorded in decimal and should only be approved
   after a designated expert, appointed by the IESG area director, has
   been consulted.  The intention is that any allocation will be
   accompanied by a published RFC.  However, the designated expert can
   approve allocations once it seems clear that an RFC will be
   published, allowing for the allocation of values prior to the



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   document being approved for publication as an RFC.  New items should
   be documented in a publicly and freely available specification.  We
   should also allow external specifications to allocate and use the
   IS-IS Authentication Type Codes maintained by this registry.

   Initial values for the "IS-IS Authentication Type Codes for TLV 10"
   registry are given below; future assignments are to be made through
   Expert Review.  Assignments consist of an authentication type name
   and its associated value.

   +---------------------------------------------+-------+-------------+
   | Authentication Type Code                    | Value | Reference   |
   +---------------------------------------------+-------+-------------+
   | Reserved                                    | 0     | [ISO-10589] |
   | Cleartext Password                          | 1     | [ISO-10589] |
   | ISO 10589 Reserved                          | 2     | [ISO-10589] |
   | HMAC-MD5 Authentication                     | 54    | RFC 5304    |
   | Routeing Domain private authentication      | 255   | [ISO-10589] |
   | method                                      |       |             |
   +---------------------------------------------+-------+-------------+

5.  Acknowledgements

   The authors would like to thank (in alphabetical order) Stephen
   Farrell, Dave Katz, Steven Luong, Tony Przygienda, Nai-Ming Shen, and
   Henk Smit for their comments and suggestions on this document.

6.  References

6.1.  Normative References

   [ISO-10589]  ISO, "Intermediate System to Intermediate System intra-
                domain routeing information exchange protocol for use in
                conjunction with the protocol for providing the
                connectionless-mode network service (ISO 8473)",
                International Standard 10589:2002, Second Edition, 2002.

   [RFC2104]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
                Hashing for Message Authentication", RFC 2104,
                February 1997.

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.








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6.2.  Informative References

   [Bell06a]    Bellare, M., "New Proofs for NMAC and HMAC: Security
                without Collision-Resistance", Preliminary Version, in
                Proceedings of Crypto 2006, Lecture Notes in Computer
                Science, Vol. 4117, August 2006.

   [Bell06b]    Bellare, M., "New Proofs for NMAC and HMAC: Security
                without Collision-Resistance", August 2006, <http://
                www-cse.ucsd.edu/users/mihir/papers/hmac-new.html>.

   [DoS]        Voydock, V. and S. Kent, "Security Mechanisms in High-
                level Networks", ACM Computing Surveys Vol. 15, No. 2,
                June 1983.

   [Dobb96a]    Dobbertin, H., "Cryptanalysis of MD5 Compress",
                EuroCrypt Rump Session 1996, May 1996.

   [Dobb96b]    Dobbertin, H., "The Status of MD5 After a Recent
                Attack", CryptoBytes, Vol. 2, No. 2, 1996.

   [Dobb98]     Dobbertin, H., "Cryptanalysis of MD4", Journal of
                Cryptology, Vol. 11, No. 4, 1998.

   [FLN07]      Fouque, P., Leurent, G., and P. Nguyen, "Full Key-
                Recovery Attacks on HMAC/NMAC-MD5 and NMAC-MD5",
                Proceedings of Crypto 2007, August 2007.

   [RFC1195]    Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
                dual environments", RFC 1195, December 1990.

   [RFC1704]    Haller, N. and R. Atkinson, "On Internet
                Authentication", RFC 1704, October 1994.

   [RFC2154]    Murphy, S., Badger, M., and B. Wellington, "OSPF with
                Digital Signatures", RFC 2154, June 1997.

   [RFC3359]    Przygienda, T., "Reserved Type, Length and Value (TLV)
                Codepoints in Intermediate System to Intermediate
                System", RFC 3359, August 2002.

   [RFC3563]    Zinin, A., "Cooperative Agreement Between the ISOC/IETF
                and ISO/IEC Joint Technical Committee 1/Sub Committee 6
                (JTC1/SC6) on IS-IS Routing Protocol Development",
                RFC 3563, July 2003.






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   [RFC3567]    Li, T. and R. Atkinson, "Intermediate System to
                Intermediate System (IS-IS) Cryptographic
                Authentication", RFC 3567, July 2003.

   [RFC4593]    Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
                Routing Protocols", RFC 4593, October 2006.

   [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                May 2008.

   [WY05]       Wang, X. and H. Yu, "How to Break MD5 and Other Hash
                Functions", Proceedings of EuroCrypt 2005, Lecture Notes
                in Computer Science, Vol. 3494, 2005.

Authors' Addresses

   Tony Li
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA  95134
   USA

   Phone: +1 408 750 5160
   EMail: tony.li@tony.li


   R. Atkinson
   Extreme Networks, Inc.
   3585 Monroe St.
   Santa Clara, CA  95051
   USA

   Phone: +1 408 579 2800
   EMail: rja@extremenetworks.com
















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