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rfc:rfc4822

Network Working Group R. Atkinson Request for Comments: 4822 Extreme Networks Obsoletes: 2082 M. Fanto Updates: 2453 NIST Category: Standards Track February 2007

                 RIPv2 Cryptographic 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.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

IESG Note

 In the interests of encouraging rapid migration away from Keyed-MD5
 and its known weakness, the IESG has approved this document even
 though it does not meet the guidelines in BCP 107 (RFC 4107).
 However, the IESG stresses that automated key management should be
 used to establish session keys and urges that the future work on key
 management described in Section 5.6 of this document should be
 performed as soon as possible.

Abstract

 This note describes a revision to the RIPv2 Cryptographic
 Authentication mechanism originally specified in RFC 2082.  This
 document obsoletes RFC 2082 and updates RFC 2453.  This document adds
 details of how the SHA family of hash algorithms can be used with
 RIPv2 Cryptographic Authentication, whereas the original document
 only specified the use of Keyed-MD5.  Also, this document clarifies a
 potential issue with an active attack on this mechanism and adds
 significant text to the Security Considerations section.

Atkinson & Fanto Standards Track [Page 1] RFC 4822 RIPv2 Cryptographic Authentication February 2007

1. Introduction

 Growth in the Internet has made us aware of the need for improved
 authentication of routing information.  RIPv2 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 [Bell89].  Cleartext passwords, originally
 specified for use with RIPv2, are widely understood to be vulnerable
 to easily deployed passive attacks [HA94].
 The original RIPv2 cryptographic authentication specification, RFC
 2082 [AB97], used the Keyed-MD5 cryptographic mechanism.  While there
 are no openly published attacks on that mechanism, some reports
 [Dobb96a, Dobb96b] create concern about the ultimate strength of the
 MD5 cryptographic hash function.  Further, some end users,
 particularly several different governments, require the use of the
 SHA hash function family rather than any other such function for
 policy reasons.  Finally, the original specification uses a hashing
 construction widely believed to be weaker than the HMAC construction
 used with the algorithms added in this revision of the specification.
 This document obsoletes the original specification, RFC 2082 [AB97].
 This specification differs from RFC 2082 by adding support for the
 SHA family of hash algorithms and the HMAC technique, while retaining
 the original Keyed-MD5 algorithm and mode.  As the original RIPv2
 Cryptographic Authentication mechanism was algorithm-independent,
 backwards compatibility is retained.  This requirement for backwards
 compatibility precludes making significant protocol changes.  So,
 this document limits changes to the addition of support for an
 additional family of cryptographic algorithms.  The original
 specification has been very widely implemented, is known to be widely
 interoperable, and is also widely deployed.
 The authors do NOT believe that this specification is the final
 answer to RIPv2 authentication and encourage the reader to consult
 the Security Considerations section of this document for more
 details.
 If RIPv2 authentication is disabled, then only simple
 misconfigurations are detected.  The original RIPv2 authentication
 mechanism relied upon reused cleartext passwords.  Use of cleartext
 password authentication can protect against accidental
 misconfigurations if that were the only concern, but is not helpful
 from a security perspective.  By simply capturing information on the
 wire -- straightforward even in a remote environment -- a hostile

Atkinson & Fanto Standards Track [Page 2] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 entity can read the cleartext RIPv2 password and use that knowledge
 to inject false information into the routing system via the RIPv2
 routing protocol.
 This mechanism is intended to reduce the risk of a successful passive
 attack upon RIPv2 deployments.  That is, deployment of this mechanism
 greatly reduces the vulnerability of the RIPv2-based routing system
 from a passive attack.  When cryptographic authentication is enabled,
 we transmit the output of a keyed cryptographic one-way function in
 the authentication field of the RIPv2 packet, instead of sending a
 cleartext reusable password in the RIPv2 packet.  The RIPv2
 Authentication Key is known only to the authorized parties of the
 RIPv2 session.  The RIPv2 Authentication Key is never sent over the
 network in the clear.
 In this way, protection is afforded against forgery or message
 modification.  While it is possible to replay a message until the
 sequence number changes, a sequence number can be used to reduce
 replay risks.  The mechanism does not provide confidentiality, since
 messages stay in the clear.  Since the objective of a routing
 protocol is to advertise the routing topology, confidentiality is not
 normally required for routing protocols.
 Other relevant rationales for the approach are that MD5 and SHA-1 are
 both being used for other purposes and are therefore generally
 already present in IP routers, as is some form of password
 management.

1.1. Terminology

 In this document, the words "MUST", "MUST NOT", "REQUIRED", "SHALL",
 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
 described in [BCP14] and indicate requirement levels for compliant or
 conformant implementations.

2. Implementation Approach

 Implementation requires use of a special packet format, special
 authentication procedures, and also management controls.
 Implementers need to remember that the Security Considerations
 section is an integral part of this specification and contains
 important parts of this specification.

Atkinson & Fanto Standards Track [Page 3] RFC 4822 RIPv2 Cryptographic Authentication February 2007

2.1. RIPv2 PDU Format

 The basic RIPv2 message format provides for an 8-octet header with an
 array of 20-octet records as its data content.  When RIPv2
 Cryptographic Authentication is enabled, the same header and content
 are used as with the original RIPv2 specification, but the 16-octet
 "Authentication" password field of the original RIPv2 specification
 is reused to contain a packet offset to the Authentication Data, a
 Key Identifier, the Authentication Data Length, and a non-decreasing
 sequence number.
    AUTHENTICATION TYPE
       The "Authentication Type" is Cryptographic Hash Function, which
       is indicated by the value 3.
    RIPv2 PACKET LENGTH
       An unsigned 16-bit offset from the start of the RIPv2 header to
       the end of the regular RIPv2 packet (not including the
       authentication trailer).
    KEY IDENTIFIER
       An unsigned 8-bit field that contains the Key Identifier or
       Key-ID.  This, in combination with the network interface,
       identifies the RIPv2 Security Association in use for this
       packet.  The RIPv2 Security Association, which is defined in
       Section 2.2 below, includes the Authentication Key that was
       used to create the Authentication Data for this RIPv2 message
       and other parameters.  In implementations supporting more than
       one authentication algorithm, the RIPv2 Security Association
       also includes information about which authentication algorithm
       is in use for this message.  A RIPv2 Security Association is
       always associated with an interface, rather than with a router.
       The actual cryptographic key is part of the RIPv2 Security
       Association.
    AUTHENTICATION DATA LENGTH
       An unsigned 8-bit field that contains the length in octets of
       the trailing Authentication Data field.  The presence of this
       field helps provide cryptographic algorithm independence.
    AUTHENTICATION DATA
       This field contains the cryptographic Authentication Data used
       to validate this packet.  The length of this field is stored in
       the AUTHENTICATION DATA LENGTH field above.

Atkinson & Fanto Standards Track [Page 4] RFC 4822 RIPv2 Cryptographic Authentication February 2007

    SEQUENCE NUMBER
       An unsigned 32-bit sequence number.  The sequence number MUST
       be non-decreasing for all messages sent from a given source
       router with a given Key ID value.
 The authentication trailer contains the Authentication Data, which is
 the output of the keyed cryptographic hash function.  See later
 subsections of this section for details on computing this field.
  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
 +---------------+---------------+-------------------------------+
 |  Command (1)  | Version (1)   |        Routing Domain (2)     |
 +---------------+---------------+-------------------------------+
 |             0xFFFF            |  Authentication Type=0x0003   |
 +---------------+---------------+---------------+---------------+
 |     RIPv2 Packet Length       |   Key ID      | Auth Data Len |
 +---------------+---------------+---------------+---------------+
 |               Sequence Number (non-decreasing)                |
 +---------------+---------------+---------------+---------------+
 |                      reserved must be zero                    |
 +---------------+---------------+---------------+---------------+
 |                      reserved must be zero                    |
 +---------------+---------------+---------------+---------------+
 |                                                               |
 ~            (RIPv2 Packet Length - 24) bytes of Data           ~
 |                                                               |
 +---------------+---------------+---------------+---------------+
 |             0xFFFF            |            0x0001             |
 +---------------+---------------+---------------+---------------+
 | Authentication Data (variable length; 20 bytes with HMAC-SHA1)|
 +---------------+---------------+---------------+---------------+

2.2. RIPv2 Security Association

 Understanding the RIPv2 Security Association concept is central to
 understanding this specification.  A RIPv2 Security Association
 contains the set of shared authentication configuration parameters
 needed by the legitimate sender or any legitimate receiver.
 An implementation MUST be able to support at least 2 concurrent RIPv2
 Security Associations on each RIP interface.  This is a functional
 requirement for supporting key rollover.  Support for key rollover is
 mandatory.
 The RIPv2 Security Association, defined below, is selected by the
 sender based on the outgoing router interface.  Each RIPv2 Security
 Association has a lifetime and other configuration parameters

Atkinson & Fanto Standards Track [Page 5] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 associated with it.  In normal operation, a RIPv2 Security
 Association is never used outside its lifetime.  Certain abnormal
 cases are discussed later in this document.
 The minimum data items in a RIPv2 Security Association are as
 follows:
    KEY-IDENTIFIER (KEY-ID)
       The unsigned 8-bit KEY-ID value is used to identify the RIPv2
       Security Association in use for this packet.
       The receiver uses the combination of the interface the packet
       was received upon and the KEY-ID value to uniquely identify the
       appropriate Security Association.
       The sender selects which RIPv2 Security Association to use
       based on the outbound interface for this RIPv2 packet and then
       places the correct KEY-ID value into that packet.  If multiple
       valid and active RIPv2 Security Associations exist for a given
       outbound interface at the time a RIPv2 packet is sent, the
       sender may use any of those security associations to protect
       the packet.
    AUTHENTICATION ALGORITHM
       This specifies the cryptographic algorithm and algorithm mode
       used with the RIPv2 Security Association.  This information is
       never sent in cleartext over the wire.  Because this
       information is not sent on the wire, the implementer chooses an
       implementation specific representation for this information.
       At present, the following values are possible: KEYED-MD5,
       HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512.
    AUTHENTICATION KEY
       This is the value of the cryptographic authentication key used
       with the associated Authentication Algorithm.  It MUST NOT ever
       be sent over the network in cleartext via any protocol.  The
       length of this key will depend on the Authentication Algorithm
       in use.  Operators should take care to select unpredictable and
       strong keys, avoiding any keys known to be weak for the
       algorithm in use. [ESC05] contains helpful information on both
       key generation techniques and cryptographic randomness.

Atkinson & Fanto Standards Track [Page 6] RFC 4822 RIPv2 Cryptographic Authentication February 2007

    SEQUENCE NUMBER
       This is an unsigned 32-bit number.  For a given KEY-ID value
       and sender, this number MUST NOT decrease.  In normal
       operation, the operator should rekey the RIPv2 session prior to
       reaching the maximum value.  The initial value used in the
       sequence number is arbitrary.  Receivers SHOULD keep track of
       the most recent sequence number received from a given sender.
    START TIME
       This is a local representation of the day and time that this
       Security Association first becomes valid.
    STOP TIME
       This is a local representation of the day and time that this
       Security Association becomes invalid (i.e., when it expires).
       It is permitted, but not recommended, for an operator to
       configure this to "never expire".  The "never expire" value is
       not recommended operational practice because it reduces
       security as compared with periodic rekeying.  Normally, a RIPv2
       Security Association is deleted at its STOP TIME.  However,
       there are certain pathological cases, which are discussed in
       Section 5.1.
 The authentication trailer consists of the Authentication Data, which
 is the output of the keyed cryptographic hash function.  See later
 subsections of this section for details on computing this field.

2.3. Basic Authentication Processing

 When the authentication type is "Cryptographic Hash Function",
 message processing is changed in message creation and reception as
 compared with the original RIPv2 specification in [Mal94].
 This section describes the message processing generically.
 Additional algorithm-dependent processing that is required is
 described in separate, subsequent sections of this document.  As of
 this writing, there are 2 kinds of algorithm-dependent processing.
 One covers the "Keyed-MD5" algorithm.  The other covers the
 "HMAC-SHA1" family of algorithms.

2.3.1. Message Generation

 The RIPv2 Packet is created as usual, with these exceptions:
 (1) The UDP checksum SHOULD be calculated, but MAY be set to zero
     because any of the cryptographic authentication mechanisms in
     this specification will provide stronger integrity protection
     than the standard UDP checksum.

Atkinson & Fanto Standards Track [Page 7] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 (2) The Authentication Type field indicates Cryptographic
     Authentication (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.
 See also Section 2.2 above on RIPv2 Security Association for other
 important background information.
 When creating the RIPv2 Packet, the following process is followed:
 (1) The Packet Length field of the RIPv2 header indicates the size of
     the main body of the RIPv2 packet.
 (2) An appropriate RIPv2 Security Association is selected for use
     with this packet, based on the outbound interface for the packet.
     Any valid RIPv2 Security Association for that outbound interface
     may be used.  The Authentication Data Offset, Key Identifier, and
     Authentication Data Length fields are filled in appropriately.
 (3) Algorithm-dependent processing occurs now, either for the
     "Keyed-MD5" algorithm or for the "HMAC-SHA1" algorithm family.
     See the respective sub-sections (below) for details of this
     algorithm-dependent processing.
 (4) The resulting Authentication Data value is written into the
     Authentication Data field.  The trailing pad (if any) is not
     actually transmitted, as it is entirely predictable from the
     message length and Authentication Algorithm in use.

2.3.2. Message Reception

 When the message is received, the process is reversed:
 (1) The received Authentication Data is set aside and stored for
     later use,
 (2) The appropriate RIPv2 Security Association is determined from the
     value of the Key Identifier field and the interface the packet
     was received on.  If there is no valid RIPv2 Security Association
     for the received Key Identifier on the interface that the packet
     was received on, then:
     (a) all processing of the incoming packet ceases, and
     (b) a security event SHOULD be logged by the RIPv2 subsystem of
         the receiving system.  That security event should indicate at

Atkinson & Fanto Standards Track [Page 8] RFC 4822 RIPv2 Cryptographic Authentication February 2007

         least the day/time that the bad packet was received, the
         Source IP Address of the received RIPv2 packet, the Key-ID
         field value, the interface the bad packet arrived upon, and
         the fact that no valid RIPv2 Security Association was found
         for that interface and Key-ID combination.
 (3) Algorithm-dependent processing is performed, using the algorithm
     specified by the appropriate RIPv2 Security Association for this
     packet.  This results in calculation of the Authentication Data
     based on the information in the received RIPv2 packet and
     information from the appropriate RIPv2 Security Association for
     that packet.
 (4) The calculated Authentication Data result is compared with the
     received Authentication Data.
 (5) If the calculated authentication data result does not match the
     received Authentication Data field, then:
     (a) the message MUST be discarded without being processed, and
     (b) a security event SHOULD be logged by the RIPv2 subsystem of
         the receiving system.  That security event SHOULD indicate at
         least the day/time that the bad packet was received, the
         Source IP Address of the received RIPv2 packet, the Key-ID
         field value, the interface the bad packet arrived upon, and
         the fact that RIPv2 Authentication failed upon receipt of the
         packet.
 (6) If the neighbor has been heard from recently enough to have
     viable routes in the local routing table, and the received
     sequence number is less than the last sequence number received,
     then the message MUST be discarded unprocessed.  If the received
     sequence number is less than the last sequence number received,
     that fact SHOULD be logged as a security event.  This logged
     security event SHOULD indicate at least the day/time that the bad
     packet was received, the Source IP Address of the received RIPv2
     packet, the Key-ID field value, and the fact that an out-of-order
     RIPv2 sequence number was received.
     When connectivity to the neighbor has been lost, the receiver
     SHOULD be ready to accept either:
  1. a message with a sequence number of zero.
  1. a message with a higher sequence number than the last

received sequence number.

Atkinson & Fanto Standards Track [Page 9] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 (7) Acceptable messages are now truncated to the RIPv2 message
     itself, minus the authentication trailer, and are processed
     normally (i.e., in accordance with the RIPv2 base specification
     in RFC 2453 [Mal98]).  The last received sequence number for this
     RIPv2 Security Association and sender is also updated.
 NOTA BENE: A router that has forgotten its current sequence number
 but remembers its Security Association MUST send its first packet
 with a sequence number of zero.  This leaves a small opening for a
 replay attack.  To reduce the risk of such attacks by precluding the
 situation where a router has forgotten its current sequence number,
 implementers SHOULD provide non-volatile storage for all components
 of a RIPv2 Security Association, and receiving systems SHOULD provide
 non-volatile storage for the last received sequence number from each
 sender.  See also the Security Considerations section of this
 document.

2.4. Keyed-MD5 Algorithm-Dependent Processing

 This section describes the algorithm-dependent processing steps
 applicable when the "Keyed-MD5" authentication algorithm is in use.
 The RIPv2 Authentication Key is always 16 octets when "Keyed-MD5" is
 in use.
 (1) The RIPv2 Authentication Key is appended to the RIPv2 packet in
     memory.
 (2) The Trailing Pad for MD5 and message length fields are added in
     memory.  The diagram below shows how these additions appear when
     appended in memory:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Authentication Key                        |
    /                      (16 octets long)                         /
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       zero or more pad octets (as defined by RFC 1321)        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   64-bit message length MSW                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   64-bit message length LSW                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 (3) The Authentication Data is then calculated according to the MD5
     algorithm defined by RFC 1321 [Rivest92].

Atkinson & Fanto Standards Track [Page 10] RFC 4822 RIPv2 Cryptographic Authentication February 2007

2.5. HMAC-SHA1 Algorithm-Dependent Processing

 This section describes the processing steps for HMAC Authentication.
 While HMAC was originally documented in [KMC97], for this
 specification, the terminology used in [FIPS-198] is used.  While the
 current specification only provides full details for HMAC
 Authentication using the National Institute of Standards and
 Technology (NIST) SHA-1 algorithm (and its direct derivatives), this
 same basic process could be used with other cryptographic hash
 functions in the future.  Because the RIPv2 packet is only hashed
 once, the overhead of the double hashing in this process is
 negligible.
 The US NIST Secure Hash Standard (SHS), defined by [FIPS-180-2],
 includes specifications for SHA-1, SHA-256, SHA-384, and SHA-512.
 This specification defines processing for each of these.
 The output of the cryptographic computations (e.g., HMAC-SHA1) is NOT
 truncated for RIPv2 Cryptographic Authentication.
 The Authentication Data Length is equal to the Message Digest Size
 for the hash algorithm in use.
 Any key value known to be weak with an algorithm defined by the NIST
 Secure Hash Standard MUST NOT be used with such an algorithm in an
 implementation of this specification.  US NIST is the authoritative
 source for public information on weak keys for those algorithms.
 In the algorithm description below, the following nomenclature, which
 is consistent with [FIPS-198], is used:
       H    is the specific hashing algorithm,
            for example, SHA-1 or SHA-256.
       Ko   is the cryptographic key used with the hash algorithm.
       B    is the block-size of H, measured in octets, not bits.
            Note 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, not bits.
            For example, with SHA-1, L == 20.
       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.

Atkinson & Fanto Standards Track [Page 11] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 (1) PREPARATION OF KEY
     In this application, Ko is always L octets long.
     If the Authentication Key is L octets long, then Ko is set equal
     to the Authentication Key.  If the Authentication Key is more
     than L octets long, then Ko is set to H(Authentication Key).  If
     the Authentication Key is less than L octets long, then Ko is set
     to the Authentication Key with zeros appended to the end of the
     Authentication Key such that Ko is L octets long.
 (2) FIRST HASH
     First, the RIPv2 packet's Authentication Data field is filled
     with the value Apad.
     Then, a first hash, also known as the inner hash, is computed as
     follows:
             First-Hash = H(Ko XOR Ipad || (RIPv2 Packet))
 (3) SECOND HASH
     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 Data field of the RIPv2 packet.  The
     length of the Authentication Data field is always identical to
     the message digest size of the hash function H that is being
     used.
     This also implies that use of hash functions with larger output
     sizes will also increase the size of the packet as transmitted on
     the wire.

3. Management Procedures

 Key management is an important component of this mechanism and proper
 implementation is central to providing the intended level of risk
 reduction.

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 configured or stored using protocols (e.g., RADIUS) or
 cryptographic algorithms that have known flaws.

Atkinson & Fanto Standards Track [Page 12] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 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.  Implementations MUST associate a specific
 Security Association lifetime (i.e., date/time first valid and
 date/time no longer valid) and a key identifier with each key.
 Implementations also MUST support manual key distribution.  An
 example of manual key distribution is having the privileged user
 typing in the key, key lifetime, and key identifier on the router
 console.  An operator may configure the Security Association lifetime
 to infinite, which means that the session is never rekeyed.  However,
 instead, it is strongly recommended that operators rekey regularly,
 using a moderately short Security Association lifetime (e.g., 24
 hours).
 This specification requires support for at least two authentication
 algorithms, so the implementation MUST require that the
 authentication algorithm be specified for each key when the other key
 information is entered.  Manual deletion of active Security
 Associations MUST be supported.
 It is likely that the IETF will define a standard key management
 protocol for use with routing protocols.  It is strongly desirable to
 use an IETF standards-track key management protocol to distribute
 RIPv2 Authentication Keys among communicating RIPv2 implementations.
 Such a protocol would provide scalability and significantly reduce
 the human administrative burden.  The Key-ID field can be used as a
 hook between RIPv2 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 RIPv2 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 RIPv2 Authentication Key on a regular basis.  To maintain routing
 stability during such changes, implementations MUST be able to store
 and use more than one RIPv2 Authentication Key on a given interface
 at the same time.
 Each key will have its own Key Identifier (KEY-ID), which is stored
 locally.  The combination of the Key Identifier and the interface
 associated with the message uniquely identifies the Authentication
 Algorithm and RIPv2 Authentication Key in use.

Atkinson & Fanto Standards Track [Page 13] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 As noted above in Section 2.3.1, the party creating the RIPv2 message
 will select a valid RIPv2 Security Association from the set of valid
 RIPv2 Security Associations for that interface.  The receiver MUST
 use the Key Identifier and receiving interface to determine which
 RIPv2 Security Association to use for authentication of the received
 message.  More than one RIPv2 Security Association MAY be associated
 with an interface at the same time.  The receiver MUST NOT simply try
 all RIPv2 Security Associations (i.e., keys) that might be configured
 for RIPv2 on the receiving interface, as that creates an easily
 exploited denial-of-service attack on the RIP subsystem of the
 receiver.  (At least one widely used implementation of the previous
 version of this specification violates these requirements as of the
 publication date of this document and has consequent security
 vulnerabilities.)
 Hence, it is possible to have fairly smooth RIPv2 Security
 Association (i.e., key) rollovers, without losing legitimate RIPv2
 messages due to an invalid shared key and without requiring people to
 change all the keys at once.  To ensure a smooth rollover, each
 communicating RIPv2 system must be updated with the new RIPv2
 Security Association (including the new key) several minutes before
 the current RIPv2 Security Association will expire and several
 minutes before the new RIPv2 Security Association lifetime begins.
 Also, the new RIPv2 Security Association should have a lifetime that
 starts several minutes before the old RIPv2 Security Association
 expires.  This gives time for each system to learn of the new
 security association before that security association will be used.
 It also ensures that the new security association will begin use and
 the current security association will go out of use before the
 current security association's lifetime expires.  For the duration of
 the overlap in security association lifetimes, a system may receive
 messages corresponding to either security association and
 successfully authenticate the message.  The Key-ID in the received
 message is used to select the appropriate security association (i.e.,
 key) to be used for authentication.

4. Conformance Requirements

 For this specification, the term "conformance" has identical meaning
 to the phrase "full compliance".
 The Keyed MD5 authentication algorithm and the HMAC-SHA1 algorithm
 MUST be implemented by all conforming implementations.  In addition,
 the HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 algorithms SHOULD be
 implemented.  MD5 is defined in [Rivest92].  SHA-1, SHA-256, SHA-384,
 and SHA-512 have been defined by the US NIST in [FIPS-180-2].

Atkinson & Fanto Standards Track [Page 14] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 A conforming implementation MAY also support additional
 authentication algorithms, provided those additional algorithms are
 publicly and openly specified.
 Manual key distribution as described above MUST be supported by all
 conforming implementations.  All implementations MUST support the
 smooth key rollover described under "Key Management Procedures".
 This also means that implementations MUST support at least 2
 concurrent RIPv2 Security Associations.
 The user documentation provided with the implementation ought to
 contain clear instructions on how to configure the implementation
 such that smooth key rollover occurs successfully.
 Implementations SHOULD support a standard key management protocol for
 secure distribution of RIPv2 Authentication Keys once such a key
 management protocol is standardized by the IETF.
 The Security Considerations section of this document is an integral
 part of the specification, not just a discussion of the protocol.

5. Security Considerations

 This entire memo describes and specifies an authentication mechanism
 for the RIPv2 routing protocol that is believed to be secure against
 passive attacks.  The term "passive attack" is defined in RFC 1704
 [HA94].  The analysis contained in RFC 1704 motivated this work.
 Passive attacks are clearly widespread in the Internet at present
 [HA94].
 Protection against active attacks is incomplete in this current
 specification.  The main issue relative to active attacks lies in the
 need to support the case where another router has recently rebooted
 and that router lacks the non-volatile storage needed to remember the
 RIPv2 Security Association(s) and last received RIPv2 sequence
 number(s) across that reboot.

5.1. Known Pathological Cases

 Two known pathological cases exist that MUST be handled by
 implementations.  Both of these are failures of the network manager.
 Each of these should be exceedingly rare in normal operation.
 (1) During key rollover, devices might exist that have not yet been
     successfully configured with the new key.  Therefore, routers
     SHOULD implement an algorithm that detects the set of RIPv2
     Security Associations being used by its neighbors, and transmit
     its messages using both the new and old RIPv2 Security

Atkinson & Fanto Standards Track [Page 15] RFC 4822 RIPv2 Cryptographic Authentication February 2007

     Associations (i.e., keys) until all of the neighbors are using
     the new security association or the lifetime of the old security
     association expires.  Under normal circumstances, this elevated
     transmission rate will exist for a single RIP update interval.
 (2) In the event that the last RIPv2 Security Association of an
     interface expires, it is unacceptable to revert to an
     unauthenticated condition, and not advisable to disrupt routing.
     Therefore, the router MUST send a "last RIPv2 Security
     Association expiration" notification to the network manager
     (e.g., via SYSLOG, SNMP, and/or other means) and SHOULD treat
     that last Security Association as having an infinite lifetime
     until the lifetime is extended, the Security Association is
     deleted by network management, or a new security association is
     configured.
 In some circumstances, the practice described in (2) can leave an
 opening to an active attack on the RIPv2 routing subsystem.
 Therefore, any actual occurrence of a RIPv2 Security Association
 expiration MUST cause a security event to be logged by the
 implementation.  This log item MUST include at least a note that the
 RIPv2 Authentication Key expired, the RIP routing protocol
 instance(s) affected, the routing interfaces affected, the Key-ID
 that is affected, and the current date/time.  Operators are
 encouraged to check such logs as an operational security practice to
 help detect active attacks on the RIPv2 routing subsystem.  Further,
 implementations SHOULD provide a configuration knob ("fail secure")
 to let a network operator prefer to have the RIPv2 routing fail when
 the last key expires, rather than continue using RIPv2 in an insecure
 manner.

5.2 Network Management Considerations

 Also, the use of SNMP, even SNMPv3 with cryptographic authentication
 and cryptographic confidentiality enabled, to modify or configure the
 RIPv2 Security Associations, or any component of the security
 association (for example, the cryptographic key), is NOT RECOMMENDED.
 This practice would create a potential for a cascading vulnerability,
 whereby a compromise in the SNMP security implementation would
 necessarily lead to a compromise not only of the local routing table
 (which could be accessed via SNMP) but also of all other routers that
 receive RIPv2 packets (directly or indirectly) from the compromised
 router.

Atkinson & Fanto Standards Track [Page 16] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 Similarly, the use of protocols not designed and evaluated for use in
 key management (e.g., RADIUS, Diameter) to configure the security
 association is also NOT RECOMMENDED.  Reading the Security
 Associations via SNMP is allowed, but the information is to be
 treated as security-sensitive and protected by using the priv mode.
 Also, the use of SNMP to configure which form of RIPv2 authentication
 is in use is also NOT RECOMMENDED because of a similar cascading
 failure issue.  Any future revision of the RIPv2 Management
 Information Base (MIB) [MB94] should consider making the
 rip2IfConfAuthType object read-only.  Further, this object would need
 a new enum value to accommodate the RIPv2 cryptographic
 authentication type.  In addition, the compliance statement for this
 MIB does not have a MIN-ACCESS for this object.  At a minimum, if the
 MIB is updated, a new compliance statement SHOULD be written for this
 object that allows this object to be implemented as read-only.  For
 the rip2ifConfAuthKey object, since this object always returns ''H
 when read, the object's MIN-ACCESS in any revised compliance
 statement SHOULD be not-accessible if the MIB is updated.
 Further, for similar reasons, any future revisions to the RIPv2
 Management Information Base (MIB) SHOULD deprecate or omit any
 objects that would permit the writing of any RIPv2 Security
 Association or RIPv2 Security Association component (e.g., the
 cryptographic key).
 Also, it is RECOMMENDED that any future revisions to the RIPv2
 Management Information Base (MIB) consider adding MIB objects to hold
 information about any RIPv2 security events that might have occurred,
 and MIB objects that could be used to read the set of security events
 that have been logged by the RIPv2 subsystem.  For each security
 event mentioned in this document, it is also RECOMMENDED that
 appropriate notifications be included, with a MAX-ACCESS of
 Accessible-for-notify, in any future versions of the RIPv2 MIB
 module.

5.3. Key Management Considerations

 For the past several years, manual configuration (e.g., via a
 console) has been commonly used to create and modify RIPv2 Security
 Associations.  There are a number of large-scale RIP deployments
 today that successfully use manual configuration of RIPv2 Security
 Associations.  There are also sites that use scripts (e.g., combining
 Tcl/Expect, PERL, and SSHv2) with a site-specific configuration
 database and secure console connections to dynamically manage all
 aspects of their router configurations, including their RIPv2
 Security Associations.  This last approach is similar to the current
 IETF approach to Network Configuration (NetConf) standards.

Atkinson & Fanto Standards Track [Page 17] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 Recent IETF Multicast Security (MSEC) working group efforts into
 multicast key management appear promising.  Several large RIPv2
 deployments happen to also have deployed the Kerberos authentication
 system.  Recent IETF work into the use of Kerberos for Internet Key
 Negotiation (KINK) also seems relevant; one might use Kerberos to
 support RIPv2 key management functions for use at sites that have
 already deployed Kerberos.  It is hoped that in the future the IETF
 will standardize a key management protocol suitable for managing
 RIPv2 Security Associations.

5.4. Assurance Considerations

 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 RIPv2 implementations.  This mechanism also depends on
 the RIPv2 Authentication Key being kept confidential by all parties.
 If any of these are incorrect or insufficiently secure, then no real
 security will be provided to the users of this mechanism.
 Use of high-assurance development methods is RECOMMENDED for
 implementations of this specification, in order to reduce the risk of
 subtle implementation flaws that might adversely impact the
 operational risk reduction that this specification seeks to provide.

5.5. Confidentiality and Traffic Analysis Considerations

 Confidentiality is not provided by this mechanism.  It is generally
 considered that an IP routing protocol does not require
 confidentiality, as the purpose of any routing protocols is to
 disseminate information about the topology of the network.
 Protection against traffic analysis is also not provided.  Mechanisms
 such as bulk link encryption SHOULD be used when protection against
 traffic analysis is required [CKHD89].

5.6. Other Security Considerations

 Separately, the receipt of a RIPv2 packet using cryptographic
 authentication but containing an invalid or unknown Key-ID value
 might indicate an active attack on the RIP routing subsystem and is a
 significant security event.  Therefore, any actual receipt of a RIPv2
 packet using cryptographic authentication and containing an unknown,
 expired, or otherwise invalid KEY-ID value SHOULD cause a security
 event to be logged by the implementation.  This log item SHOULD
 include at least the fact that the invalid KEY-ID was received, the
 source IP address of the packet containing the invalid KEY-ID, the

Atkinson & Fanto Standards Track [Page 18] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 interface(s) the packet was received on, the KEY-ID received, and the
 current date/time.
 A subtle user-interface consideration also should be noted.  If a
 user interface only permits the entry of human-readable text (e.g., a
 password in US-ASCII format) for use as a cryptographic key,
 significant numbers of bits of the cryptographic key in use become
 predictable, thereby reducing the strength of the key in this
 context.  For this reason, implementations of this specification
 SHOULD support the entry of RIPv2 cryptographic authentication keys
 in hexadecimal format.

5.7. Future Security Directions

 Specification and deployment of a standards-track key management
 protocol that supports this RIPv2 cryptographic authentication
 mechanism would be a significant next step in operational risk
 reduction and might actually increase the ease of deployment and
 operation of this mechanism.  Such specification is beyond the scope
 of this document.  Recent IETF work in MSEC and KINK working groups
 appears promising in this regard.  Recent IETF work in the NETCONF
 working group towards standardizing methods for secure configuration
 management of routers is also relevant.
 Finally, we observe that this mechanism is not the final word on
 RIPv2 authentication.  Rather, it is believed that this particular
 mechanism represents a significant risk reduction over previous
 methods (e.g., plaintext passwords), while remaining straightforward
 to implement correctly and also straightforward to deploy.
 User communities that believe this mechanism is not adequate to their
 needs are encouraged to consider using digital signatures with RIPv2.
 [MBW97] specifies the use of OSPF with Digital signatures; that
 document might be a starting point for creating such a specification
 for the RIPv2 protocol.  Digital signatures are significantly more
 expensive computationally and are also significantly more difficult
 to deploy operationally, as compared with the mechanism specified
 here.  However, it appears likely that much of the mechanism in this
 document could be reused with digital signatures.

6. Acknowledgments

 Fred Baker was co-author of the earlier RIPv2 MD5 Authentication
 document [AB97].  This document is a direct derivative of that
 earlier document, though it has been significantly reworked.  The
 current authors would like to thank Bill Burr, Tim Polk, John Kelsey,
 and Morris Dworkin of (US) NIST for review of versions of this
 document.

Atkinson & Fanto Standards Track [Page 19] RFC 4822 RIPv2 Cryptographic Authentication February 2007

7. Normative References

 [BCP14]      Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [Mal98]      Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
              1998.
 [FIPS-180-2] National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002,
              <http://csrc.nist.gov/publications/fips/fips180-2/
              fips180-2.pdf>.
 [FIPS-198]   National Institute of Standards and Technology, "The
              Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB
              198, March 2002, <http://csrc.nist.gov/publications/
              fips/fips198/fips-198a.pdf>.

8. Informative References

 [AB97]       Baker, F. and R. Atkinson, "RIP-2 MD5 Authentication",
              RFC 2082, January 1997.
 [Bell89]     S. Bellovin, "Security Problems in the TCP/IP Protocol
              Suite", ACM Computer Communications Review, Volume 19,
              Number 2, pp. 32-48, April 1989.
 [CKHD89]     Cole Jr, Raymond, Donald Kallgren, Richard Hale, and
              John R. Davis, "Multilevel Secure Mixed-Media
              Communication Networks", Proceedings of the IEEE
              Military Communications Conference (MILCOM '89), IEEE,
              1989.
 [Dobb96a]    Dobbertin, H., "Cryptanalysis of MD5 Compress",
              Technical Report, 2 May 1996.  (Presented at Rump
              Session of EuroCrypt 1996.)
 [Dobb96b]    Dobbertin, H., "The Status of MD5 After a Recent
              Attack", CryptoBytes, Vol. 2, No. 2, Summer 1996.
 [ESC05]      Eastlake, D., 3rd, Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC
              4086, June 2005.
 [HA94]       Haller, N. and R. Atkinson, "On Internet
              Authentication", RFC 1704, October 1994.

Atkinson & Fanto Standards Track [Page 20] RFC 4822 RIPv2 Cryptographic Authentication February 2007

 [KMC97]      Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
              Keyed-Hashing for Message Authentication", RFC 2104,
              February 1997.
 [Mal94]      Malkin, G., "RIP Version 2 - Carrying Additional
              Information", RFC 1723, November 1994.
 [MB94]       Malkin, G. and F. Baker, "RIP Version 2 MIB Extension",
              RFC 1724, November 1994.
 [MBW97]      Murphy, S., Badger, M., and B. Wellington, "OSPF with
              Digital Signatures", RFC 2154, June 1997.
 [Rivest92]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC
              1321, April 1992.

Authors' Addresses

 R. Atkinson
 Extreme Networks
 3585 Monroe Street
 Santa Clara, CA 95051
 USA
 Phone: +1 (408) 579-2800
 EMail: rja@extremenetworks.com
 M. Fanto
 (US) National Institute of Standards and Technology
 Gaithersburg, MD 20878
 USA
 Phone: +1 (301) 975-2000
 EMail: mattjf@umd.edu
 Web:   http://csrc.nist.gov

Atkinson & Fanto Standards Track [Page 21] RFC 4822 RIPv2 Cryptographic Authentication February 2007

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Atkinson & Fanto Standards Track [Page 22]

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