Network Working Group                                          F. Baker
Request for Comments: 2082                                  R. Atkinson
Category: Standards Track                                 Cisco Systems
                                                           January 1997


                        RIP-2 MD5 Authentication

Status of this Memo

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

Table of Contents

   1 Use of Imperatives ...........................................    1
   2 Introduction .................................................    2
   3 Implementation Approach ......................................    3
   3.1 RIP-2 PDU Format ...........................................    3
   3.2 Processing Algorithm .......................................    5
   3.2.1 Message Generation .......................................    6
   3.2.2 Message Reception ........................................    7
   4 Management Procedures ........................................    7
   4.1 Key Management Requirements ................................    7
   4.2 Key Management Procedures ..................................    8
   4.3 Pathological Cases .........................................    9
   5 Conformance Requirements .....................................    9
   6 Acknowledgments ..............................................   10
   7 References ...................................................   10
   8 Security Considerations ......................................   11
   9 Chairman's Address ...........................................   11
   10 Authors' Addresses ..........................................   12

1.  Use of Imperatives

   Throughout this document, the words that are used to define the
   significance of particular requirements are capitalized.  These words
   are:

   MUST

      This word or the adjective "REQUIRED" means that the item is an
      absolute requirement of this specification.

   MUST NOT

      This phrase means that the item is an absolute prohibition of this
      specification.

   SHOULD

      This word or the adjective "RECOMMENDED" means that there may
      exist valid reasons in particular circumstances to ignore this
      item, but the full implications should be understood and the case
      carefully weighed before choosing a different course.

   SHOULD NOT

      This phrase means that there may exist valid reasons in particular
      circumstances when the listed behavior is acceptable or even
      useful, but the full implications should be understood and the
      case carefully weighed before implementing any behavior described
      with this label.

   MAY
      This word or the adjective "OPTIONAL" means that this item is
      truly optional.  One vendor may choose to include the item because
      a particular marketplace requires it or because it enhances the
      product, for example; another vendor may omit the same item.

2.  Introduction

   Growth in the Internet has made us aware of the need for improved
   authentication of routing information.  RIP-2 provides for
   unauthenticated service (as in classical RIP), or password
   authentication.  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 RIP-2, 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 RIP-2 use an authentication algorithm, as was
   originally proposed for SNMP Version 2, augmented by a sequence
   number.  Keyed MD5 is proposed as the standard authentication
   algorithm for RIP-2, 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., Keyed MD5) rather than the secret RIP-2 Authentication Key.
   This output is a one-way function of a message and a secret RIP-2
   Authentication Key.  This RIP-2 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 Keyed MD5 is
   being used for OSPF cryptographic authentication, and is therefore
   present in routers already, as is some form of password management.
   A similar approach has been standardized for use in IP-layer
   authentication. [7]

3.  Implementation Approach

   Implementation requires three issues to be addressed:

   (1)  A changed packet format,

   (2)  Authentication procedures, and

   (3)  Management controls.

3.1.  RIP-2 PDU Format

   The basic RIP-2 message format provides for an 8 byte header with an
   array of 20 byte records as its data content.  When Keyed MD5 is
   used, the same header and content are used, except that the 16 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 3 (1 and 2 indicate "IP Route" and
        "Password", respectively).

   (2)  A 16 bit offset from the RIP-2 header to the MD5 digest (if no
        other trailer fields are ever defined, this value equals the
        RIP-2 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 RIP-2 message.  In
        implementations supporting more than one authentication
        algorithm, the Key-ID also indicates the authentication
        algorithm in use for this message. A key is associated with
        an interface.

   (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., Keyed SHA) to be substituted for
        Keyed MD5 if desired.

   (5)  An unsigned 32 bit sequence number.  The sequence number MUST be
        non-decreasing for all messages sent with the same Key ID.

   The trailer consists of the Authentication Data, which is the output
   of the Keyed Message Digest Algorithm.  When the Authentication
   Algorithm is Keyed 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 1321.

3.2.  Processing Algorithm

   When the authentication type is "Keyed Message Digest", message
   processing is changed in message creation and reception.

       0                   1                   2                   3 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Command (1)   | Version (1)   |       Routing Domain (2)      |
   +---------------+---------------+-------------------------------+
   |             0xFFFF            | AuType=Keyed Message Digest   |
   +-------------------------------+-------------------------------+
   |    RIP-2 Packet Length        |    Key ID    | Auth Data Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Sequence Number (non-decreasing)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               reserved must be zero                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               reserved must be zero                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /    (RIP-2 Packet Length - 24) bytes of Data                   /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             0xFFFF            |       0x01                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /  Authentication Data (var. length; 16 bytes with Keyed MD5)   /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In memory, the following trailer is appended by the MD5 algorithm and
   treated as though it were part of the message.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              sixteen octets of MD5 "secret"                   |
   /                                                               /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | zero or more pad bytes (defined by RFC 1321 when MD5 is used) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        64 bit message length MSW              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        64 bit message length LSW              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.1.  Message Generation

   The RIP-2 Packet is created as usual, with these exceptions:

   (1) The UDP checksum need not be calculated, but MAY be set to
       zero.

   (2) The authentication type field indicates the Keyed Message Digest
       Algorithm (3).

   (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 RIP-2 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.  Since the key's algorithm is
   related to the key itself, 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 RIP-2 header's packet length field indicates the standard
        RIP-2 portion of the packet.

   (2)  The Authentication Data Offset, Key Identifier, and
        Authentication Data size fields are filled in appropriately.

   (3)  The RIP-2 Authentication Key, which is 16 bytes long when the
        Keyed MD5 algorithm is used, is now appended to the data.  For
        all algorithms, the RIP-2 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 Keyed MD5 is the
        algorithm in use, these are calculated per RFC 1321.

   (5)  The digest is written over the RIP-2 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.

3.2.2.  Message Reception

   When the message is received, the process is reversed:

   (1)  The digest is set aside,

   (2)  The appropriate algorithm and key are determined from the value
        of the Key Identifier field,

   (3)  The RIP-2 Authentication Key is written into the appropriate
        number (16 when Keyed MD5 is used) of bytes starting at the
        offset indicated,

   (4)  Appropriate padding is added as needed, and

   (5)  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 has been heard
   from recently enough to have viable routes in the route table and the
   received sequence number is less than the last one received, the
   message likewise is discarded unprocessed.  When connectivity to the
   neighbor has been lost, the receiver SHOULD be ready to accept
   either:
   - a message with a sequence number of zero
   - a message with a higher sequence number than the last received
     sequence number.

   A router that has forgotten its current sequence number but remembers
   its key and Key-ID MUST send its first packet with a sequence number
   of zero.  This leaves a small opening for a replay attack.  Router
   vendors are encouraged to provide stable storage for keys, key
   lifetimes, Key-IDs, and the related sequence numbers.

   Acceptable messages are now truncated to RIP-2 message itself and
   treated normally.

4.  Management Procedures

4.1.  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.

   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., date/time first valid and date/time no longer valid) and a key
   identifier with each key, 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 RIP-2 Authentication Keys among communicating
   RIP-2 implementations.  Such a protocol would provide scalability and
   significantly reduce the human administrative burden. The Key ID can
   be used as a hook between RIP-2 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 RIP-2 implementations should
   such a flaw be discovered, integrated key management protocol
   techniques were deliberately omitted from this specification.

4.2.  Key Management Procedures

   As with all security methods using keys, it is necessary to change
   the RIP-2 Authentication Key on a regular basis.  To maintain routing
   stability during such changes, implementations MUST be able to store
   and use more than one RIP-2 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
   RIP-2 Authentication Key in use.

   As noted above in Section 2.2.1, the party creating the RIP-2 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 RIP-2 Authentication Key
   rollovers without losing legitimate RIP-2 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 RIP-2
   system must be updated with the new key several minutes before the
   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 RIP-2 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. The Key-ID in the received message is used to select the
   appropriate key for authentication.

4.3.  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 SHOULD
   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 update 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
   lifetime is extended, the key is deleted by network management, or a
   new key is configured.

5.  Conformance Requirements

   To conform to this specification, an implementation MUST support all
   of its aspects.  The Keyed MD5 authentication algorithm MUST be
   implemented by all conforming implementations. MD5 is defined in
   RFC-1321.  A conforming implementation MAY also support other
   authentication algorithms such as Keyed 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.

   Implementations SHOULD support a standard key management protocol for
   secure distribution of RIP-2 Authentication Keys once such a key
   management protocol is standardized by the IETF.

6.  Acknowledgments

   This work was done by the RIP-2 Working Group, of which Gary Malkin
   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 sources.

7.  References

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

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

   [3]  Malkin, G., and F. Baker, "RIP Version 2 MIB Extension",
        RFC 1389, Xylogics, Inc., Advanced Computer Communications,
        January 1993.

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

   [6]  Braden, R., Clark, D., Crocker, S., and C. Huitema, "Report
        of IAB Workshop on Security in the Internet Architecture",
        RFC 1636, June 1994.

   [7]  Atkinson, R., "IP Authentication Header", RFC 1826, August 1995.

   [8]  Atkinson, R., "IP Encapsulating Security Payload", RFC 1827,
        August 1995.

8.  Security Considerations

   This entire memo describes and specifies an authentication mechanism
   for the RIP-2 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 because active attacks are becoming more common.

   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 RIP-2 implementations. This mechanism also depends on
   the RIP-2 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 RIP-2
   configuration parameters (e.g. routing table, RIP-2 Authentication
   Key) manageable via SNMP could be compromised as well.  Changing
   Authentication Keys using non-encrypted SNMP is no more secure than
   sending passwords in the clear.

   Confidentiality is not provided by this mechanism.  Recent work in
   the IETF provides a standard mechanism for IP-layer encryption. [8]
   That mechanism might be used to provide confidentiality for RIP-2 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 RIP
   Version 2 that was raised during the IAB's recent security review
   [6].

9.  Chairman's Address

   Gary Scott Malkin
   Xylogics, Inc.
   53 Third Avenue
   Burlington, MA 01803

   Phone:  (617) 272-8140
   EMail:  gmalkin@Xylogics.COM

10.  Authors' Addresses

   Fred Baker
   cisco Systems
   519 Lado Drive
   Santa Barbara, California 93111

   Phone: (805) 681 0115
   Email: fred@cisco.com


   Randall Atkinson
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134-1706

   Phone: (408) 526-6566
   EMail: rja@cisco.com