Network Working Group D. Eastlake 3rd
Request for Comments: 4635 Motorola Laboratories
Category: Standards Track August 2006 HMAC SHA TSIG Algorithm Identifiers
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) The Internet Society (2006).
Use of the Domain Name System TSIG resource record requires
specification of a cryptographic message authentication code.
Currently, identifiers have been specified only for HMAC MD5 (Hashed
Message Authentication Code, Message Digest 5) and GSS (Generic
Security Service) TSIG algorithms. This document standardizes
identifiers and implementation requirements for additional HMAC SHA
(Secure Hash Algorithm) TSIG algorithms and standardizes how to
specify and handle the truncation of HMAC values in TSIG.
Table of Contents
1. Introduction ....................................................22. Algorithms and Identifiers ......................................23. Specifying Truncation ...........................................33.1. Truncation Specification ...................................44. TSIG Truncation Policy and Error Provisions .....................45. IANA Considerations .............................................56. Security Considerations .........................................57. Normative References ............................................68. Informative References. .........................................7
[RFC2845] specifies a TSIG Resource Record (RR) that can be used to
authenticate DNS (Domain Name System [STD13]) queries and responses.
This RR contains a domain name syntax data item that names the
authentication algorithm used. [RFC2845] defines the
HMAC-MD5.SIG-ALG.REG.INT name for authentication codes using the HMAC
(Hashed Message Authentication Code) [RFC2104] algorithm with the MD5
(Message Digest 5) [RFC1321] hash algorithm. IANA has also
registered "gss-tsig" as an identifier for TSIG authentication where
the cryptographic operations are delegated to the Generic Security
Service (GSS) [RFC3645].
Note that use of TSIG presumes prior agreement, between the resolver
and server involved, as to the algorithm and key to be used.
In Section 2, this document specifies additional names for TSIG
authentication algorithms based on US NIST SHA (United States,
National Institute of Science and Technology, Secure Hash Algorithm)
algorithms and HMAC and specifies the implementation requirements for
In Section 3, this document specifies the effect of inequality
between the normal output size of the specified hash function and the
length of MAC (Message Authentication Code) data given in the TSIG
RR. In particular, it specifies that a shorter-length field value
specifies truncation and that a longer-length field is an error.
In Section 4, policy restrictions and implications related to
truncation are described and specified, as is a new error code to
indicate truncation shorter than that permitted by policy.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "MAY", in
this document are to be interpreted as described in [RFC2119].
2. Algorithms and Identifiers
TSIG Resource Records (RRs) [RFC2845] are used to authenticate DNS
queries and responses. They are intended to be efficient symmetric
authentication codes based on a shared secret. (Asymmetric
signatures can be provided using the SIG RR [RFC2931]. In
particular, SIG(0) can be used for transaction signatures.) Used
with a strong hash function, HMAC [RFC2104] provides a way to
calculate such symmetric authentication codes. The only specified
HMAC-based TSIG algorithm identifier has been HMAC-MD5.SIG-
ALG.REG.INT, based on MD5 [RFC1321].
The use of SHA-1 [FIPS180-2, RFC3174], which is a 160-bit hash, as
compared with the 128 bits for MD5, and additional hash algorithms in
the SHA family [FIPS180-2, RFC3874, RFC4634] with 224, 256, 384, and
512 bits may be preferred in some cases. This is because
increasingly successful cryptanalytic attacks are being made on the
Use of TSIG between a DNS resolver and server is by mutual agreement.
That agreement can include the support of additional algorithms and
criteria as to which algorithms and truncations are acceptable,
subject to the restriction and guidelines in Sections 3 and 4 below.
Key agreement can be by the TKEY mechanism [RFC2930] or some other
mutually agreeable method.
The current HMAC-MD5.SIG-ALG.REG.INT and gss-tsig identifiers are
included in the table below for convenience. Implementations that
support TSIG MUST also implement HMAC SHA1 and HMAC SHA256 and MAY
implement gss-tsig and the other algorithms listed below.
SHA-1 truncated to 96 bits (12 octets) SHOULD be implemented.
3. Specifying Truncation
When space is at a premium and the strength of the full length of an
HMAC is not needed, it is reasonable to truncate the HMAC output and
use the truncated value for authentication. HMAC SHA-1 truncated to
96 bits is an option available in several IETF protocols, including
IPsec and TLS.
The TSIG RR [RFC2845] includes a "MAC size" field, which gives the
size of the MAC field in octets. However, [RFC2845] does not specify
what to do if this MAC size differs from the length of the output of
HMAC for a particular hash function. Truncation is indicated by a
MAC size less than the HMAC size, as specified below.
3.1. Truncation Specification
The specification for TSIG handling is changed as follows:
1. If "MAC size" field is greater than HMAC output length:
This case MUST NOT be generated and, if received, MUST cause
the packet to be dropped and RCODE 1 (FORMERR) to be returned.
2. If "MAC size" field equals HMAC output length:
Operation is as described in [RFC2845], and the entire output
HMAC output is present.
3. "MAC size" field is less than HMAC output length but greater than
that specified in case 4, below:
This is sent when the signer has truncated the HMAC output to
an allowable length, as described in RFC 2104, taking initial
octets and discarding trailing octets. TSIG truncation can only
be to an integral number of octets. On receipt of a packet with
truncation thus indicated, the locally calculated MAC is similarly
truncated and only the truncated values are compared for
authentication. The request MAC used when calculating the TSIG
MAC for a reply is the truncated request MAC.
4. "MAC size" field is less than the larger of 10 (octets) and half
the length of the hash function in use:
With the exception of certain TSIG error messages described in
RFC 2845, Section 3.2, where it is permitted that the MAC size be
zero, this case MUST NOT be generated and, if received, MUST cause
the packet to be dropped and RCODE 1 (FORMERR) to be returned.
The size limit for this case can also, for the hash functions
mentioned in this document, be stated as less than half the hash
function length for hash functions other than MD5 and less than 10
octets for MD5.
4. TSIG Truncation Policy and Error Provisions
Use of TSIG is by mutual agreement between a resolver and server.
Implicit in such "agreement" are criterion as to acceptable keys and
algorithms and, with the extensions in this document, truncations.
Note that it is common for implementations to bind the TSIG secret
key or keys that may be in place at a resolver and server to
particular algorithms. Thus, such implementations only permit the
use of an algorithm if there is an associated key in place. Receipt
of an unknown, unimplemented, or disabled algorithm typically results
in a BADKEY error.
Local policies MAY require the rejection of TSIGs, even though
they use an algorithm for which implementation is mandatory.
When a local policy permits acceptance of a TSIG with a particular
algorithm and a particular non-zero amount of truncation, it SHOULD
also permit the use of that algorithm with lesser truncation (a
longer MAC) up to the full HMAC output.
Regardless of a lower acceptable truncated MAC length specified by
local policy, a reply SHOULD be sent with a MAC at least as long as
that in the corresponding request, unless the request specified a MAC
length longer than the HMAC output.
Implementations permitting multiple acceptable algorithms and/or
truncations SHOULD permit this list to be ordered by presumed
strength and SHOULD allow different truncations for the same
algorithm to be treated as separate entities in this list. When so
implemented, policies SHOULD accept a presumed stronger algorithm and
truncation than the minimum strength required by the policy.
If a TSIG is received with truncation that is permitted under
Section 3 above but the MAC is too short for the local policy in
force, an RCODE of 22 (BADTRUNC) MUST be returned.
5. IANA Considerations
This document (1) registers the new TSIG algorithm identifiers listed
in Section 2 with IANA and (2) allocates the BADTRUNC RCODE 22 in
Section 4 [RFC2845].
6. Security Considerations
For all of the message authentication code algorithms listed herein,
those producing longer values are believed to be stronger; however,
while there have been some arguments that mild truncation can
strengthen a MAC by reducing the information available to an
attacker, excessive truncation clearly weakens authentication by
reducing the number of bits an attacker has to try to break the
authentication by brute force [RFC2104].
Significant progress has been made recently in cryptanalysis of hash
function of the types used herein, all of which ultimately derive
from the design of MD4. While the results so far should not effect
HMAC, the stronger SHA-1 and SHA-256 algorithms are being made
mandatory due to caution.
See the Security Considerations section of [RFC2845]. See also the
Security Considerations section of [RFC2104] from which the limits on
truncation in this RFC were taken.
7. Normative References
[FIPS180-2] "Secure Hash Standard", (SHA-1/224/256/384/512) US
Federal Information Processing Standard, with Change
Notice 1, February 2004.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC
1321, April 1992.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for
DNS (TSIG)", RFC 2845, May 2000.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm
1 (SHA1)", RFC 3174, September 2001.
[RFC3874] Housley, R., "A 224-bit One-way Hash Function: SHA-224",
RFC 3874, September 2004.
[RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA)", RFC 4634, July 2006.
[STD13] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
8. Informative References.
[RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000.
[RFC3645] Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J.,
and R. Hall, "Generic Security Service Algorithm for
Secret Key Transaction Authentication for DNS (GSS-
TSIG)", RFC 3645, October 2003.
Donald E. Eastlake 3rd
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-786-7554 (w)
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