Internet Engineering Task Force (IETF)                         M. Bhatia
Request for Comments: 6094                                Alcatel-Lucent
Category: Informational                                        V. Manral
ISSN: 2070-1721                                              IP Infusion
                                                           February 2011


    Summary of Cryptographic Authentication Algorithm Implementation
                   Requirements for Routing Protocols

Abstract

   The routing protocols Open Shortest Path First version 2 (OSPFv2),
   Intermediate System to Intermediate System (IS-IS), and Routing
   Information Protocol (RIP) currently define cleartext and MD5
   (Message Digest 5) methods for authenticating protocol packets.
   Recently, effort has been made to add support for the SHA (Secure
   Hash Algorithm) family of hash functions for the purpose of
   authenticating routing protocol packets for RIP, IS-IS, and OSPF.

   To encourage interoperability between disparate implementations, it
   is imperative that we specify the expected minimal set of algorithms,
   thereby ensuring that there is at least one algorithm that all
   implementations will have in common.

   Similarly, RIP for IPv6 (RIPng) and OSPFv3 support IPsec algorithms
   for authenticating their protocol packets.

   This document examines the current set of available algorithms, with
   interoperability and effective cryptographic authentication
   protection being the principal considerations.  Cryptographic
   authentication of these routing protocols requires the availability
   of the same algorithms in disparate implementations.  It is desirable
   that newly specified algorithms should be implemented and available
   in routing protocol implementations because they may be promoted to
   requirements at some future time.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6094.

Copyright Notice

   Copyright (c) 2011 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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Intermediate System to Intermediate System (IS-IS) ..............4
      2.1. Authentication Scheme Selection ............................4
      2.2. Authentication Algorithm Selection .........................5
   3. Open Shortest Path First Version 2 (OSPFv2) .....................5
      3.1. Authentication Scheme Selection ............................6
      3.2. Authentication Algorithm Selection .........................6
   4. Open Shortest Path First Version 3 (OSPFv3) .....................7
   5. Routing Information Protocol Version 2 (RIPv2) ..................7
      5.1. Authentication Scheme Selection ............................7
      5.2. Authentication Algorithm Selection .........................8
   6. Routing Information Protocol for IPv6 (RIPng) ...................8
   7. Security Considerations .........................................9
   8. Acknowledgements ................................................9
   9. References .....................................................10
      9.1. Normative References ......................................10
      9.2. Informative References ....................................10

1.  Introduction

   Most routing protocols include three different types of
   authentication schemes: Null authentication, cleartext password, and
   cryptographic authentication.  Null authentication is equivalent to
   having no authentication scheme at all.

   In a cleartext scheme, also known as a "simple password" scheme, the
   password is exchanged completely unprotected, and anyone with
   physical access to the network can learn the password and compromise
   the integrity of the routing domain.  The simple password scheme
   protects against accidental establishment of routing sessions in a
   given domain, but beyond that it offers no additional protection.

   In a cryptographic authentication scheme, routers share a secret key
   that is used to generate a message authentication code for each of
   the protocol packets.  Today, routing protocols that implement
   message authentication codes often use a Keyed-MD5 [RFC1321] digest.
   The recent escalating series of attacks on MD5 raise concerns about
   its remaining useful lifetime.

   These attacks may not necessarily result in direct vulnerabilities
   for Keyed-MD5 digests as message authentication codes because the
   colliding message may not correspond to a syntactically correct
   protocol packet.  The known collision, pre-image, and second
   pre-image attacks [RFC4270] on MD5 may not increase the effectiveness
   of the key recovery attacks on HMAC-MD5.  Regardless, there is a need
   felt to deprecate MD5 [RFC1321] as the basis for the Hashed Message
   Authentication Code (HMAC) algorithm in favor of stronger digest
   algorithms.

   In light of these considerations, there are proposals to replace
   HMAC-MD5 with keyed HMAC-SHA [SHS] digests where HMAC-MD5 is
   currently mandated in RIPv2 [RFC2453] IS-IS [ISO] [RFC1195], and
   Keyed-MD5 in OSPFv2 [RFC2328].

   OSPFv3 [RFC5340] and RIPng [RFC2080] rely on the IPv6 Authentication
   Header (AH) [RFC4302] and IPv6 Encapsulating Security Payload (ESP)
   [RFC4303] in order to provide integrity, authentication, and/or
   confidentiality.

   However, the nature of cryptography is that algorithmic improvement
   is an ongoing process, as is the exploration and refinement of attack
   vectors.  An algorithm believed to be strong today may be
   demonstrated to be weak tomorrow.  Given this, the choice of
   preferred algorithm should favor the minimization of the likelihood
   of it being compromised quickly.

   It should be recognized that preferred algorithm(s) will change over
   time to adapt to the evolving threats.  At any particular time, the
   mandatory-to-implement algorithm(s) might not be specified in the
   base protocol specification.  As protocols are extended, the
   preference for presently stronger algorithms presents a problem
   regarding the question of interoperability of existing and future
   implementations with respect to standards, and also regarding
   operational preference for the configuration as deployed.

   It is expected that an implementation should support the changing of
   security (authentication) keys.  Changing the symmetric key used in
   any HMAC algorithm on a periodic basis is good security practice.
   Operators need to plan for this.

   Implementations can support in-service key change so that no control
   packets are lost.  During an in-service/in-band key change, more than
   one key can be active for receiving packets for a session.  Some
   protocols support a key identifier that allows the two peers of a
   session to have multiple keys simultaneously for a session.

   However, these protocols currently manage keys manually (i.e., via
   operator intervention) or dynamically based on some timer or security
   protocol.

2.  Intermediate System to Intermediate System (IS-IS)

   The IS-IS specification allows for authentication of its Protocol
   Data Units (PDUs) via the authentication TLV (TLV 10) in the PDU.
   The base specification [ISO] had provisions only for cleartext
   passwords.  [RFC5304] extends the authentication capabilities by
   providing cryptographic authentication for IS-IS PDUs.  It mandates
   support for HMAC-MD5.

   [RFC5310] adds support for the use of any cryptographic hash function
   for authenticating IS-IS PDUs.  In addition to this, [RFC5310] also
   details how IS-IS can use the HMAC construct along with the Secure
   Hash Algorithm (SHA) family of cryptographic hash functions to secure
   IS-IS PDUs.

2.1.  Authentication Scheme Selection

   In order for IS-IS implementations to securely interoperate, they
   must support one or more authentication schemes in common.  This
   section specifies the preference for standards-conformant IS-IS
   implementations that use accepted authentication schemes.

   The earliest interoperability requirement for authentication as
   stated by [ISO] [RFC1195] required all implementations to support a

   cleartext password.  This authentication scheme's utility is limited
   to precluding the accidental introduction of a new IS into a
   broadcast domain.  Operators should not use this scheme, as it
   provides no protection against an attacker with access to the
   broadcast domain: anyone can determine the secret password through
   inspection of the PDU.  This mechanism does not provide any
   significant level of security and should be avoided.

   [RFC5304] defined the cryptographic authentication scheme for IS-IS.
   HMAC-MD5 was the only algorithm specified; hence, it is mandated.
   [RFC5310] defined a generic cryptographic scheme and added support
   for additional algorithms.  Implementations should support [RFC5310],
   as it defines the generic cryptographic authentication scheme.

2.2.  Authentication Algorithm Selection

   For IS-IS implementations to securely interoperate, they must have
   support for one or more authentication algorithms in common.

   This section details the authentication algorithm requirements for
   standards-conformant IS-IS implementations.

   The following are the available options for authentication
   algorithms:

   o  [RFC5304] mandates the use of HMAC-MD5.

   o  [RFC5310] does not require a particular algorithm but instead
      supports any digest algorithm (i.e., cryptographic hash
      functions).

   As noted earlier, there is a desire to deprecate MD5.  IS-IS
   implementations will likely migrate to an authentication scheme
   supported by [RFC5310], because it is algorithm agnostic.  Possible
   digest algorithms include SHA-1, SHA-224, SHA-256, SHA-384, and
   SHA-512.  Picking at least one mandatory-to-implement algorithm is
   imperative to ensuring interoperability.

3.  Open Shortest Path First Version 2 (OSPFv2)

   [RFC2328] includes three different types of authentication schemes:
   Null authentication, cleartext password (defined as "simple password"
   in [RFC2328]), and cryptographic authentication.  Null authentication
   is semantically equivalent to no authentication.

   In the cryptographic authentication scheme, the OSPFv2 routers on a
   common network/subnet are configured with a shared secret that is
   used to generate a Keyed-MD5 digest for each packet.  A monotonically

   increasing sequence number scheme is used to protect against replay
   attacks.

   [RFC5709] adds support for the use of the SHA family of hash
   algorithms for authentication of OSPFv2 packets.

3.1.  Authentication Scheme Selection

   For OSPF implementations to securely interoperate, they must have one
   or more authentication schemes in common.

   While all implementations will have Null authentication since it's
   mandated by [RFC2328], its use is not appropriate in any context
   where the operator wishes to authenticate OSPFv2 packets in their
   network.

   While all implementations will also support a cleartext password
   since it's mandated by [RFC2328], its use is only appropriate when
   the operator wants to preclude the accidental introduction of a
   router into the domain.  This scheme is patently not useful when an
   operator wants to authenticate the OSPFv2 packets.

   Cryptographic authentication is a mandatory scheme defined in
   [RFC2328], and all conformant implementations must support this.

3.2.  Authentication Algorithm Selection

   For OSPFv2 implementations to securely interoperate, they must
   support one or more cryptographic authentication algorithms in
   common.

   The following are the available options for authentication
   algorithms:

   o  [RFC2328] specifies the use of Keyed-MD5.

   o  [RFC5709] specifies the use of HMAC-SHA-1, HMAC-SHA-256,
      HMAC-SHA-384, and HMAC-SHA-512, and also mandates support for
      HMAC-SHA-256 (HMAC-SHA-1 is optional).

   As noted earlier, there is a desire to deprecate MD5.  Some
   alternatives for MD5 are listed in [RFC5709].

   Possible digest algorithms include SHA-1, SHA-256, SHA-384, and
   SHA-512.  Picking one mandatory-to-implement algorithm is imperative
   to ensuring interoperability.

4.  Open Shortest Path First Version 3 (OSPFv3)

   OSPFv3 [RFC5340] relies on the IPv6 Authentication Header (AH)
   [RFC4302] and IPv6 Encapsulating Security Payload (ESP) [RFC4303] in
   order to provide integrity, authentication, and/or confidentiality.

   [RFC4552] mandates the use of ESP for authenticating OSPFv3 packets.
   The implementations could also provide support for using AH to
   protect these packets.

   The algorithm requirements for AH and ESP are described in [RFC4835]
   as follows:

   o  [RFC2404] mandates HMAC-SHA-1-96.

   o  [RFC3566] indicates AES-XCBC-MAC-96 as a "should", but it's likely
      that this will be mandated at some future time.

5.  Routing Information Protocol Version 2 (RIPv2)

   RIPv2, originally specified in [RFC1388] and then in [RFC1723], has
   been updated and published as STD 56, [RFC2453].  If the Address
   Family Identifier of the first (and only the first) entry in the
   RIPv2 message is 0xFFFF, then the remainder of the entry contains the
   authentication information.  The [RFC2453] version of the protocol
   provides for authenticating packets using a cleartext password
   (defined as "simple password" in [RFC2453]) not more than 16 octets
   in length.

   [RFC2082] added support for Keyed-MD5 authentication, where a digest
   is appended to the end of the RIP packet.  [RFC4822] obsoleted
   [RFC2082] and added the SHA family of hash algorithms to the list of
   cryptographic authentications that can be used to protect RIPv2,
   whereas [RFC2082] previously specified only the use of Keyed-MD5.

5.1.  Authentication Scheme Selection

   For RIPv2 implementations to securely interoperate, they must support
   one or more authentication schemes in common.

   While all implementations will support a cleartext password since
   it's mandated by [RFC2453], its use is only appropriate when the
   operator wants to preclude the accidental introduction of a router
   into the domain.  This scheme is patently not useful when an operator
   wants to authenticate the RIPv2 packets.

   [RFC2082] mandates the use of an authentication scheme that uses
   Keyed-MD5.  However, [RFC2082] has been obsoleted by [RFC4822].
   Compliant implementations must provide support for an authentication
   scheme that uses Keyed-MD5 but should recognize that this is
   superseded by cryptographic authentication as defined in [RFC4822].

   Implementations should provide support for [RFC4822], as it specifies
   the RIPv2 cryptographic authentication schemes.

5.2.  Authentication Algorithm Selection

   For RIPv2 implementations to securely interoperate, they must support
   one or more authentication algorithms in common.

   The following are the available options for authentication
   algorithms:

   o  [RFC2082] specifies the use of Keyed-MD5.

   o  [RFC4822] specifies the use of Keyed-MD5, HMAC-SHA-1,
      HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512.

   As noted earlier, there is a desire to deprecate MD5.  Some
   alternatives for MD5 are listed in [RFC4822].  Possible digest
   algorithms include SHA-1, SHA-256, SHA-384, and SHA-512.  Picking one
   mandatory-to-implement algorithm is imperative to ensuring
   interoperability.

6.  Routing Information Protocol for IPv6 (RIPng)

   RIPng [RFC2080] relies on the IPv6 Authentication Header (AH)
   [RFC4302] and IPv6 Encapsulating Security Payload (ESP) [RFC4303] in
   order to provide integrity, authentication, and/or confidentiality.

   The algorithm requirements for AH and ESP are described in [RFC4835]
   as follows:

   o  [RFC2404] mandates HMAC-SHA-1-96.

   o  [RFC3566] indicates AES-XCBC-MAC-96 as a "should", but it's likely
      that this will be mandated at some future time.

7.  Security Considerations

   The cryptographic mechanisms referenced in this document provide only
   authentication algorithms.  These algorithms do not provide
   confidentiality.  Encrypting the content of the packet and thereby
   providing confidentiality is not considered in the definition of the
   routing protocols.

   The cryptographic strength of the HMAC depends upon the cryptographic
   strength of the underlying hash function and on the size and quality
   of the key.  The feasibility of attacking the integrity of routing
   protocol messages protected by keyed digests may be significantly
   more limited than that of other data; however, preference for one
   family of algorithms over another may also change independently of
   the perceived risk to a particular protocol.

   To ensure greater security, the keys used should be changed
   periodically, and implementations must be able to store and use more
   than one key at the same time.  Operational experience suggests that
   the lack of periodic rekeying is a source of significant exposure and
   that the lifespan of shared keys in the network is frequently
   measured in years.

   While simple password schemes are well represented in the document
   series and in conformant implementations of the protocols, the
   inability to offer either integrity or identity protection are
   sufficient reason to strongly discourage their use.

   This document concerns itself with the selection of cryptographic
   algorithms for use in the authentication of routing protocol packets
   being exchanged between adjacent routing processes.  The
   cryptographic algorithms identified in this document are not known to
   be broken at the current time, and ongoing cryptographic research so
   far leads us to believe that they will likely remain secure in the
   foreseeable future.  We expect that new revisions of this document
   will be issued in the future to reflect current thinking on the
   algorithms that various routing protocols should employ to ensure
   interoperability between disparate implementations.

8.  Acknowledgements

   We would like to thank Joel Jaeggli, Sean Turner, and Adrian Farrel
   for their comments and feedback on this document, which resulted in
   significant improvement of the same.

9.  References

9.1.  Normative References

   [ISO]       "Intermediate system to Intermediate system routing
               information exchange protocol for use in conjunction with
               the protocol for providing the connectionless-mode
               network service", ISO/IEC 10589:1992 (ISO 8473).

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

   [RFC2080]   Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
               January 1997.

   [RFC2328]   Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC2453]   Malkin, G., "RIP Version 2", STD 56, RFC 2453,
               November 1998.

   [RFC4822]   Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
               Authentication", RFC 4822, February 2007.

   [RFC4835]   Manral, V., "Cryptographic Algorithm Implementation
               Requirements for Encapsulating Security Payload (ESP) and
               Authentication Header (AH)", RFC 4835, April 2007.

   [RFC5304]   Li, T. and R. Atkinson, "IS-IS Cryptographic
               Authentication", RFC 5304, October 2008.

   [RFC5310]   Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
               and M. Fanto, "IS-IS Generic Cryptographic
               Authentication", RFC 5310, February 2009.

   [RFC5340]   Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
               for IPv6", RFC 5340, July 2008.

   [RFC5709]   Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
               Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
               Authentication", RFC 5709, October 2009.

9.2.  Informative References

   [RFC1321]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
               April 1992.

   [RFC1388]   Malkin, G., "RIP Version 2 Carrying Additional
               Information", RFC 1388, January 1993.

   [RFC1723]   Malkin, G., "RIP Version 2 - Carrying Additional
               Information", RFC 1723, November 1994.

   [RFC2082]   Baker, F. and R. Atkinson, "RIP-2 MD5 Authentication",
               RFC 2082, January 1997.

   [RFC2404]   Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
               ESP and AH", RFC 2404, November 1998.

   [RFC3566]   Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96
               Algorithm and Its Use With IPsec", RFC 3566,
               September 2003.

   [RFC4270]   Hoffman, P. and B. Schneier, "Attacks on Cryptographic
               Hashes in Internet Protocols", RFC 4270, November 2005.

   [RFC4302]   Kent, S., "IP Authentication Header", RFC 4302,
               December 2005.

   [RFC4303]   Kent, S., "IP Encapsulating Security Payload (ESP)",
               RFC 4303, December 2005.

   [RFC4552]   Gupta, M. and N. Melam, "Authentication/Confidentiality
               for OSPFv3", RFC 4552, June 2006.

   [SHS]       "Secure Hash Standard (SHS)", National Institute of
               Standards and Technology (NIST) FIPS Publication 180-3,
               October 2008.

Authors' Addresses

   Manav Bhatia
   Alcatel-Lucent
   Bangalore
   India

   EMail: manav.bhatia@alcatel-lucent.com


   Vishwas Manral
   IP Infusion
   1188 E. Arques Ave.
   Sunnyvale, CA  94089
   USA

   Phone: 408-400-1900
   EMail: vishwas@ipinfusion.com