faqs.org - Internet FAQ Archives

RFC 5709 - OSPFv2 HMAC-SHA Cryptographic Authentication

Or Display the document by number

Network Working Group                                          M. Bhatia
Request for Comments: 5709                                Alcatel-Lucent
Updates: 2328                                                  V. Manral
Category: Standards Track                                    IP Infusion
                                                                M. Fanto
                                                     Aegis Data Security
                                                                R. White
                                                               M. Barnes
                                                           Cisco Systems
                                                                   T. Li
                                                             R. Atkinson
                                                        Extreme Networks
                                                            October 2009

              OSPFv2 HMAC-SHA Cryptographic Authentication


   This document describes how the National Institute of Standards and
   Technology (NIST) Secure Hash Standard family of algorithms can be
   used with OSPF version 2's built-in, cryptographic authentication
   mechanism.  This updates, but does not supercede, the cryptographic
   authentication mechanism specified in RFC 2328.

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.

Copyright and License Notice

   Copyright (c) 2009 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
   (http://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
   described in the BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

1.  Introduction

   A variety of risks exist when deploying any routing protocol
   [Bell89].  This document provides an update to OSPFv2 Cryptographic
   Authentication, which is specified in Appendix D of RFC 2328.  This
   document does not deprecate or supercede RFC 2328.  OSPFv2, itself,
   is defined in RFC 2328 [RFC2328].

   This document adds support for Secure Hash Algorithms (SHA) defined
   in the US NIST Secure Hash Standard (SHS), which is defined by NIST
   FIPS 180-2.  [FIPS-180-2] includes SHA-1, SHA-224, SHA-256, SHA-384,
   and SHA-512.  The Hashed Message Authentication Code (HMAC)
   authentication mode defined in NIST FIPS 198 is used [FIPS-198].

   It is believed that [RFC2104] is mathematically identical to
   [FIPS-198] and it is also believed that algorithms in [RFC4634] are
   mathematically identical to [FIPS-180-2].

   The creation of this addition to OSPFv2 was driven by operator
   requests that they be able to use the NIST SHS family of algorithms
   in the NIST HMAC mode, instead of being forced to use the Keyed-MD5
   algorithm and mode with OSPFv2 Cryptographic Authentication.
   Cryptographic matters are discussed in more detail in the Security
   Considerations section of this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Background

   All OSPF protocol exchanges can be authenticated.  The OSPF packet
   header (see Appendix A.3.1 of RFC 2328) includes an Authentication
   Type field and 64 bits of data for use by the appropriate
   authentication scheme (determined by the Type field).

   The authentication type is configurable on a per-interface (or,
   equivalently, on a per-network/subnet) basis.  Additional
   authentication data is also configurable on a per-interface basis.

   OSPF authentication types 0, 1, and 2 are defined by RFC 2328.  This
   document provides an update to RFC 2328 that is only applicable to
   Authentication Type 2, "Cryptographic Authentication".

3.  Cryptographic Authentication with NIST SHS in HMAC Mode

   Using this authentication type, a shared secret key is configured in
   all routers attached to a common network/subnet.  For each OSPF
   protocol packet, the key is used to generate/verify a "message
   digest" that is appended to the end of the OSPF packet.  The message
   digest is a one-way function of the OSPF protocol packet and the
   secret key.  Since the secret key is never sent over the network in
   the clear, protection is provided against passive attacks [RFC1704].

   The algorithms used to generate and verify the message digest are
   specified implicitly by the secret key.  This specification discusses
   the computation of OSPFv2 Cryptographic Authentication data when any
   of the NIST SHS family of algorithms is used in the Hashed Message
   Authentication Code (HMAC) mode.  Please also see RFC 2328, Appendix

   With the additions in this document, the currently valid algorithms
   (including mode) for OSPFv2 Cryptographic Authentication include:

           Keyed-MD5               (defined in RFC 2328, Appendix D)
           HMAC-SHA-1              (defined here)
           HMAC-SHA-256            (defined here)
           HMAC-SHA-384            (defined here)
           HMAC-SHA-512            (defined here)

   Of the above, implementations of this specification MUST include
   support for at least:


   and SHOULD include support for:


   and SHOULD also (for backwards compatibility with existing
   implementations and deployments) include support for:


   and MAY also include support for:


   An implementation of this specification MUST allow network operators
   to configure ANY authentication algorithm supported by that
   implementation for use with ANY given KeyID value that is configured
   into that OSPFv2 router.

3.1.  Generating Cryptographic Authentication

   The overall cryptographic authentication process defined in Appendix
   D of RFC 2328 remains unchanged.  However, the specific cryptographic
   details (i.e., SHA rather than MD5, HMAC rather than Keyed-Hash) are
   defined herein.  To reduce the potential for confusion, this section
   minimises the repetition of text from RFC 2328, Appendix D, which is
   incorporated here by reference [RFC2328].

   First, following the procedure defined in RFC 2328, Appendix D,
   select the appropriate OSPFv2 Security Association for use with this
   packet and set the KeyID field to the KeyID value of that OSPFv2
   Security Association.

   Second, set the Authentication Type to Cryptographic Authentication,
   and set the Authentication Data Length field to the length (measured
   in bytes, not bits) of the cryptographic hash that will be used.
   When any NIST SHS algorithm is used in HMAC mode with OSPFv2
   Cryptographic Authentication, the Authentication Data Length is equal
   to the normal hash output length (measured in bytes) for the specific
   NIST SHS algorithm in use.  For example, with NIST SHA-256, the
   Authentication Data Length is 32 bytes.

   Third, the 32-bit cryptographic sequence number is set in accordance
   with the procedures in RFC 2328, Appendix D that are applicable to
   the Cryptographic Authentication type.

   Fourth, the message digest is then calculated and appended to the
   OSPF packet, as described below in Section 3.3.  The KeyID,
   Authentication Algorithm, and Authentication Key to be used for
   calculating the digest are all components of the selected OSPFv2
   Security Association.  Input to the authentication algorithm consists
   of the OSPF packet and the secret key.

3.2.  OSPFv2 Security Association

   This document uses the term OSPFv2 Security Association (OSPFv2 SA)
   to refer to the authentication key information defined in Section D.3
   of RFC 2328.  The OSPFv2 protocol does not include an in-band
   mechanism to create or manage OSPFv2 Security Associations.  The
   parameters of an OSPFv2 Security Association are updated to be:

   Key Identifier (KeyID)
      This is an 8-bit unsigned value used to uniquely identify an
      OSPFv2 SA and is configured either by the router administrator
      (or, in the future, possibly by some key management protocol
      specified by the IETF).  The receiver uses this to locate the
      appropriate OSPFv2 SA to use.  The sender puts this KeyID value in
      the OSPF packet based on the active OSPF configuration.

   Authentication Algorithm
      This indicates the authentication algorithm (and also the
      cryptographic mode, such as HMAC) to be used.  This information
      SHOULD never be sent over the wire in cleartext form.  At present,
      valid values are Keyed-MD5, HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-
      384, and HMAC-SHA-512.

   Authentication Key
      This is the cryptographic key used for cryptographic
      authentication with this OSPFv2 SA.  This value SHOULD never be
      sent over the wire in cleartext form.  This is noted as "K" in
      Section 3.3 below.

   Key Start Accept
      The time that this OSPF router will accept packets that have been
      created with this OSPF Security Association.

   Key Start Generate
      The time that this OSPF router will begin using this OSPF Security
      Association for OSPF packet generation.

   Key Stop Generate
      The time that this OSPF router will stop using this OSPF Security
      Association for OSPF packet generation.

   Key Stop Accept
      The time that this OSPF router will stop accepting packets
      generated with this OSPF Security Association.

   In order to achieve smooth key transition, KeyStartAccept SHOULD be
   less than KeyStartGenerate and KeyStopGenerate SHOULD be less than
   KeyStopAccept.  If KeyStopGenerate and KeyStopAccept are left

   unspecified, the key's lifetime is infinite.  When a new key replaces
   an old, the KeyStartGenerate time for the new key MUST be less than
   or equal to the KeyStopGenerate time of the old key.

   Key storage SHOULD persist across a system restart, warm or cold, to
   avoid operational issues.  In the event that the last key associated
   with an interface expires, it is unacceptable to revert to an
   unauthenticated condition, and not advisable to disrupt routing.
   Therefore, the router should send a "last Authentication Key
   expiration" notification to the network manager and treat the key as
   having an infinite lifetime until the lifetime is extended, the key
   is deleted by network management, or a new key is configured.

3.3.  Cryptographic Aspects

   This describes the computation of the Authentication Data value when
   any NIST SHS algorithm is used in the HMAC mode with OSPFv2
   Cryptographic Authentication.

   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 Authentication Key for the OSPFv2 security

      Ko   is the cryptographic key used with the hash algorithm.

      B    is the block size of H, measured in octets
           rather than bits.  Note well 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.

      Implementation note:
         This definition of Apad means that Apad is always the same
         length as the hash output.

       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.

       First, the OSPFv2 packet's Authentication Trailer (which is the
       appendage described in RFC 2328, Section D.4.3, Page 233, items
       (6)(a) and (6)(d)) is filled with the value Apad, and the
       Authentication Type field is set to 2.

       Then, a First-Hash, also known as the inner hash, is computed as

             First-Hash = H(Ko XOR Ipad || (OSPFv2 Packet))

       Implementation Notes:
          Note that the First-Hash above includes the Authentication
          Trailer containing the Apad value, as well as the OSPF packet,
          as per RFC 2328, Section D.4.3.

       The definition of Apad (above) ensures it is always the same
       length as the hash output.  This is consistent with RFC 2328.
       The "(OSPFv2 Packet)" mentioned in the First-Hash (above) does
       include the OSPF Authentication Trailer.

       The digest length for SHA-1 is 20 bytes; for SHA-256, 32 bytes;
       for SHA-384, 48 bytes; and for SHA-512, 64 bytes.

       Then a Second-Hash, also known as the outer hash, is computed as

             Second-Hash = H(Ko XOR Opad || First-Hash)

   (4) RESULT
       The resulting Second-Hash becomes the Authentication Data that is
       sent in the Authentication Trailer of the OSPFv2 packet.  The
       length of the Authentication Trailer is always identical to the
       message digest size of the specific hash function H that is being

       This also means that the use of hash functions with larger output
       sizes will also increase the size of the OSPFv2 packet as
       transmitted on the wire.

       Implementation Note:
          RFC 2328, Appendix D specifies that the Authentication Trailer
          is not counted in the OSPF packet's own Length field, but is
          included in the packet's IP Length field.

3.4.  Message Verification

   Message verification follows the procedure defined in RFC 2328,
   except that the cryptographic calculation of the message digest
   follows the procedure in Section 3.3 above when any NIST SHS
   algorithm in the HMAC mode is in use.  Kindly recall that the
   cryptographic algorithm/mode in use is indicated implicitly by the
   KeyID of the received OSPFv2 packet.

   Implementation Notes:
      One must save the received digest value before calculating the
      expected digest value, so that after that calculation the received
      value can be compared with the expected value to determine whether
      to accept that OSPF packet.

      RFC 2328, Section D.4.3 (6) (c) should be read very closely prior
      to implementing the above.  With SHA algorithms in HMAC mode, Apad
      is placed where the MD5 key would be put if Keyed-MD5 were in use.

3.5.  Changing OSPFv2 Security Associations

   Using KeyIDs makes changing the active OSPFv2 SA convenient.  An
   implementation can choose to associate a lifetime with each OSPFv2 SA
   and can thus automatically switch to a different OSPFv2 SA based on
   the lifetimes of the configured OSPFv2 SA(s).

   After changing the active OSPFv2 SA, the OSPF sender will use the
   (different) KeyID value associated with the newly active OSPFv2 SA.
   The receiver will use this new KeyID to select the appropriate (new)
   OSPFv2 SA to use with the received OSPF packet containing the new
   KeyID value.

   Because the KeyID field is present, the receiver does not need to try
   all configured OSPFv2 Security Associations with any received OSPFv2
   packet.  This can mitigate some of the risks of a Denial-of-Service
   (DoS) attack on the OSPF instance, but does not entirely prevent all
   conceivable DoS attacks.  For example, an on-link adversary still
   could generate OSPFv2 packets that are syntactically valid but that
   contain invalid Authentication Data, thereby forcing the receiver(s)
   to perform expensive cryptographic computations to discover that the
   packets are invalid.

4.  Security Considerations

   This document enhances the security of the OSPFv2 routing protocol by
   adding, to the existing OSPFv2 Cryptographic Authentication method,
   support for the algorithms defined in the NIST Secure Hash Standard
   (SHS) using the Hashed Message Authentication Code (HMAC) mode, and
   by adding support for the Hashed Message Authentication Code (HMAC)

   This provides several alternatives to the existing Keyed-MD5
   mechanism.  There are published concerns about the overall strength
   of the MD5 algorithm ([Dobb96a], [Dobb96b], [Wang04]).  While those
   published concerns apply to the use of MD5 in other modes (e.g., use
   of MD5 X.509v3/PKIX digital certificates), they are not an attack
   upon Keyed-MD5, which is what OSPFv2 specified in RFC 2328.  There
   are also published concerns about the SHA algorithm [Wang05] and also
   concerns about the MD5 and SHA algorithms in the HMAC mode ([RR07],
   [RR08]).  Separately, some organisations (e.g., the US government)
   prefer NIST algorithms, such as the SHA family, over other algorithms
   for local policy reasons.

   The value Apad is used here primarily for consistency with IETF
   specifications for HMAC-SHA authentication of RIPv2 SHA [RFC4822] and
   IS-IS SHA [RFC5310] and to minimise OSPF protocol processing changes
   in Section D.4.3 of RFC 2328 [RFC2328].

   The quality of the security provided by the Cryptographic
   Authentication option depends completely on the strength of the
   cryptographic algorithm and cryptographic mode in use, the strength
   of the key being used, and the correct implementation of the security
   mechanism in all communicating OSPF implementations.  Accordingly,
   the use of high assurance development methods is recommended.  It
   also requires that all parties maintain the secrecy of the shared
   secret key.  [RFC4086] provides guidance on methods for generating
   cryptographically random bits.

   This mechanism is vulnerable to a replay attack by any on-link node.
   An on-link node could record a legitimate OSPF packet sent on the
   link, then replay that packet at the next time the recorded OSPF
   packet's sequence number is valid.  This replay attack could cause
   significant routing disruptions within the OSPF domain.

   Ideally, for example, to prevent the preceding attack, each OSPF
   Security Association would be replaced by a new and different OSPF
   Security Association before any sequence number were reused.  As of
   the date this document was published, no form of automated key
   management has been standardised for OSPF.  So, as of the date this
   document was published, common operational practice has been to use
   the same OSPF Authentication Key for very long periods of time.  This
   operational practice is undesirable for many reasons.  Therefore, it
   is clearly desirable to develop and standardise some automated key-
   management mechanism for OSPF.

   Because all of the currently specified algorithms use symmetric
   cryptography, one cannot authenticate precisely which OSPF router
   sent a given packet.  However, one can authenticate that the sender
   knew the OSPF Security Association (including the OSPFv2 SA's
   parameters) currently in use.

   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 the routing system.

   The technology in this document enhances an authentication mechanism
   for OSPFv2.  The mechanism described here is not perfect and need not
   be perfect.  Instead, this mechanism represents a significant
   increase in the work function of an adversary attacking OSPFv2, as
   compared with plain-text authentication or null authentication, while
   not causing undue implementation, deployment, or operational
   complexity.  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 2328 and limits itself to adding support for
   additional cryptographic hash functions.  Also, some large network
   operators have indicated they prefer to retain the basic mechanism
   defined in RFC 2328, rather than migrate to IP Security, due to
   deployment and operational considerations.  If all the OSPFv2 routers
   supported IPsec, then IPsec tunnels could be used in lieu of this
   mechanism [RFC4301].  This would, however, relegate the topology to
   point-to-point adjacencies over the mesh of IPsec tunnels.

   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.  Use of full digital signatures would
   enable precise authentication of the OSPF router originating each
   OSPF link-state advertisement, and thereby provide much stronger
   integrity protection for the OSPF routing domain.

5.  IANA Considerations

   The OSPF Authentication Codes registry entry for Cryptographic
   Authentication (Registry Code 2) has been updated to refer to this
   document as well as to RFC 2328.

6.  Acknowledgements

   The authors would like to thank Bill Burr, Tim Polk, John Kelsey, and
   Morris Dworkin of (US) NIST for review of portions of this document
   that are directly derived from the closely related work on RIPv2
   Cryptographic Authentication [RFC4822].

   David Black, Nevil Brownlee, Acee Lindem, and Hilarie Orman (in
   alphabetical order by last name) provided feedback on earlier
   versions of this document.  That feedback has greatly improved both
   the technical content and the readability of the current document.

   Henrik Levkowetz's Internet Draft tools were very helpful in
   preparing this document and are much appreciated.

7.  References

7.1.  Normative References

   [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.

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

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

7.2.  Informative References

   [Bell89]     Bellovin, S., "Security Problems in the TCP/IP Protocol
                Suite", ACM Computer Communications Review, Volume 19,
                Number 2, pp. 32-48, April 1989.

   [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.

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

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

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

   [RFC4086]    Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                "Randomness Requirements for Security", BCP 106, RFC
                4086, June 2005.

   [RFC4301]    Kent, S. and K. Seo, "Security Architecture for the
                Internet Protocol", RFC 4301, December 2005.

   [RFC4634]    Eastlake 3rd, D. and T. Hansen, "US Secure Hash
                Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006.

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

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

   [RR07]       Rechberger, C. and V. Rijmen, "On Authentication with
                HMAC and Non-random Properties", Financial Cryptography
                and Data Security, Lecture Notes in Computer Science,
                Volume 4886/2008, Springer-Verlag, Berlin, December

   [RR08]       Rechberger, C. and V. Rijmen, "New Results on NMAC/HMAC
                when Instantiated with Popular Hash Functions", Journal
                of Universal Computer Science, Volume 14, Number 3, pp.
                347-376, 1 February 2008.

   [VK83]       Voydock, V. and S. Kent, "Security Mechanisms in High-
                level Networks", ACM Computing Surveys, Vol. 15, No. 2,
                June 1983.

   [Wang04]     Wang, X., et alia, "Collisions for Hash Functions MD4,
                MD5, HAVAL-128, and RIPEMD", August 2004, IACR,

   [Wang05]     Wang, X., et alia, "Finding Collisions in the Full SHA-
                1" Proceedings of Crypto 2005, Lecture Notes in Computer
                Science, Volume 3621, pp. 17-36, Springer-Verlag,
                Berlin, August 31, 2005.

Authors' Addresses

   Manav Bhatia

   EMail: manav.bhatia@alcatel-lucent.com

   Vishwas Manral
   IP Infusion
   Almora, Uttarakhand

   EMail: vishwas@ipinfusion.com

   Matthew J. Fanto
   Aegis Data Security
   Dearborn, MI

   EMail: mfanto@aegisdatasecurity.com

   Russ I. White
   Cisco Systems
   7025 Kit Creek Road
   P.O. Box 14987
   RTP, NC
   27709 USA

   EMail: riw@cisco.com

   M. Barnes
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA
   95134  USA

   EMail: mjbarnes@cisco.com

   Tony Li
   300 Holger Way
   San Jose, CA
   95134  USA

   EMail: tony.li@tony.li

   Randall J. Atkinson
   Extreme Networks
   3585 Monroe Street
   Santa Clara, CA
   95051  USA

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


User Contributions:

Comment about this RFC, ask questions, or add new information about this topic: