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     DRAFT                   OSPF MD5 Authentication             October 1994
                             OSPF MD5 Authentication
                           Fri Oct 14 09:40:36 PDT 1994
                                    Fred Baker
                         Advanced Computer Communications
                                 Randall Atkinson
                         Information Technology Division
                            Naval Research Laboratory
                               Status of this Memo
     This document is an Internet Draft.  Internet Drafts are working
     documents of the Internet Engineering Task Force (IETF), its Areas, and
     its Working Groups.  Note that other groups may also distribute working
     documents as Internet Drafts.
     Internet Drafts are valid for a maximum of six months and may be
     updated, replaced, or obsoleted by other documents at any time.  It is
     inappropriate to use Internet Drafts as reference material or to cite
     them other than as a "work in progress".
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     1.  Introduction
     Growth in the Internet has made us aware of the need for improved
     authentication of routing information.  OSPF provides two authentication
     mechanisms for use in an area: "No Authentication" and "Simple
     Password".  Both are vulnerable to passive attacks currently widespread
     in the Internet.  Well-understood security issues exist in routing
     protocols [4].  Clear text passwords, currently specified for use with
     OSPF, are no longer considered sufficient [5].
     If authentication is disabled, then only simple misconfigurations are
     detected.  Simple passwords transmitted in the clear will further
     protect against the honest neighbor, but are useless in the general
     case.  By simply capturing information on the wire - straightforward
     even in a remote environment - a hostile process can learn the password
     and overcome the network.
     We propose that OSPF use an authentication algorithm, as in SNMP Version
     2, augmented by a sequence number.  MD5 is proposed as the standard
     authentication algorithm for OSPF, but the mechanism is intended to be
     algorithm-independent.  While this mechanism is not unbreakable (no
     known mechanism is), it provides a greatly enhanced probability that a
     system being attacked will detect and ignore hostile messages.  This is
     because we transmit the output of an authentication algorithm (e.g.,
     MD5) rather than the secret OSPF Authentication Key.  This output is a
     one-way function of a message and a secret OSPF Authentication Key.
     This OSPF Authentication Key is never sent over the network in the
     clear, thus providing protection against the passive attacks now
     commonplace in the Internet.
     In this way, protection is afforded against forgery or message
     modification.  It is possible to replay a message until the sequence
     number changes, but the sequence number makes replay in the long term
     less of an issue.  The mechanism does not afford confidentiality, since
     messages stay in the clear; however, the mechanism is also exportable
     from most countries, which test a privacy algorithm would fail.
     Other relevant rationales for the approach are that MD5 is used in SNMP
     Version 2, and is therefore present in routers already, as is some form
     of password management.  A similar approach has been proposed for
     authentication in IP version 6 (IPv6).
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     2.  Implementation Approach
     Implementation requires three issues to be addressed:
      (1)   A changed packet format,
      (2)   Authentication procedures, and
      (3)   Management controls.
     2.1.  OSPF PDU Format
     The basic OSPF message format provides for a 24 byte header with an
     arbitrary data content.  When MD5 is used, the same header and content
     are used, except that the eight byte "authentication key" field is
     reused to describe a "Keyed Message Digest" trailer.  This consists in
     five fields:
      (1)   The "Authentication Type" is Keyed Message Digest Algorithm,
            indicated by the value 2.
      (2)   A 16 bit offset from the OSPF header to the MD5 digest (if no
            other trailer fields are ever defined, this value equals the OSPF
            Data Length).
      (3)   An unsigned 8-bit field that contains the Key Identifier or Key-
            ID.  This identifies the key used to create the Authentication
            Data for this OSPF message.  A key is associated with an
      (4)   An unsigned 8-bit field that contains the length in octets of the
            trailing Authentication Data field.  The presence of this field
            permits other algorithms (e.g., SHA) to be substituted for MD5 if
      (5)   An unsigned 32 bit non-decreasing sequence number.
     The trailer consists of the Authentication Data, which is the output of
     the Keyed Message Digest Algorithm.  When the Authentication Algorithm
     is MD5, the output data is 16 bytes; during digest calculation, this is
     effectively followed by a pad field and a length field as defined by RFC
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     2.2.  Processing Algorithm
     When the authentication type is "Keyed Message Digest", message
     processing is changed in message creation and reception.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   Version #   |     type      |        OSPF Data Length       |
     |                          Router ID                            |
     |                           Area ID                             |
     |   Reserved - Must be Zero     | AuType=Keyed Message Digest Fn|
     |   Reserved - Must be Zero     |    Key ID    | Auth Data Len  |
     |               Sequence Number (non-decreasing)                |
     |                                                               |
     /           (OSPF Data Length-24) bytes Data                    /
     |                                                               |
     / Authentication Data  (var. length; 16 bytes when MD5 is used) /
     In memory, the following trailer is appended by the MD5 algorithm and
     treated as though it were part of the message.
     | zero or more pad bytes (defined by RFC 1321 when MD5 is used) |
     |                        64 bit message length MSW              |
     |                        64 bit message length LSW              |
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     2.2.1.  Message Generation
     The OSPF Packet is created as usual, with these exceptions:
      (1)   The OSPF checksum is not calculated, but is set to zero.
      (2)   The authentication type field indicates the Keyed Message Digest
            Algorithm (2).
      (3)   The authentication "password" field is reused to store a packet
            offset to the Authentication Data, a Key Identifier, the
            Authentication Data Length, and a non-decreasing sequence number.
     The value used in the sequence number is arbitrary, but two suggestions
     are the time of the message's creation or a simple message counter.
     The OSPF Authentication Key is selected by the sender based on the
     outgoing interface.  Each key has a lifetime associated with it.  No key
     is ever used outside its lifetime.  If more than one key is currently
     alive, then the youngest key (the key whose lifetime most recently
     started) SHOULD be used.  Since the key's algorithm is an attribute of
     the key, stored in the sender and receiver along with it, the Key ID
     effectively indicates which authentication algorithm is in use if the
     implementation supports more than one authentication algorithm.
      (1)   The OSPF header's packet length field indicates the standard OSPF
            portion of the packet.
      (2)   The Authentication Data Offset, Key Identifier, and
            Authentication Data size fields are filled in appropriately.
      (3)   The OSPF Authentication Key, which is 16 bytes long when the MD5
            algorithm is used, is now appended to the data.  For all
            algorithms, the OSPF Authentication Key is never longer than the
            output of the algorithm in use.
      (4)   Trailing pad and length fields are added and the digest
            calculated using the indicated algorithm.  When MD5 is the
            algorithm in use, these are calculated per RFC 1321.
      (5)   The digest is written over the OSPF Authentication Key.  When MD5
            is used, this digest will be 16 bytes long.
     The trailing pad is not actually transmitted, as it is entirely
     predictable from the message length and algorithm in use.
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     2.2.2.  Message Reception
     When the message is received, the process is reversed:
      (1)   The OSPF Checksum is not calculated,
      (2)   The digest is set aside,
      (3)   The appropriate algorithm and key are determined from the value
            of the Key Identifier field,
      (4)   The OSPF Authentication Key is written into the appropriate
            number (16 when MD5 is used) of bytes starting at the offset
      (5)   Appropriate padding is added as needed, and
      (6)   A new digest calculated using the indicated algorithm.
     If the calculated digest does not match the received digest, the message
     is discarded unprocessed.  If the neighbor is in a state other than DOWN
     or ATTEMPT and the received sequence number is less than the last one
     received, the message likewise is discarded unprocessed.  The received
     sequence number must, of course, be stored by neighbor and zeroed upon a
     "neighbor down" event.  Acceptable messages are now truncated to "OSPF
     Data Length" and treated normally.
     3.  Management Procedures
     3.1.  Key Management Requirements
     It is strongly desirable that a hypothetical security breach in one
     Internet protocol not automatically compromise other Internet protocols.
     The Authentication Key of this specification SHOULD NOT be stored using
     protocols or algorithms that have known flaws or fail to afford perfect
     forward secrecy.
     Implementations MUST support the storage of more than one key at the
     same time, although it is recognized that only one key will normally be
     active on an interface.  They MUST associate a specific lifetime (i.e.,
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     DRAFT                   OSPF MD5 Authentication             October 1994
     data/time first valid and data/time no longer valid) with each key, the
     key identifier, and MUST support manual key distribution (e.g., the
     privileged user manually typing in the key, key lifetime, and key
     identifier on the router console).  The lifetime may be infinite.  If
     more than one algorithm is supported, then the implementation MUST
     require that the algorithm be specified for each key at the time the
     other key information is entered.  Keys that are out of date MAY be
     deleted at will by the implementation without requiring human
     intervention.  Manual deletion of active keys SHOULD also be supported.
     It is likely that the IETF will define a standard key management
     protocol.  It is strongly desirable to use that key management protocol
     to distribute OSPF Authentication Keys among communicating OSPF
     implementations.  Such a protocol would provide scalability and
     significantly reduce the human administrative burden.  The Key ID can be
     used as a hook between OSPF and such a future protocol.  Key management
     protocols have a long history of subtle flaws that are often discovered
     long after the protocol was first described in public.  To avoid having
     to change all OSPF implementations should such a flaw be discovered,
     integrated key management protocol techniques were deliberately omitted
     from this specification.
     3.2.  Key Management Procedures
     As with all security methods using keys, it is necessary to change the
     OSPF Authentication Key on a regular basis.  To maintain routing
     stability during such changes, implementations are required to store and
     support the use of more than one OSPF Authentication Key on a given
     interface at the same time.
     Each key will have its own Key Identifier, which is stored locally.  The
     combination of the Key Identifier and the interface associated with the
     message uniquely identifies the Authentication Algorithm and OSPF
     Authentication Key in use.
     As noted above in Section 2.2.1, the party creating the OSPF message
     will select a valid key from the set of valid keys for that interface.
     The receiver will use the Key Identifier and interface to determine
     which key to use for authentication of the received message.  More than
     one key may be associated with an interface at the same time.
     Hence it is possible to have fairly smooth OSPF Authentication Key
     rollovers without losing legitimate OSPF messages because the stored key
     is incorrect and without requiring people to change all the keys at
     once.  To ensure a smooth rollover, each communicating OSPF system must
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     DRAFT                   OSPF MD5 Authentication             October 1994
     be updated with the new key several minutes before they current key will
     expire and several minutes before the new key lifetime begins.  The new
     key should have a lifetime that starts several minutes before the old
     key expires.  This gives time for each system to learn of the new OSPF
     Authentication Key before that key will be used.  It also ensures that
     the new key will begin being used and the current key will go out of use
     before the current key's lifetime expires.  For the duration of the
     overlap in key lifetimes, a system may receive messages using either key
     and authenticate the message.
     3.3.  Pathological Cases
     Two pathological cases exist which must be handled, which are failures
     of the network manager.  Both of these should be exceedingly rare.
     During key switchover, devices may exist which have not yet been
     successfully configured with the new key.  Therefore, routers MAY
     implement (and would be well advised to implement) an algorithm that
     detects the set of keys being used by its neighbors, and transmits its
     messages using both the new and old keys until all of the neighbors are
     using the new key or the lifetime of the old key expires.  Under normal
     circumstances, this elevated transmission rate will exist for a single
     HELLO interval.
     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 lfietime is
     extended, the key is deleted by network management, or a new key is
     4.  Conformance Requirements
     To conform to this specification, an implementation MUST support all of
     its aspects.  The MD5 authentication algorithm defined in RFC-1321 MUST
     be implemented by all conforming implementations.  A conforming
     implementation MAY also support other authentication algorithms such as
     NIST's Secure Hash Algorithm (SHA).  Manual key distribution as
     described above MUST be supported by all conforming implementations.
     All implementations MUST support the smooth key rollover described under
     "Key Change Procedures."
     The user documentation provided with the implementation MUST contain
     clear instructions on how to ensure that smooth key rollover occurs.
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     Implementations SHOULD support a standard key management protocol for
     secure distribution of OSPF Authentication Keys once such a key
     management protocol is standardized by the IETF.
     5.  Acknowledgments
     This work was done by the OSPF Working Group, of which John Moy is the
     Chair.  This suggestion was originally made by Christian Huitema on
     behalf of the IAB.  Jeff Honig (Cornell) and Dennis Ferguson (ANS) built
     the first operational prototype, proving out the algorithms.  The
     authors gladly acknowledge significant inputs from each of these
     6.  References
     [1]  Moy, John, "OSPF Version 2", RFC 1583, March 1994.
     [2]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
     [3]  F.  Baker, R.  Coltun, "OSPF Version 2 Management Information
          Base", RFC 1253, August 1991
     [4]  S.  Bellovin, "Security Problems in the TCP/IP Protocol Suite", ACM
          Computer Communications Review, Volume 19, Number 2, pp.32-48,
          April 1989.
     [5]  N.  Haller, R.  Atkinson, "Internet Authentication Guidelines",
          RFC-XXXX (already submitted to RFC Editor), September 1994.
     7.  Security Considerations
     This entire memo describes and specifies an authentication mechanism for
     the OSPF routing protocol that is believed to be secure against active
     and passive attacks.  Passive attacks are clearly widespread in the
     Internet at present.  Protection against active attacks is also needed
     even though such attacks are not currently widespread.
     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
     OSPF implementations.  This mechanism also depends on the OSPF
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     Authentication Key being kept confidential by all parties.  If any of
     these incorrect or insufficiently secure, then no real security will be
     provided to the users of this mechanism.
     Specifically with respect to the use of SNMP, compromise of SNMP
     security has the necessary result that the various OSPF configuration
     parameters (e.g. routing table, OSPF Authentication Key) managable via
     SNMP could be compromised as well.  Changing Authentication Keys using
     non-encrypted SNMP is no more secure than sending passwords in the
     Confidentiality is not provided by this mechanism.  Work is underway
     within the IETF to specify a standard mechanism for IP-layer encryption.
     That mechanism might be used to provide confidentiality for OSPF in the
     future.  Protection against traffic analysis is also not provided.
     Mechanisms such as bulk link encryption might be used when protection
     against traffic analysis is required.
     The memo is written to address a security consideration in OSPF Version
     2 that was raised during the IAB's recent security review [cite RFC
     8.  Author's Address
          Fred Baker
          Cisco Systems
          519 Lado Drive
          Santa Barbara, California 93111
     Phone: (805) 964 8007
     Email: fred@cisco.com
          Randall Atkinson
          Information Technology Division
          Naval Research Laboratory
          Washington, DC 20375-5320
     Voice: (DSN) 354-8590
     Fax: (DSN) 354-7942
     Email: atkinson@itd.nrl.navy.mil
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     DRAFT                   OSPF MD5 Authentication             October 1994
     Table of Contents
     1 Introduction ....................................................    2
     2 Implementation Approach .........................................    3
     2.1 OSPF PDU Format ...............................................    3
     2.2 Processing Algorithm ..........................................    4
     2.2.1 Message Generation ..........................................    5
     2.2.2 Message Reception ...........................................    6
     3 Management Procedures ...........................................    6
     3.1 Key Management Requirements ...................................    6
     3.2 Key Management Procedures .....................................    7
     3.3 Pathological Cases ............................................    8
     4 Conformance Requirements ........................................    8
     5 Acknowledgments .................................................    9
     6 References ......................................................    9
     7 Security Considerations .........................................    9
     8 Author's Address ................................................   10
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