DRAFT RIP-II Cryptographic Authentication Oct 1995
RIP-II Cryptographic Authentication
draft-ietf-ripv2-md5-02.txt |
Wed Oct 11 1995 |
Fred Baker
Cisco Systems
fred@cisco.com
Randall Atkinson
Naval Research Laboratory
atkinson@itd.nrl.navy.mil
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups. Note that other groups may also distribute working
documents as Internet Drafts.
Internet Drafts are valid for a maximum of six months and may be
updated, replaced, or obsoleted by other documents at any time. It is
inappropriate to use Internet Drafts as reference material or to cite
them other than as a "work in progress".
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1. Introduction
Growth in the Internet has made us aware of the need for improved
authentication of routing information. RIP-II 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-II, 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-II use an authentication algorithm, as in SNMP
Version 2, augmented by a sequence number. Keyed MD5 is proposed as |
the standard authentication algorithm for RIP-II, 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-II Authentication Key. This output is a one-way function of a
message and a secret RIP-II Authentication Key. This RIP-II
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 confidentiality algorithm would fail.
Other relevant rationales for the approach are that Keyed MD5 is used |
in SNMP Version 2, and is therefore present in routers already, as is
some form of password management. A similar approach has recently been |
standardised for use in IP-layer authentication [7]. |
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2. Implementation Approach
Implementation requires three issues to be addressed:
(1) A changed packet format,
(2) Authentication procedures, and
(3) Management controls.
2.1. RIP-II PDU Format
The basic RIP-II 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-II header to the record containing the
cryptogtaphic digest. This value effectively points to the end of
routing data in the packet..
(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-II 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 non-decreasing sequence number.
The trailer consists of the Authentication Data, which is the output of
the Keyed Message Digest Algorithm. When the Authentication Algorithm
is Keyed MD5, the output data is 16 bytes; during digest calculation, |
this is effectively followed by a pad field and a length field as
defined by RFC 1321. The field is contained in a record reminiscient
of other entiries, to be kind to ancient RIP implementations, but the
actual length of the digest varies by algorithm.
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2.2. 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-II Packet Length | Key ID | Auth Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (non-decreasing) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved must be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved must be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ (RIP-II Packet length-24) bytes Data /
| |
+---------------+---------------+-------------------------------+
| 0xFFFF | 0x01 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Authentication Data (var. length; 16 bytes with Keyed MD5) / |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MD5 algorithm logically appends the following information to the
packet during the MD5 calculation.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more pad bytes (defined by RFC 1321 when MD5 is used) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 bit message length MSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 bit message length LSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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2.2.1. Message Generation
The RIP-II Packet is created as usual, with these exceptions:
(1) The UDP checksum need not be calculated. If it is not, it MUST 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-II 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. If more than one key is currently
alive, then the youngest key (the key whose lifetime most recently
started) SHOULD be used. Since the key's algorithm is an attribute of
the key, 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-II header's packet length field indicates the standard
RIP-II portion of the packet.
(2) The Authentication Data Offset, Key Identifier, and Authentication
Data size fields are filled in appropriately.
(3) The RIP-II 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-II Authentication Key is never longer than the
output of the algorithm in use.
(4) Trailing pad and length fields are added and the digest calculated
using the indicated algorithm. When Keyed MD5 is the algorithm in use, |
these are calculated per RFC 1321.
(5) The digest is written over the RIP-II Authentication Key. When Keyed |
MD5 is used, this digest will be 16 bytes long.
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The trailing pad is not actually transmitted, as it is entirely
predictable from the message length and algorithm in use.
2.2.2. 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-II 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. The received sequence number must, of course,
be stored by neighbor and zeroed whenever it determines that
connectivity to the neighbor has been lost. Acceptable messages are now
truncated to RIP-II message itself and treated normally.
3. Management Procedures
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 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
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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.
Note that there are four "times" that are important with respect to a
key:
+ The time the system starts accepting received packets signed with
the key (KeyStartReceive).
+ The time the system starts signing packets with the key
(KeyStartSign).
+ The time the system stops signing packets with the key, which is to
say, the time it starts signing with the next key (KeyStopSign).
+ The time the system stops accepting received packets signed with the
key (KeyStopReceive).
The times SHOULD be in the order listed, which is to say that none of
these times occurs before the one mentioned before it. There needs to
be some distance between starts and between stops in order to get a
seamless transition. Each system sends with whichever key has the most
recent "start" time, and makes its first attempt at validation of
incoming traffic with the same key. If this validation fails and
another (older) key is also active, the system should attempt to
validate with any other active keys it may possess.
Note that the concept of a "key lifetime" does not require a hardware
time of day clock or the use of NTP, although one or the other is
advised; it merely requires that the earliest and latest times that the
key is valid must be programmable in a way the router understands.
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-II Authentication Keys among communicating RIP-II
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-II 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-II implementations should such a flaw be
discovered, integrated key management protocol techniques were
deliberately omitted from this specification.
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3.2. Key Management Procedures
As with all security methods using keys, it is necessary to change the
RIP-II Authentication Key on a regular basis. To maintain routing
stability during such changes, implementations are required to store and
support the use of more than one RIP-II 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-II
Authentication Key in use.
As noted above in Section 2.2.1, the party creating the RIP-II 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-II Authentication Key
rollovers without losing legitimate RIP-II 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-II system
must be updated with the new key several minutes before they 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-II 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.
Key storage SHOULD persist across a system restart, warm or cold, to
avoid operational issues. Key lifetime is an obvious issue, to be
solved by the implementation. Obvious solutions include the use of the
Network Time Protocol, hardware time of day clocks, or waiting some
period of time before emitting the initial RIP REQUEST to determine what
key other systems are signing with. The matter is left for the
implementor.
3.3. Pathological Cases
Two pathological cases exist which must be handled, which are failures
of the network manager. Both of these should be exceedingly rare.
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During key switchover, devices may exist which have not yet been
successfully configured with the new key. Therefore, routers MAY
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.
4. 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-II Authentication Keys once such a key
management protocol is standardized by the IETF.
5. Acknowledgments
This work was done by the RIP-II 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.
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6. References
[1] Malkin, Gary, "RIP Version 2 Carrying Additional Information", RFC
1388, January 1993.
[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC 1389,
Xylogics, Inc., Advanced Computer Communications, January 1993.
[4] S. Bellovin, "Security Problems in the TCP/IP Protocol Suite", ACM
Computer Communications Review, Volume 19, Number 2, pp.32-48,
April 1989.
[5] N. Haller, R. Atkinson, "On Internet Authentication", Request for
Comments 1704, DDN Network Information Center, October 1994.
[6] R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of IAB
Workshop on Security in the Internet Architecture", Request for
Comments 1636, DDN Network Information Center, June 1994.
[7] R. Atkinson, "IP Authentication Header", RFC-1826, August 1995. |
|
[8] R. Atkinson, "IP Encapsulating Security Payload", RFC-1827, |
August 1995. |
7. Security Considerations
This entire memo describes and specifies an authentication mechanism for
the RIP-II 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
even though such attacks are not currently widespread.
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-II implementations. This mechanism also depends on the RIP-II
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-II configuration
parameters (e.g. routing table, RIP-II Authentication Key) managable via
SNMP could be compromised as well. Changing Authentication Keys using
non-encrypted SNMP is no more secure than sending passwords in the
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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-II 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-II
Version 2 that was raised during the IAB's recent security review [6].
8. Chairman's Address
Gary Scott Malkin
Xylogics, Inc.
53 Third Avenue
Burlington, MA 01803
Phone: (617) 272-8140
EMail: gmalkin@Xylogics.COM
9. Author's Address
Fred Baker
Cisco Systems
519 Lado Drive
Santa Barbara, California 93111
Phone: (805) 681 0115
Email: fred@cisco.com
Randall Atkinson
Information Technology Division
Naval Research Laboratory
Washington, DC 20375-5337 |
Phone: (202) 404-8590 |
Email: atkinson@itd.nrl.navy.mil |
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Table of Contents
1 Introduction .................................................... 2
2 Implementation Approach ......................................... 3
2.1 RIP-II PDU Format ............................................. 3
2.2 Processing Algorithm .......................................... 4
2.2.1 Message Generation .......................................... 5
2.2.2 Message Reception ........................................... 6
3 Management Procedures ........................................... 6
3.1 Key Management Requirements ................................... 6
3.2 Key Management Procedures ..................................... 8
3.3 Pathological Cases ............................................ 8
4 Conformance Requirements ........................................ 9
5 Acknowledgments ................................................. 9
6 References ...................................................... 10
7 Security Considerations ......................................... 10
8 Chairman's Address .............................................. 11
9 Author's Address ................................................ 11
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