Internet Draft M. Bhatia
<draft-ietf-ospf-hmac-sha-07.txt> Alcatel-Lucent
Category: Standards-Track V. Manral
Expires: 31 Jan 2010 IP Infusion
Updates: RFC 2328 M. Fanto
Aegis Data Security
R. White
Cisco Systems
T. Li
Ericsson
M. Barnes
Cisco Systems
R. Atkinson
Extreme Networks
31 August 2009
OSPFv2 HMAC-SHA Cryptographic Authentication
<draft-ietf-ospf-hmac-sha-07.txt>
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Abstract
This document describes how the NIST Secure Hash Standard family of
algorithms can be used with OSPF version 2's built-in cryptographic
authentication mechanism. This updates, but does not supercede,
the cryptographic authentication mechanism specified in RFC 2328.
1. INTRODUCTION
A variety of risks exist when depoying any routing
protocol.[Bell89] This document provides an update to OSPFv2
Cryptographic Authentication, which is specified in Appendix D
of RFC 2328. This document does not deprecate or supercede
RFC 2328. OSPFv2 itself is defined in RFC 2328. [RFC 2328]
This document adds support for Secure Hash Algorithms defined in
the US NIST Secure Hash Standard (SHS) as defined by NIST FIPS
180-2. [FIPS-180-2] includes SHA-1, SHA-224, SHA-256, SHA-384,
and SHA-512. The HMAC authentication mode defined in NIST FIPS
198 is used. [FIPS-198]
It is believed that [RFC 2104] is mathematically identical to
[FIPS-198] and also believed that algorithms in [RFC 4684] are
mathematically identical to [FIPS-180-2].
The creation of this addition to OSPFv2 was driven by operator
requests that they be able to use the NIST SHS family of
algorithms in the NIST HMAC mode, instead of being forced
to use the Keyed-MD5 algorithm and mode with OSPFv2 Cryptographic
Authentication. Cryptographic matters are discussed in more
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detail in the Security Considerations section of 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 RFC 2119. [RFC 2119]
2. Background
All OSPF protocol exchanges can be authenticated. The OSPF
packet header (see Section A.3.1 of RFC-2328) includes an
Authentication Type field, and 64-bits of data for use by
the appropriate authentication scheme (determined by the
type field).
The authentication type is configurable on a per-interface
(or equivalently, on a per-network/subnet) basis. Additional
authentication data is also configurable on a per-interface
basis.
OSPF Authentication types 0, 1, and 2 are defined by RFC 2328.
This document provides an update to RFC 2328 that is only
applicable to Authentication Type 2, "Cryptographic
Authentication".
3. Cryptographic authentication with NIST SHS in HMAC mode
Using this authentication type, a shared secret key is configured
in all routers attached to a common network/subnet. For each
OSPF protocol packet, the key is used to generate/verify a
"message digest" that is appended to the end of the OSPF packet.
The message digest is a one-way function of the OSPF protocol
packet and the secret key. Since the secret key is never sent
over the network in the clear, protection is provided against
passive attacks. [RFC 1704]
The algorithms used to generate and verify the message digest
are specified implicitly by the secret key. This specification
discusses the computation of OSPF Cryptographic Authentication
data when any of the NIST SHS family of algorithms is used in
the Hashed Message Authentication Code (HMAC) mode.
Please also see RFC 2328, Appendix D.
With the additions in this document, the currently valid algorithms
(including mode) for OSPFv2 Cryptographic Authentication include:
Keyed-MD5 (defined in RFC-2328, Appendix D)
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HMAC-SHA-1 (defined here)
HMAC-SHA-256 (defined here)
HMAC-SHA-384 (defined here)
HMAC-SHA-512 (defined here)
Of the above, implementations of this specification MUST
include support for at least:
HMAC-SHA-256
and SHOULD include support for:
HMAC-SHA-1
and SHOULD also (for backwards compatibility with existing
implementations and deployments) include support for:
Keyed-MD5
and MAY also include support for:
HMAC-SHA-384
HMAC-SHA-512
An implementation of this specification MUST allow network
operators to configure ANY authentication algorithm supported
by that implementation for use with ANY given Key-ID value
that is configured into that OSPFv2 router.
3.1. Generating Cryptographic Authentication
The overall cryptographic authentication process defined in
Appendix D of RFC 2328 remains unchanged. However, the specific
cryptographic details (i.e. SHA rather than MD5, HMAC rather
than Keyed-Hash) are defined herein. To reduce the potential for
confusion, this section minimises the repetition of text from RFC
2328, Appendix D, which is incorporated here by reference.[RFC
2328]
First, following the procedure defined in RFC 2328, Appendix D,
select the appropriate OSPFv2 Security Association for use with
this packet and set the Key-ID field to the KeyID value of that
OSPFv2 Security Association.
Second, set the Authentication Type to cryptographic
authentication, and set the Authentication Data Length field to
the length (measured in bytes, not bits) of the cryptographic
hash that will be used. When any NIST SHS algorithm is used in
HMAC mode with OSPFv2 Cryptographic Authentication, the
Authentication Data Length is equal to the normal hash output
length (measured in bytes) for the specific NIST SHS algorithm in
use. For example, with NIST SHA-256, the Authentication Data
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Length is 32 bytes.
Third, The 32-bit Cryptographic sequence number is set in
accordance with the procedures in RFC 2328, Appendix D
applicable to the Cryptographic Authentication type.
Fourth, The message digest is then calculated and appended
to the OSPF packet, as described below in Section 3.3. The
KeyID, Authentication Algorithm, and Key to be used for
calculating the digest are all components of the selected
OSPFv2 Security Association. Input to the authentication
algorithm consists of the OSPF packet and the secret key.
3.2 OSPFv2 Security Association
This document uses the term OSPFv2 Security Association
(OSPFv2 SA) to refer to the authentication key information
defined in Section D.3, pages 228 and 229, of RFC 2328.
The OSPFv2 protocol does not include an in-band mechanism
to create or manage OSPFv2 Security Associations. The
parameters of an OSPFv2 Security Association are updated
to be:
Key Identifier (KeyID)
This is an 8-bit unsigned value used to
uniquely identify an OSPFv2 SA and is
configured either by the router administrator
(or, in the future, possibly by some key
management protocol specified by the
IETF). The receiver uses this to locate
the appropriate OSPFv2 SA to use. The
sender puts this KeyID value in the OSPF
packet based on the active OSPF configuration.
Authentication Algorithm
This indicates the authentication algorithm
(and also the cryptographic mode, such as HMAC)
to be used. This information SHOULD never be
sent over the wire in cleartext form.
At present valid values are: Keyed-MD5,
HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-384,
and HMAC-SHA-512.
Authentication Key
This is the cryptographic key used for
cryptographic authentication with this
OSPFv2 SA. This value SHOULD never be
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sent over the wire in cleartext form.
This is noted as "K" in Section 3.3 below.
Key Start Accept
The time that this OSPF router will accept
packets that have been created with this
OSPF Security Association.
Key Start Generate
The time that this OSPF router will begin
using this OSPF Security Association for
OSPF packet generation.
Key Stop Generate
The time that this OSPF router will stop
using this OSPF Security Association for
OSPF packet generation.
Key Stop Accept
The time that this OSPF router will stop
accepting packets generated with this
OSPF Security Association.
In order to achieve smooth key transition, KeyStartAccept SHOULD
be less than KeyStartGenerate and KeyStopGenerate SHOULD be less
than KeyStopAccept. If KeyStopGenerate and KeyStopAccept are left
unspecified, the key's lifetime is infinite. When a new key
replaces an old, the KeyStartGenerate time for the new key MUST
be less than or equal to the KeyStopGenerate time of the old key.
Key storage SHOULD persist across a system restart, warm or cold,
to avoid operational issues. 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.
3.3 Cryptographic Aspects
This describes the computation of the Authentication Data value
when any NIST SHS algorithm is used in the HMAC mode with OSPFv2
Cryptographic Authentication.
In the algorithm description below, the following nomenclature,
which is consistent with [FIPS-198], is used:
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H is the specific hashing algorithm (e.g. SHA-256).
K is the authentication key for the OSPFv2 security
association.
Ko is the cryptographic key used with the hash algorithm.
B is the block size of H, measured in octets,
rather than bits. Note well 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.
Implementation note:
This definition of Apad means that Apad always
is the same length as the hash output.
(1) PREPARATION OF 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 OSPFv2 packet's Authentication Trailer, which
is the appendage described in RFC 2328, Section D.4.3,
Page 233, items (6)(a) and (6)(d), is filled with the value
Apad, and the Authentication Type field is set to 2.
Then, a first hash, also known as the inner hash, is computed
as follows:
First-Hash = H(Ko XOR Ipad || (OSPFv2 Packet))
Implementation Notes:
Note that the First-Hash above includes the Authentication
Trailer containing the Apad value, as well as the OSPF
packet, as per RFC 2328, Section D.4.3.
The definition of Apad (above) ensures it is always the same
length as the hash output. This is consistent with RFC 2328.
The "(OSPFv2 Packet)" mentioned in the First Hash (above)
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does include the OSPF Authentication Trailer.
The digest length for SHA-1 is 20 bytes, for SHA-256 is
32 bytes, for SHA-384 is 48 bytes, and for SHA-512 is
64-bytes.
(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 Trailer of the OSPFv2 packet.
The length of the Authentication Trailer is always identical
to the message digest size of the specific hash function H
that is being used.
This also means that the use of hash functions with larger
output sizes will also increase the size of the OSPFv2 packet
as transmitted on the wire.
Implementation Note:
RFC 2328, Appendix D specifies that the Authentication
Trailer is not counted in the OSPF packet's own length
field, but is included in the packet's IP length field.
3.4. Message verification
Message verification follows the procedure defined in RFC 2328,
except that the cryptographic calculation of the message digest
follows the procedure in Section 3.3 above when any NIST SHS
algorithm in the HMAC mode is in use. Kindly recall that the
cryptographic algorithm/mode in use is indicated implicitly
by the Key-ID of the received OSPFv2 packet.
Implementation Notes:
One must save the received digest value before calculating
the expected digest value, so that after that calculation
the received value can be compared with the expected
value to determine whether to accept that OSPF packet.
RFC 2328, Section D.4.3 (6) (c) should be read very
closely prior to implementing the above. With SHA
algorithms in HMAC mode, Apad is placed where the MD5
key would be put if Keyed-MD5 were in use.
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3.5 Changing OSPFv2 Security Associations
Using KeyIDs makes changing the active OSPFv2 SA convenient.
An implementation can choose to associate a lifetime with
each OSPFv2 SA and can thus automatically switch to a different
OSPFv2 SA based on the lifetimes of the configured OSPFv2 SA(s).
After changing the active OSPFv2 SA, the OSPF sender will use
the (different) KeyID value associated with the newly active
OSPFv2 SA. The receiver will use this new KeyID to select
the appropriate (new) OSPFv2 SA to use with the received OSPF
packet containing the new KeyID value.
Because the KeyID field is present, the receiver does not need
to try all configured OSPFv2 Security Associations with any
received OSPFv2 packet. This can mitigate some of the risks
of a Denial-of-Service attack on the OSPF instance, but does
not entirely prevent all conceivable DoS attacks. For example,
an on-link adversary still could generate OSPFv2 packets that
are synactically valid, but contain invalid Authentication
Data, thereby forcing the receiver(s) to perform expensive
cryptographic computations to discover that the packets are
invalid.
4. Security Considerations
This document enhances the security of the OSPFv2 routing
protocol by adding support for the algorithms defined in
the NIST Secure Hash Standard (SHS) using the Hashed
Message Authentication Code (HMAC) mode to the existing
OSPFv2 Cryptographic Authentication method, and support
for the Hashed Message Authentication Code (HMAC) mode.
This provides several alternatives to the existing Keyed-MD5
mechanism. There are published concerns about the overall
strength of the MD5 algorithm. [Dobb96a, Dobb96b, Wang04]
While those published concerns apply to the use of MD5 in
other modes (e.g. use of MD5 X.509v3/PKIX digital certificates),
they are not an attack upon Keyed-MD5, which is what OSPFv2
specified in RFC 2328. There are also published concerns
about the SHA algorithm [Wang05] and also concerns about
the MD5 and SHA algorithms in the HMAC mode [RR07, RR08].
Separately, some organisations (e.g. US Government)
prefer NIST algorithms, such as the SHA family, over
other algorithms for local policy reasons.
The value Apad is used here primarily for consistency with
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IETF specifications for HMAC-SHA authentication of RIPv2 SHA
[RFC 4822] and IS-IS SHA [RFC 5310] and to minimise OSPF
protocol processing changes in Section D.4.3 of RFC 2328.
[RFC 2328]
The quality of the security provided by the Cryptographic
Authentication option depends completely on the strength
of the cryptographic algorithm and cryptographic mode in use,
the strength of the key being used, and the correct
implementation of the security mechanism in all communicating
OSPF implementations. Accordingly, the use of high assurance
development methods is recommended. It also requires that
all parties maintain the secrecy of the shared secret key.
[RFC 4086] provides guidance on methods for generating
cryptographically random bits.
This mechanism is vulnerable to a replay attack by any on-link
node. An on-link node could record a legitimate OSPF packet
sent on the link, then replay that packet at the next time
the recorded OSPF packet's sequence number is valid. This
replay attack could cause significant routing disruptions
within the OSPF domain.
Ideally, for example to prevent the preceding attack, each
OSPF Security Association would be replaced by a new and
different OSPF Security Association before any sequence number
were reused. As of the date this document was published,
no form of automated key management has been standardised
for OSPF. So, as of the date this document was published,
common operational practice has been to use the same OSPF
authentication key for very long periods of time. This
operational practice is undesirable for many reasons.
Therefore, it is clearly desirable to develop and
standardise some automated key management mechanism for
OSPF.
Because all of the currently specified algorithms use
symmetric cryptography, one cannot authenticate precisely
which OSPF router sent a given packet. However, one can
authenticate that the sender knew the OSPF Security
Association (including the OSPFv2 SA's parameters)
currently in use.
Because a routing protocol contains information that need
not be kept secret, privacy is not a requirement. However,
authentication of the messages within the protocol is of
interest, to reduce the risk of an adversary compromising
the routing system by deliberately injecting false
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information into the routing system.
The technology in this document enhances an authentication
mechanism for OSPFv2. The mechanism described here is not
perfect and need not be perfect. Instead, this mechanism
represents a significant increase in the work function of
an adversary attacking OSPFv2, as compared with plain-text
authentication or null authentication, while not causing
undue implementation, deployment, or operational complexity.
Denial of service attacks are not generally preventable
in a useful networking protocol. [VK83]
Because of implementation considerations, including the
need for backwards compatibility, this specification uses
the same mechanism as specified in RFC 2328 and limits
itself to adding support for additional cryptographic hash
functions. Also, some large network operators have indicated
they prefer to retain the basic mechanism defined in RFC 2328,
rather than migrate to IP Security, due to deployment and
operational considerations. If all the OSPFv2 routers
supported IPsec, then IPsec tunnels could be used in lieu
of this mechanism.[RFC 4301] This would, however, relegate
the topology to point-to-point adjacencies over the mesh
of IPsec tunnels.
If a stronger authentication were believed to be required,
then the use of a full digital signature [RFC 2154] would be
an approach that should be seriously considered. Use of full
digital signatures would enable precise authentication of the
OSPF router originating each OSPF link-state advertisement,
and thereby provide much stronger integrity protection for
the OSPF routing domain.
5. IANA CONSIDERATIONS
The OSPF Authentication Codes registry entry for Cryptographic
Authentication (Registry Code 2) must be updated to refer to
this document as well as RFC 2328.
6. ACKNOWLEDGEMENTS
The authors would 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 [RFC 4822].
David Black, Nevil Brownlee, Acee Lindem, and Hilarie Orman (in
alphabetical order by last name) provided feedback on earlier
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versions of this document. That feedback has greatly improved
both the technical content and the readability of the current
draft.
Henrik Levkowetz's Internet Draft tools were very helpful
in preparing this draft and are much appreciated.
7. REFERENCES
7.1 Normative References
[FIPS-180-2] US National Institute of Standards & Technology,
"Secure Hash Standard (SHS)", FIPS PUB 180-2,
August 2002.
[FIPS-198] US National Institute of Standards & Technology,
"The Keyed-Hash Message Authentication Code (HMAC)",
FIPS PUB 198, March 2002.
[RFC 2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP-14, March 1997.
[RFC 2328] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
7.2 Informative References
[Bell89] S. Bellovin, "Security Problems in the TCP/IP Protocol
Suite", ACM Computer Communications Review, Volume 19,
Number 2, pp. 32-48, April 1989.
[Dobb96a] Dobbertin, H, "Cryptanalysis of MD5 Compress",
Technical Report, 2 May 1996. (Presented at the
Rump Session of EuroCrypt 1996.)
[Dobb96b] Dobbertin, H, "The Status of MD5 After a Recent
Attack", CryptoBytes, Vol. 2, No. 2, Summer 1996.
[RFC 1704] N. Haller and R. Atkinson, "On Internet
Authentication", RFC 1704, October 1994.
[RFC 2104] Krawczyk, H. et alia, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104,
February 1997.
[RFC 2154] Murphy, S., Badger, M. and B. Wellington,
"OSPF with Digital Signatures", RFC 2154, June 1997.
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[RFC 4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP-106,
RFC 4086, June 2005.
[RFC 4301] Kent, S. & K. Seo, "Security Architecture for
the Internet Protocol", RFC 4301, December 2005.
[RFC 4684] Eastlake 3rd, D., & T. Hansen, "US Secure Hash
Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006.
[RFC 4822] R. Atkinson, M. Fanto, "RIPv2 Cryptographic
Authentication", RFC 4822, February 2007.
[RFC 5310] M. Bhatia, V. Manral, T. Li, R. Atkinson, R. White,
& M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, February 2009.
[RR07] Rechberger, Christian & Vincent Rijmen, "On
Authentication with HMAC and Non-random Properties",
Financial Cryptography and Data Security,
Lecture Notes in Computer Science, Volume 4886/2008,
Springer-Verlag, Berlin, December 2007.
[RR08] Rechberger, Christian & Vincent Rijmen, "New
Results on NMAC/HMAC when Instantiated with Popular
Hash Functions", Journal of Universal Computer Science,
Volume 14, Number 3, pp. 347-376, 1 February 2008.
[VK83] Voydock, V. and S. Kent, "Security Mechanisms in
High-level Networks", ACM Computing Surveys,
Vol. 15, No. 2, June 1983.
[Wang04] Wang, X. et alia, "Collisions for Hash Functions MD4,
MD5, HAVAL-128, and RIPEMD", August 2004, IACR.
http://eprint.iacr.org/2004/199
[Wang05] Wang, X. et alia, "Finding Collisions in the Full SHA-1"
Proceedings of Crypto 2005, Lecture Notes in Computer
Science, Volume 3621, pp. 17-36, Springer-Verlag, Berlin,
August 31, 2005.
AUTHORS
Manav Bhatia
Alcatel-Lucent
Bangalore,
India
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EMail: manav@alcatel-lucent.com
Vishwas Manral
IP Infusion
Almora, Uttarakhand
India
EMail: vishwas@ipinfusion.com
Matthew J. Fanto
Aegis Data Security
Dearborn, MI
USA
EMail: mfanto@aegisdatasecurity.com
Russ I. White
Cisco Systems
7025 Kit Creek Road
P.O. Box 14987
RTP, NC
27709 USA
EMail: riw@cisco.com
Tony Li
Ericsson
300 Holger Way
San Jose, CA
95134 USA
Email: tony.li@tony.li
M. Barnes
Cisco Systems
225 West Tasman Drive
San Jose, CA
95134 USA
Email: mjbarnes@cisco.com
Randall J. Atkinson
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Extreme Networks
3585 Monroe Street
Santa Clara, CA
95051 USA
Phone: +1 (408) 579-2800
EMail: rja@extremenetworks.com
Expires: 31 JAN 2010
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