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

Baker & Atkinson Standards Track [Page 1] RFC 2082 RIP-2 MD5 Authentication January 1997

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

Baker & Atkinson Standards Track [Page 2] RFC 2082 RIP-2 MD5 Authentication January 1997

 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
 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).

Baker & Atkinson Standards Track [Page 3] RFC 2082 RIP-2 MD5 Authentication January 1997

 (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.

Baker & Atkinson Standards Track [Page 4] RFC 2082 RIP-2 MD5 Authentication January 1997

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              |

Baker & Atkinson Standards Track [Page 5] RFC 2082 RIP-2 MD5 Authentication January 1997

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
 (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.

Baker & Atkinson Standards Track [Page 6] RFC 2082 RIP-2 MD5 Authentication January 1997

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
 - 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.

Baker & Atkinson Standards Track [Page 7] RFC 2082 RIP-2 MD5 Authentication January 1997

 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.

Baker & Atkinson Standards Track [Page 8] RFC 2082 RIP-2 MD5 Authentication January 1997

 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
 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

Baker & Atkinson Standards Track [Page 9] RFC 2082 RIP-2 MD5 Authentication January 1997

 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
 [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.

Baker & Atkinson Standards Track [Page 10] RFC 2082 RIP-2 MD5 Authentication January 1997

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

9. Chairman's Address

 Gary Scott Malkin
 Xylogics, Inc.
 53 Third Avenue
 Burlington, MA 01803
 Phone:  (617) 272-8140
 EMail:  gmalkin@Xylogics.COM

Baker & Atkinson Standards Track [Page 11] RFC 2082 RIP-2 MD5 Authentication January 1997

10. Authors' Addresses

 Fred Baker
 cisco Systems
 519 Lado Drive
 Santa Barbara, California 93111
 Phone: (805) 681 0115
 Randall Atkinson
 cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134-1706
 Phone: (408) 526-6566

Baker & Atkinson Standards Track [Page 12]

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