    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 Notice

       Copyright (C) The Internet Society (2006).

    Abstract

       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 ....................................................2
       2. Algorithms and Identifiers ......................................2
       3. Specifying Truncation ...........................................3
          3.1. Truncation Specification ...................................4
       4. TSIG Truncation Policy and Error Provisions .....................4
       5. IANA Considerations .............................................5
       6. Security Considerations .........................................5
       7. Normative References ............................................6
       8. Informative References. .........................................7

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    1.  Introduction

       [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
       those algorithms.

       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].

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       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
       shorter hashes.

       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.

          Mandatory      HMAC-MD5.SIG-ALG.REG.INT
          Optional       gss-tsig
          Mandatory      hmac-sha1
          Optional       hmac-sha224
          Mandatory      hmac-sha256
          Optional       hamc-sha384
          Optional       hmac-sha512

       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.

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    RFC 4635          HMAC SHA TSIG Algorithm Identifiers        August 2006

    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

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    RFC 4635          HMAC SHA TSIG Algorithm Identifiers        August 2006

       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

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       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,
                   February 1997.

       [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.

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    RFC 4635          HMAC SHA TSIG Algorithm Identifiers        August 2006

    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.

    Author's Address

       Donald E. Eastlake 3rd
       Motorola Laboratories
       155 Beaver Street
       Milford, MA 01757 USA

       Phone: +1-508-786-7554 (w)
       EMail: Donald.Eastlake@motorola.com

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    RFC 4635          HMAC SHA TSIG Algorithm Identifiers        August 2006

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