Network Working Group R. Atkinson

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.

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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

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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.

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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.

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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

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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.

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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.

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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

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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:

- a message with a sequence number of zero.

- a message with a higher sequence number than the last
received sequence number.

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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].

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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.

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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.

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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.

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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].

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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

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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.

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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.

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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

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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.

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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: [email protected]

M. Fanto
(US) National Institute of Standards and Technology
Gaithersburg, MD 20878
USA

Phone: +1 (301) 975-2000
EMail: [email protected]
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]