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Versions: 00 01 02 03 04 05 06 07 RFC 5709

Internet Draft                                                M. Bhatia
<draft-ietf-ospf-hmac-sha-04.txt>                        Alcatel-Lucent
Category: Standards-Track                                     V. Manral
Expires 06 Nov 2009                                         IP Infusion
Updates: RFC 2328                                              M. Fanto
                                                    Aegis Data Security
                                                               R. White
                                                          Cisco Systems
                                                                  T. Li
                                                              M. Barnes
                                                          Cisco Systems
                                                            R. Atkinson
                                                       Extreme Networks

                                                             6 May 2009
              OSPFv2 HMAC-SHA Cryptographic Authentication

Status of this Memo

   Distribution of this memo is unlimited.

   This Internet-Draft is submitted to IETF in full conformance
   with the provisions of BCP 78 and BCP 79.

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   Internet-Drafts are working documents of the Internet Engineering
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   This document describes how the 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.


   A variety of risks exist when depoying 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. [RFC 2328]

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

   It is believed that [RFC 2104] is mathematically identical to
   [FIPS-198] and also believed that algorithms in [RFC 4684] are
   mathematically identical to [FIPS-180-2].  It is believed that
   [RFC 3874] is mathematically identical to SHA-224 as specified
   in [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

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   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",
   and "OPTIONAL" in this document are to be interpreted as
   described in RFC 2119.  [RFC 2119]

2. Background

   All OSPF protocol exchanges are authenticated.  The OSPF packet
   header (see Section 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

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. [RFC 1704]

   The algorithms used to generate and verify the message digest
   are specified implicitly by the secret key. This specification
   discusses the computation of OSPF 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 D.

   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)

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        HMAC-SHA-1          (defined here)
        HMAC-SHA-224        (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:
        HMAC-SHA-1, HMAC-SHA-224, HMAC-SHA-384, & HMAC-SHA-512

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

   An implementation of this specification MUST allow network
   operators to configure ANY algorithm and mode supported
   by that implementation for use with ANY given Key-ID 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.[RFC

   First, following the procedure defined in RFC 2328, Appendix D,
   select the appropriate OSPFv2 Security Association for use with
   this packet and set the Key-ID 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

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   accordance with the procedures in RFC 2328, Appendix D
   applicable to the Cryptographic Authentication type.

   Fourth, The message digest is then calculated and appended to
   the OSPF packet, as described below in Section 3.2.  The KeyID,
   Authentication Algorithm, Algorithm Mode, and 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

   RFC 2328 defined an OSPFv2 Security Association (OSPFv2 SA) in
   Section D.3, pages 228 and 229.  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
             to be used.  THis information should never
             be sent over the wire in cleartext form.
             Currently valid values are:   MD5, SHA-1,
             SHA-224, SHA-256, SHA-384, and SHA-512.

   Authentication Mode
             This indicates the authentication mode to
             be used with this OSPFv2 SA.  For MD5,
             the only currently valid value is Keyed-Hash.
             For the SHA family of algorithms, the only
             currently valid value is HMAC.

   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 3.2 above.

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   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 selected OSPFv2 key.

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      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 always
        is 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 follows:
           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-224 is

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       28 bytes, for SHA-256 is 32 bytes, for SHA-384 is 48 bytes,
       and for SHA-512 is 64-bytes.

     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 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 used.

     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 Key-ID 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.

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   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 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 synactically valid, but contain invalid Authentication
   Data, thereby forcing the receiver(s) to perform expensive
   cryptographic computations to discover that the packets are

4. Security Considerations

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

   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. 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
   [RFC 4822] and IS-IS SHA [RFC 5310] and to minimise OSPF
   protocol processing changes in Section D.4.3 of RFC 2328.

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   [RFC 2328]

   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.
   [RFC 4086] provides guidance on methods for generating
   cryptographically random bits.

   This mechanism is vulnerable to a replay attack.  The
   cryptographic sequence numbers typically will be initialised
   to zero when a router first boots, typically will be
   initialised to zero when forming an adjacency with a new
   neighbor, and might be reset to zero when a router reboots
   (e.g. if the router does not store the sequence information
   in non-volatile storage).  This means that anyone on the link
   might copy and replay packets with a sequence number of zero.
   However, receivers can mitigate this attack by briefly storing
   messages with an unexpected sequence number and listening
   to see if the legitimate OSPF neighbor sends an OSPF packet
   with an expected sequence number.  If such a legitimate OSPF
   packet arrives during the period the suspect OSPF packet is
   being stored, then the suspect OSPF packet should be dropped
   without being processed and a security fault ought to be
   logged noting that a replayed OSPF packet was received.

   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, 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

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   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 systems deployed by a
   given network operator also supported using the IP
   Authentication Header to protect OSPFv2, then such a network
   operator might consider using the IP Authentication Header
   in lieu of this mechanism.

   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.  Use of full
   digital signatures would enable precise authentication of the
   OSPF router originating each OSPF link-state advertisement.


   There are no IANA considerations for this document.


   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 [RFC 4822].

   Acee Lindem provided feedback on earlier versions of this
   document, which feedback has greatly improved the readability
   of the current draft.  He also pointed out the replay attack
   mitigation described above.

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


7.1 Normative References

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   [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.

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

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

7.2 Informative References

   [Bell89] S. Bellovin, "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.

   [Lindem]  Lindem, A, Private Communication, October 2008.

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

   [RFC 2104] Krawczyk, H. et alia, "HMAC: Keyed-Hashing
           for Message Authentication", RFC 2104,
           February 1997.

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

   [RFC 3874] R. Housley, "A 224-bit One-way Hash Function:
           SHA-224", RFC 3874, September 2004.

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

   [RFC 4684] Eastlake 3rd, D., & T. Hansen, "US Secure Hash
              Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006.

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

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

   [RR07]   Rechberger, Christian & Vincent 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 2007.

   [RR08]   Rechberger, Christian & Vincent 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.


   Manav Bhatia

   EMail: manav@alcatel-lucent.com

   Vishwas Manral
   IP Infusion
   Almora, Uttarakhand

   EMail: vishwas@ipinfusion.com

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   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

   Tony Li
   300 Holger Way
   San Jose, CA
   95134  USA

   Email: tony.li@tony.li

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

   Email: mjbarnes@cisco.com

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

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

   Expires: 6 NOV 2009

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