

INTERNET-DRAFT                                    Donald E. Eastlake 3rd
UPDATES RFC 2845                                   Motorola Laboratories
Expires: February 2005                                       August 2004


                  HMAC SHA TSIG Algorithm Identifiers
                  ---- --- ---- --------- -----------
                  <draft-ietf-dnsext-tsig-sha-00.txt>


Status of This Document

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

   This draft is intended to be become a Proposed Standard RFC.
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Abstract

   Use of the TSIG DNS resource record requires specification of a
   cryptographic message authentication code.  Currently identifiers
   have been specified only for the HMAC-MD5 and GSS TSIG algorithms.
   This document standardizes identifiers for additional HMAC SHA TSIG
   algorithms and standardizes how to specify the truncation of HMAC
   values.


Copyright Notice

   Copyright (C) The Internet Society 2004. All Rights Reserved.




D. Eastlake 3rd                                                 [Page 1]


INTERNET-DRAFT                                 HMAC-SHA TSIG Identifiers


Table of Contents

      Status of This Document....................................1
      Abstract...................................................1
      Copyright Notice...........................................1

      Table of Contents..........................................2

      1. Introduction............................................3

      2. Algorithms and Identifiers..............................4

      3. Specifying Truncation...................................5

      4. IANA Considerations.....................................6
      5. Security Considerations.................................6
      6. Copyright and Disclaimer................................6

      7. Normative References....................................7
      8. Informative References..................................7

      Authors Address............................................8
      Expiration and File Name...................................8





























D. Eastlake 3rd                                                 [Page 2]


INTERNET-DRAFT                                 HMAC-SHA TSIG Identifiers


1. Introduction

   [RFC 2845] specifies a TSIG Resource Record (RR) that can be used to
   authenticate DNS queries and responses. This RR contains a domain
   name syntax data item which names the authentication algorithm used.
   [RFC 2845] defines the HMAC-MD5.SIG-ALG.REG.INT name for
   authentication codes using the HMAC [RFC 2104] algorithm with the MD5
   [RFC 1321] hash algorithm. IANA has also registered "gss-tsig" as an
   identifier for TSIG authentication where the cryptographic operations
   are delegated to GSS [RFC 3645].

   In section 2, this document specifies additional names for TSIG
   authentication algorithms based on US NIST SHA algorithms and HMAC.

   In section 3, this document specifies the meaning 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 a longer length field is an error.

































D. Eastlake 3rd                                                 [Page 3]


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2. Algorithms and Identifiers

   TSIG Resource Records (RRs) [RFC 2845] 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 [RFC 2931]. In particular, SIG(0)
   can be used for transaction signatures.) Used with a strong hash
   function, HMAC [RFC 2104] 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 [RFC 1321].

   The use of SHA-1 [FIPS 180-1, RFC 3174], which is a 160 bit hash, as
   compared with the 128 bits for MD5, and additional hash algorithms in
   the SHA family [FIPS 180-2, RFC sha224] with 224, 256, 384, and 512
   bits, may be preferred in some case. Use of TSIG between a DNS
   resolver and server is by mutual agreement. That agreement can
   include the support of additional algorithms.

   For completeness in relation to HMAC based algorithms, the current
   HMAC-MD5.SIG-ALG.REG.INT identifier is included in the table below.
   Implementations which support TSIG MUST implement HMAC MD5, SHOULD
   implement HMAC SHA-1, and MAY implement gss-tsig and the other
   algorithms listed below.

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






















D. Eastlake 3rd                                                 [Page 4]


INTERNET-DRAFT                                 HMAC-SHA TSIG Identifiers


3. Specifying Truncation

   In some cases, it is reasonable to truncate the output of HMAC and
   use the truncated value for authentication. HMAC SHA-1 truncated to
   96 bits is an optional available in several IETF protocols including
   IPSEC and TLS.

   The TSIG RR [RFC 2845] includes a "MAC size" field, which gives the
   size of the MAC field in octets. But [RFC 2845] does not specify what
   to do if this MAC size differs from the length of the output of HMAC
   for a particular hash function.

   The specification for TSIG handling is changed as follows:

   1. If The "MAC size" field is larger than the HMAC output length or
      is zero: 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 the "MAC size" field equals the HMAC output length: Operation
      is as described in [RFC 2845].

   3. If the "MAC size" field is less than the HMAC output length but is
      not zero: This is sent when the signer has truncated the HMAC
      output 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 compared for authentication.

   TSIG implementations SHOULD implement SHA-1 truncated to 96 bits (12
   octets) and MAY implement any or all other truncations valid under
   case 3 above.



















D. Eastlake 3rd                                                 [Page 5]


INTERNET-DRAFT                                 HMAC-SHA TSIG Identifiers


4. IANA Considerations

   This document, on approval for publication as a standards track RFC,
   registers the new TSIG algorithm identifiers listed in Section 2 with
   IANA.



5. Security Considerations

   For all of the message authentication code algorithms listed herein,
   those producing longer values are believed to be stronger; however,
   while there are 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 force. See [RFC 2104] which recommends
   that ah HMAC never be truncated to less than half its length nor to
   less than 80 bits (10 octets).

   See also the Security Considerations section of [RFC 2845].



6. Copyright and Disclaimer

   Copyright (C) The Internet Society 2004.  This document is subject to
   the rights, licenses and restrictions contained in BCP 78 and except
   as set forth therein, the authors retain all their rights.


   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.















D. Eastlake 3rd                                                 [Page 6]


INTERNET-DRAFT                                 HMAC-SHA TSIG Identifiers


7. Normative References

   [FIPS 180-2] - "Secure Hash Standard", (SHA-1/256/384/512) US Federal
   Information Processing Standard, Draft, 1 August 2002.

   [RFC 1321] - Rivest, R., "The MD5 Message-Digest Algorithm ", RFC
   1321, April 1992.

   [RFC 2104] - Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
   Hashing for Message Authentication", RFC 2104, February 1997.

   [RFC 2434] - Narten, T. and H. Alvestrand, "Guidelines for Writing an
   IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [RFC 2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
   Wellington, "Secret Key Transaction Authentication for DNS (TSIG)",
   RFC 2845, May 2000.

   [RFC sha224] - "A 224-bit One-way Hash Function: SHA-224", R.
   Housley, December 2003, work in progress, draft-ietf-pkix-
   sha224-*.txt.



8. Informative References.

   [FIPS 180-1] - Secure Hash Standard, (SHA-1) US Federal Information
   Processing Standard, 17 April 1995.

   [RFC 2931] - Eastlake 3rd, D., "DNS Request and Transaction
   Signatures ( SIG(0)s )", RFC 2931, September 2000.

   [RFC 3174] - Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm
   1 (SHA1)", RFC 3174, September 2001.

   [RFC 3645] - 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.













D. Eastlake 3rd                                                 [Page 7]


INTERNET-DRAFT                                 HMAC-SHA TSIG Identifiers


Authors Address

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

   Telephone:   +1-508-786-7554 (w)
                +1-508-634-2066 (h)
   EMail:       Donald.Eastlake@motorola.com



Expiration and File Name

   This draft expires in February 2005.

   Its file name is draft-ietf-dnsext-tsig-sha-00.txt


































D. Eastlake 3rd                                                 [Page 8]


