Network Working Group M. Bhatia
Request for Comments: 5310 Alcatel-Lucent
Category: Standards Track V. Manral
Redback Networks Inc.
Aegis Data Security
February 2009 IS-IS Generic 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 (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document proposes an extension to Intermediate System to
Intermediate System (IS-IS) to allow the use of any cryptographic
authentication algorithm in addition to the already-documented
authentication schemes, described in the base specification and RFC
5304. IS-IS is specified in International Standards Organization
(ISO) 10589, with extensions to support Internet Protocol version 4
(IPv4) described in RFC 1195.
Although this document has been written specifically for using the
Hashed Message Authentication Code (HMAC) construct along with the
Secure Hash Algorithm (SHA) family of cryptographic hash functions,
the method described in this document is generic and can be used to
extend IS-IS to support any cryptographic hash function in the
Table of Contents
1. Introduction ....................................................21.1. Conventions Used in This Document ..........................32. IS-IS Security Association ......................................33. Authentication Procedures .......................................43.1. Authentication TLV .........................................43.2. Authentication Process .....................................53.3. Cryptographic Aspects ......................................53.4. Procedures at the Sending Side .............................73.5. Procedure at the Receiving Side ............................84. Security Considerations .........................................85. Acknowledgments .................................................96. IANA Considerations ............................................107. References .....................................................107.1. Normative References ......................................107.2. Informative References ....................................111. Introduction
The Intermediate System to Intermediate System (IS-IS) specification
([ISO], [RFC1195]) allows for authentication of its Protocol Data
Units (PDUs) via the authentication TLV 10 that is carried as a part
of the PDU. The base specification has provision for only cleartext
passwords and RFC 5304 [RFC5304] augments this to provide the
capability to use Hashed Message Authentication Code - Message Digest
5 (HMAC-MD5) authentication for its PDUs.
The first octet of the value field of TLV 10 specifies the type of
authentication to be carried out. Type 0 is reserved, Type 1
indicates a cleartext password, Type 54 indicates HMAC MD5, and Type
255 is used for routing domain private authentication methods. The
remainder of the value field contains the actual authentication data,
determined by the value of the authentication type.
This document proposes a new authentication type to be carried in TLV
10, called the generic cryptographic authentication (CRYPTO_AUTH).
This can be used to specify any authentication algorithm for
authenticating and verifying IS-IS PDUs.
This document also explains how HMAC-SHA authentication can be used
By definition, HMAC ([RFC2104], [FIPS-198]) requires a cryptographic
hash function. We propose to use any one of SHA-1, SHA-224, SHA-256,
SHA-384, or SHA-512 [FIPS-180-3] to authenticate the IS-IS PDUs.
We propose to do away with the per-interface keys and instead have
Key IDs that map to unique IS-IS Security Associations (SAs).
While at the time of this writing there are no openly published
attacks on the HMAC-MD5 mechanism, some reports ([Dobb96a],
[Dobb96b]) create concern about the ultimate strength of the MD5
cryptographic hash function.
The mechanism described in this document does not provide
confidentiality, since PDUs are sent in the clear. However, the
objective of a routing protocol is to advertise the routing topology,
and confidentiality is not normally required for routing protocols.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. IS-IS Security Association
An IS-IS Security Association contains a set of parameters shared
between any two legitimate IS-IS speakers.
Parameters associated with an IS-IS SA:
o Key Identifier (Key ID): This is a two-octet unsigned integer used
to uniquely identify an IS-IS SA, as manually configured by the
The receiver determines the active SA by looking at the Key ID
field in the incoming PDU.
The sender, based on the active configuration, selects the
Security Association to use and puts the correct Key ID value
associated with the Security Association in the IS-IS PDU. If
multiple valid and active IS-IS Security Associations exist for a
given outbound interface at the time an IS-IS PDU is sent, the
sender may use any of those Security Associations to protect the
Using Key IDs makes changing keys while maintaining protocol
operation convenient. Each Key ID specifies two independent
parts: the authentication protocol and the authentication key,
explained below. Normally, an implementation would allow the
network operator to configure a set of keys in a key chain, with
each key in the chain having a fixed lifetime. The actual
operation of these mechanisms is outside the scope of this
Note that each Key ID can indicate a key with a different
authentication protocol. This allows multiple authentication
mechanisms to be used at various times without disrupting an IS-IS
peering, including the introduction of new authentication
o Authentication Algorithm: This signifies the authentication
algorithm to be used with the IS-IS SA. 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: HMAC-SHA-1, HMAC-SHA-224, HMAC-SHA-
256, HMAC-SHA-384, and HMAC-SHA-512.
o Authentication Key: This value denotes the cryptographic
authentication key associated with the IS-IS SA. The length of
this key is variable and depends upon the authentication algorithm
specified by the IS-IS SA.
3. Authentication Procedures
3.1. Authentication TLV
A new authentication code, 3, indicates that the CRYPTO_AUTH
mechanism described in this document is in use and is inserted in the
first octet of the existing IS-IS Authentication TLV (10).
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
| Type 10 |
| Length |
| Auth Type 3 |
| Key ID |
| Authentication Data (Variable)|
Figure 13.2. Authentication Process
When calculating the CRYPTO_AUTH result for Sequence Number PDUs,
Level 1 Sequence Number PDUs SHALL use the Area Authentication
string, as in Level 1 Link State PDUs. Level 2 Sequence Number PDUs
shall use the domain authentication string, as in Level 2 Link State
IS-IS HELLO PDUs SHALL use the Link Level Authentication string,
which MAY be different from that of Link State PDUs. The CRYPTO_AUTH
result for the IS-IS HELLO PDUs SHALL be calculated after the PDU is
padded to the MTU size, if padding is not disabled. Implementations
that support the optional checksum for the Sequence Number PDUs and
IS-IS HELLO PDUs MUST NOT include the Checksum TLV.
3.3. Cryptographic Aspects
In the algorithm description below, the following nomenclature, which
is consistent with [FIPS-198] is used:
H is the specific hashing algorithm (e.g., SHA-256).
K is the password for the PDU type as per the International
Standard ISO/IEC 10589 [ISO].
Ko is the cryptographic key used with the hash algorithm.
B is the block size of H, measured in octets rather than 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 rather than bits.
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.
(1) Preparation of the Key
In this application, Ko is always L octets long.
If the Authentication Key (K) is L octets long, then Ko is equal
to K. If the Authentication Key (K) is more than L octets long,
then Ko is set to H(K). If the Authentication Key (K) is less
than L octets long, then Ko is set to the Authentication Key (K)
with zeros appended to the end of the Authentication Key (K)
such that Ko is L octets long.
(2) First Hash
First, the IS-IS packet's Authentication Data field is filled
with the value Apad, and the Authentication Type field is set to
Then, a first hash, also known as the inner hash, is computed as
First-Hash = H(Ko XOR Ipad || (IS-IS PDU))
(3) Second Hash
Then a second hash, also known as the outer hash, is computed as
Second-Hash = H(Ko XOR Opad || First-Hash)
The resulting second hash becomes the authentication data that
is sent in the Authentication Data field of the IS-IS PDU. The
length of the Authentication Data field is always identical to
the message digest size of the specific hash function H that is
This also means that the use of hash functions with larger
output sizes will also increase the size of the IS-IS PDU as
transmitted on the wire.
3.4. Procedures at the Sending Side
An appropriate IS-IS SA is selected for use with an outgoing IS-IS
PDU. This is done based on the active key at that instant. If IS-IS
is unable to find an active key, then the PDU is discarded.
If IS-IS is able to find the active key, then the key provides the
authentication algorithm (HMAC-SHA-1, HMAC-SHA-224, HMAC-SHA-256,
HMAC-SHA-384, or HMAC-SHA-512) that needs to be applied on the PDU.
An implementation MUST fill the authentication type and the length
before the authentication data is computed. The authentication data
is computed as explained in the previous section. The length of the
TLV is set as per the authentication algorithm that is being used.
The length is set to 23 for HMAC-SHA-1, 31 for HMAC-SHA-224, 35 for
HMAC-SHA-256, 51 for HMAC-SHA-384, and 67 for HMAC-SHA-512. Note
that two octets have been added to account for the Key ID and one
octet for the authentication type.
The Key ID is filled.
The Checksum and Remaining Lifetime fields are set to zero for the
Link State Packets (LSPs) before authentication is calculated.
The result of the authentication algorithm is placed in the
authentication data, following the Key ID.
The authentication data for the IS-IS IIH PDUs MUST be computed after
the IS-IS Hello (IIH) has been padded to the MTU size, if padding is
not explicitly disabled.
3.5. Procedure at the Receiving Side
The appropriate IS-IS SA is identified by looking at the Key ID from
the Authentication TLV 10 from the incoming IS-IS PDU.
Authentication-algorithm-dependent processing needs to be performed,
using the algorithm specified by the appropriate IS-IS SA for the
Before an implementation performs any processing, it needs to save
the values of the Authentication Value, the Checksum, and the
Remaining Lifetime fields.
It should then set the Authentication Value field with Apad and the
Checksum and Remaining Lifetime fields with zero before the
authentication data is computed. The calculated data is compared
with the received authentication data in the PDU, and the PDU is
discarded if the two do not match. In such a case, an error event
SHOULD be logged.
An implementation MAY have a transition mode where it includes
CRYPTO_AUTH information in the PDUs but does not verify this
information. This is provided as a transition aid for networks in
the process of migrating to the new CRYPTO_AUTH-based authentication
4. Security Considerations
This document proposes extensions to IS-IS that make it more secure
than what it is today. It does not provide confidentiality as a
routing protocol contains information that does not need to be kept
secret. It does, however, provide means to authenticate the sender
of the PDUs, which is of interest to us.
It should be noted that authentication method described in this
document is not being used to authenticate the specific originator of
a PDU, but is rather being used to confirm that the PDU has indeed
been issued by an intermediate system that had access to either the
area or domain password, depending upon the kind of PDU it is.
The mechanism described here is not perfect and does not need to be
perfect. Instead, this mechanism represents a significant increase
in the work function of an adversary attacking the IS-IS protocol,
while not causing undue implementation, deployment, or operational
The mechanism detailed in this document does not protect IS-IS
against replay attacks. An adversary could in theory replay old IIHs
and bring down the adjacency [CRYPTO] or replay old Complete Sequence
Number PDUs (CSNPs) and Partial Sequence Number PDUs (PSNPs) that
would cause a flood of LSPs in the network. Using some sort of
crypto sequence numbers in IS-IS IIHs and CSNP/PSNPs is an option to
solve this problem. Discussing this is beyond the scope of this
This document states that the remaining lifetime of the LSP MUST be
set to zero before computing the authentication, thus this field is
not authenticated. This field is excluded so that the LSPs may be
aged by the ISes in between, without requiring re-computation of the
authentication data. This can be exploited by an attacker.
There is a transition mode suggested where routers can ignore the
CRYPTO_AUTH information carried in the PDUs. The operator must
ensure that this mode is only used when migrating to the new
CRYPTO_AUTH-based authentication scheme, as this leaves the router
vulnerable to an attack.
To ensure greater security, the keys used should be changed
periodically, and implementations MUST be able to store and use more
than one key at the same time. Operators should ensure that the
authentication key is never sent over the network in cleartext via
any protocol. Care should also be taken to ensure that the selected
key is unpredictable, avoiding any keys known to be weak for the
algorithm in use. [RFC4086] contains helpful information on both key
generation techniques and cryptographic randomness.
It should be noted that the cryptographic strength of the HMAC
depends upon the cryptographic strength of the underlying hash
function and on the size and quality of the key.
If a stronger authentication were believed to be required, then the
use of a full digital signature [RFC2154] would be an approach that
should be seriously considered. It was rejected for this purpose at
this time because the computational burden of full digital signatures
is believed to be much higher than is reasonable given the current
threat environment in operational commercial networks.
The authors would like to thank Hugo Krawczyk, Arjen K. Lenstra (Bell
Labs), and Eric Grosse (Bell Labs) for educating us on some of the
finer points related to Crypto Mathematics.
We would also like to thank Bill Burr, Tim Polk, John Kelsey, and
Morris Dworkin of (US) NIST for review of portions of this document
that are directly derived from the closely related work on RIPv2
Cryptographic Authentication [RFC4822].
We would also like to mention Alfred Hoenes for his careful and
detailed review during the last call.
Lastly, we would like to acknowledge Brian and Stephen Eisenberg for
their continued support.
6. IANA Considerations
IANA has registered the value for the CRYPTO_AUTH method in the
"IS-IS Authentication Type Codes for TLV 10" subregistry established
by [RFC5304]. The value 3 denotes the CRYPTO_AUTH mechanism for
authenticating IS-IS PDUs.
| Authentication Type Code | Value | Reference |
| Cryptographic Authentication (CRYPTO_AUTH) | 3 | [RFC5310] |
7.1. Normative References
[FIPS-180-3] US National Institute of Standards & Technology,
"Secure Hash Standard (SHS)", FIPS PUB 180-3,
[FIPS-198] US National Institute of Standards & Technology, "The
Keyed-Hash Message Authentication Code (HMAC)", FIPS
PUB 198, March 2002.
[ISO] "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction
with the Protocol for providing the Connectionless-mode
Network Service (ISO 8473)", ISO/IEC 10589:1992.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2104] Krawczk, H., "HMAC: Keyed-Hashing for Message
Authentication", RFC 2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, February 2001.
[RFC5304] Li, T. and R. Atkinson, "Intermediate System to
Intermediate System (IS-IS) Cryptographic
Authentication", RFC 5304, October 2008.
7.2. Informative References
[CRYPTO] Vishwas, M., White, R., and M. Bhatia, "Issues with
existing Cryptographic Protection Methods for Routing
Protocols", Work in Progress, February 2008.
[Dobb96a] Dobbertin, H., "Cryptanalysis of MD5 Compress",
Technical Report, May 1996.
[Dobb96b] Dobbertin, H., "The Status of MD5 After a Recent
Attack", Cryptobytes, Volume 2, No 2, Summer 1996.
[RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, June 1997.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", RFC 4086, June 2005.
[RFC4822] Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
Authentication", RFC 4822, February 2007.
Redback Networks Inc.
300 Holger Way
San Jose, CA 95134
Randall J. Atkinson
3585 Monroe Street
Santa Clara, CA 95051
RTP North Carolina
Matthew J. Fanto
Aegis Data Security