RFC2082 - RIP-2 MD5 Authentication

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

RIP-2 MD5 Authentication

Status of this Memo

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

Table of Contents

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

1. Use of Imperatives

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

MUST

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

MUST NOT

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

SHOULD

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

SHOULD NOT

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

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

2. Introduction

Growth in the Internet has made us aware of the need for improved
authentication of routing information. RIP-2 provides for
unauthenticated service (as in classical RIP), or password
authentication. Both are vulnerable to passive attacks currently
widespread in the Internet. Well-understood security issues exist in
routing protocols [4]. Clear text passwords, currently specified for
use with RIP-2, are no longer considered sufficient [5].

If authentication is disabled, then only simple misconfigurations are
detected. Simple passwords transmitted in the clear will further
protect against the honest neighbor, but are useless in the general
case. By simply capturing information on the wire - straightforward
even in a remote environment - a hostile process can learn the
password and overcome the network.

We propose that RIP-2 use an authentication algorithm, as was
originally proposed for SNMP Version 2, augmented by a sequence
number. Keyed MD5 is proposed as the standard authentication
algorithm for RIP-2, but the mechanism is intended to be algorithm-
independent. While this mechanism is not unbreakable (no known

mechanism is), it provides a greatly enhanced probability that a
system being attacked will detect and ignore hostile messages. This
is because we transmit the output of an authentication algorithm
(e.g., Keyed MD5) rather than the secret RIP-2 Authentication Key.
This output is a one-way function of a message and a secret RIP-2
Authentication Key. This RIP-2 Authentication Key is never sent over
the network in the clear, thus providing protection against the
passive attacks now commonplace in the Internet.

In this way, protection is afforded against forgery or message
modification. It is possible to replay a message until the sequence
number changes, but the sequence number makes replay in the long term
less of an issue. The mechanism does not afford confidentiality,
since messages stay in the clear; however, the mechanism is also
exportable from most countries, which test a privacy algorithm would
fail.

Other relevant rationales for the approach are that Keyed MD5 is
being used for OSPF cryptographic authentication, and is therefore
present in routers already, as is some form of password management.
A similar approach has been standardized for use in IP-layer
authentication. [7]

3. Implementation Approach

Implementation requires three issues to be addressed:

(1) A changed packet format,

(2) Authentication procedures, and

(3) Management controls.

3.1. RIP-2 PDU Format

The basic RIP-2 message format provides for an 8 byte header with an
array of 20 byte records as its data content. When Keyed MD5 is
used, the same header and content are used, except that the 16 byte
"authentication key" field is reused to describe a "Keyed Message
Digest" trailer. This consists in five fields:

(1) The "Authentication Type" is Keyed Message Digest Algorithm,
indicated by the value 3 (1 and 2 indicate "IP Route" and
"Password", respectively).

(2) A 16 bit offset from the RIP-2 header to the MD5 digest (if no
other trailer fields are ever defined, this value equals the
RIP-2 Data Length).

(3) An unsigned 8-bit field that contains the Key Identifier
or Key-ID. This identifies the key used to create the
Authentication Data for this RIP-2 message. In
implementations supporting more than one authentication
algorithm, the Key-ID also indicates the authentication
algorithm in use for this message. A key is associated with
an interface.

(4) An unsigned 8-bit field that contains the length in octets of the
trailing Authentication Data field. The presence of this field
permits other algorithms (e.g., Keyed SHA) to be substituted for
Keyed MD5 if desired.

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

The trailer consists of the Authentication Data, which is the output
of the Keyed Message Digest Algorithm. When the Authentication
Algorithm is Keyed MD5, the output data is 16 bytes; during digest
calculation, this is effectively followed by a pad field and a length
field as defined by RFC1321.

3.2. Processing Algorithm

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

0 1 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Command (1) | Version (1) | Routing Domain (2) |
+---------------+---------------+-------------------------------+
| 0xFFFF | AuType=Keyed Message Digest |
+-------------------------------+-------------------------------+
| RIP-2 Packet Length | Key ID | Auth Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (non-decreasing) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved must be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved must be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ (RIP-2 Packet Length - 24) bytes of Data /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xFFFF | 0x01 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Authentication Data (var. length; 16 bytes with Keyed MD5) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sixteen octets of MD5 "secret" |
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more pad bytes (defined by RFC1321 when MD5 is used) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 bit message length MSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 bit message length LSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.1. Message Generation

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

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

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

(3) The authentication "password" field is reused to store a packet
offset to the Authentication Data, a Key Identifier, the
Authentication Data Length, and a non-decreasing sequence number.

The value used in the sequence number is arbitrary, but two
suggestions are the time of the message's creation or a simple
message counter.

The RIP-2 Authentication Key is selected by the sender based on the
outgoing interface. Each key has a lifetime associated with it. No
key is ever used outside its lifetime. Since the key's algorithm is
related to the key itself, stored in the sender and receiver along
with it, the Key ID effectively indicates which authentication
algorithm is in use if the implementation supports more than one
authentication algorithm.

(1) The RIP-2 header's packet length field indicates the standard
RIP-2 portion of the packet.

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

(3) The RIP-2 Authentication Key, which is 16 bytes long when the
Keyed MD5 algorithm is used, is now appended to the data. For
all algorithms, the RIP-2 Authentication Key is never longer than
the output of the algorithm in use.

(4) Trailing pad and length fields are added and the digest
calculated using the indicated algorithm. When Keyed MD5 is the
algorithm in use, these are calculated per RFC1321.

(5) The digest is written over the RIP-2 Authentication Key. When
MD5 is used, this digest will be 16 bytes long.

The trailing pad is not actually transmitted, as it is entirely
predictable from the message length and algorithm in use.

3.2.2. Message Reception

When the message is received, the process is reversed:

(1) The digest is set aside,

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

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

(4) Appropriate padding is added as needed, and

(5) A new digest calculated using the indicated algorithm.

If the calculated digest does not match the received digest, the
message is discarded unprocessed. If the neighbor has been heard
from recently enough to have viable routes in the route table and the
received sequence number is less than the last one received, the
message likewise is discarded unprocessed. When connectivity to the
neighbor has been lost, the receiver SHOULD be ready to accept
either:
- a message with a sequence number of zero
- a message with a higher sequence number than the last received
sequence number.

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

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

4. Management Procedures

4.1. Key Management Requirements

It is strongly desirable that a hypothetical security breach in one
Internet protocol not automatically compromise other Internet
protocols. The Authentication Key of this specification SHOULD NOT
be stored using protocols or algorithms that have known flaws.

Implementations MUST support the storage of more than one key at the
same time, although it is recognized that only one key will normally
be active on an interface. They MUST associate a specific lifetime
(i.e., date/time first valid and date/time no longer valid) and a key
identifier with each key, and MUST support manual key distribution
(e.g., the privileged user manually typing in the key, key lifetime,
and key identifier on the router console). The lifetime may be
infinite. If more than one algorithm is supported, then the
implementation MUST require that the algorithm be specified for each
key at the time the other key information is entered. Keys that are
out of date MAY be deleted at will by the implementation without
requiring human intervention. Manual deletion of active keys SHOULD
also be supported.

It is likely that the IETF will define a standard key management
protocol. It is strongly desirable to use that key management
protocol to distribute RIP-2 Authentication Keys among communicating
RIP-2 implementations. Such a protocol would provide scalability and
significantly reduce the human administrative burden. The Key ID can
be used as a hook between RIP-2 and such a future protocol. Key
management protocols have a long history of subtle flaws that are
often discovered long after the protocol was first described in
public. To avoid having to change all RIP-2 implementations should
such a flaw be discovered, integrated key management protocol
techniques were deliberately omitted from this specification.

4.2. Key Management Procedures

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

Each key will have its own Key Identifier, which is stored locally.
The combination of the Key Identifier and the interface associated
with the message uniquely identifies the Authentication Algorithm and
RIP-2 Authentication Key in use.

As noted above in Section 2.2.1, the party creating the RIP-2 message
will select a valid key from the set of valid keys for that
interface. The receiver will use the Key Identifier and interface to
determine which key to use for authentication of the received
message. More than one key may be associated with an interface at
the same time.

Hence it is possible to have fairly smooth RIP-2 Authentication Key
rollovers without losing legitimate RIP-2 messages because the stored
key is incorrect and without requiring people to change all the keys
at once. To ensure a smooth rollover, each communicating RIP-2
system must be updated with the new key several minutes before the
current key will expire and several minutes before the new key
lifetime begins. The new key should have a lifetime that starts
several minutes before the old key expires. This gives time for each
system to learn of the new RIP-2 Authentication Key before that key
will be used. It also ensures that the new key will begin being used
and the current key will go out of use before the current key's
lifetime expires. For the duration of the overlap in key lifetimes,
a system may receive messages using either key and authenticate the
message. The Key-ID in the received message is used to select the
appropriate key for authentication.

4.3. Pathological Cases

Two pathological cases exist which must be handled, which are
failures of the network manager. Both of these should be exceedingly
rare.

During key switchover, devices may exist which have not yet been
successfully configured with the new key. Therefore, routers SHOULD
implement (and would be well advised to implement) an algorithm that
detects the set of keys being used by its neighbors, and transmits
its messages using both the new and old keys until all of the
neighbors are using the new key or the lifetime of the old key
expires. Under normal circumstances, this elevated transmission rate
will exist for a single update interval.

In the event that the last key associated with an interface expires,
it is unacceptable to revert to an unauthenticated condition, and not
advisable to disrupt routing. Therefore, the router should send a
"last authentication key expiration" notification to the network
manager and treat the key as having an infinite lifetime until the
lifetime is extended, the key is deleted by network management, or a
new key is configured.

5. Conformance Requirements

To conform to this specification, an implementation MUST support all
of its aspects. The Keyed MD5 authentication algorithm MUST be
implemented by all conforming implementations. MD5 is defined in
RFC-1321. A conforming implementation MAY also support other
authentication algorithms such as Keyed Secure Hash Algorithm (SHA).
Manual key distribution as described above MUST be supported by all
conforming implementations. All implementations MUST support the

smooth key rollover described under "Key Change Procedures."

The user documentation provided with the implementation MUST contain
clear instructions on how to ensure that smooth key rollover occurs.

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

6. Acknowledgments

This work was done by the RIP-2 Working Group, of which Gary Malkin
is the Chair. This suggestion was originally made by Christian
Huitema on behalf of the IAB. Jeff Honig (Cornell) and Dennis
Ferguson (ANS) built the first operational prototype, proving out the
algorithms. The authors gladly acknowledge significant inputs from
each of these sources.

7. References

[1] Malkin, G., "RIP Version 2 Carrying Additional Information",
RFC1388, January 1993.

[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April
1992.

[3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension",
RFC1389, Xylogics, Inc., Advanced Computer Communications,
January 1993.

[4] S. Bellovin, "Security Problems in the TCP/IP Protocol Suite",
ACM Computer Communications Review, Volume 19, Number 2,
pp.32-48, April 1989.

[5] Haller, N., and R. Atkinson, "Internet Authentication
Guidelines", RFC1704, October 1994.

[6] Braden, R., Clark, D., Crocker, S., and C. Huitema, "Report
of IAB Workshop on Security in the Internet Architecture",
RFC1636, June 1994.

[7] Atkinson, R., "IP Authentication Header", RFC1826, August 1995.

[8] Atkinson, R., "IP Encapsulating Security Payload", RFC1827,
August 1995.

8. Security Considerations

This entire memo describes and specifies an authentication mechanism
for the RIP-2 routing protocol that is believed to be secure against
active and passive attacks. Passive attacks are clearly widespread in
the Internet at present. Protection against active attacks is also
needed because active attacks are becoming more common.

Users need to understand that the quality of the security provided by
this mechanism depends completely on the strength of the implemented
authentication algorithms, the strength of the key being used, and
the correct implementation of the security mechanism in all
communicating RIP-2 implementations. This mechanism also depends on
the RIP-2 Authentication Key being kept confidential by all parties.
If any of these incorrect or insufficiently secure, then no real
security will be provided to the users of this mechanism.

Specifically with respect to the use of SNMP, compromise of SNMP
security has the necessary result that the various RIP-2
configuration parameters (e.g. routing table, RIP-2 Authentication
Key) manageable via SNMP could be compromised as well. Changing
Authentication Keys using non-encrypted SNMP is no more secure than
sending passwords in the clear.

Confidentiality is not provided by this mechanism. Recent work in
the IETF provides a standard mechanism for IP-layer encryption. [8]
That mechanism might be used to provide confidentiality for RIP-2 in
the future. Protection against traffic analysis is also not
provided. Mechanisms such as bulk link encryption might be used when
protection against traffic analysis is required.

The memo is written to address a security consideration in RIP
Version 2 that was raised during the IAB's recent security review
[6].

9. Chairman's Address

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

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

10. Authors' Addresses

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

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

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

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

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