rfc3230.txt

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   If other digest-algorithm values are defined, the associated encoding
   MUST either be represented as a quoted string, or MUST NOT include
   ";" or "," in the character sets used for the encoding.

4.2 Instance digests

   An instance digest is the representation of the output of a digest
   algorithm, together with an indication of the algorithm used (and any
   parameters).

       instance-digest = digest-algorithm "="
                               <encoded digest output>

   The digest is computed on the entire instance associated with the
   message.  The instance is a snapshot of the resource prior to the
   application of of any instance manipulation or transfer-coding (see
   section 3).  The byte order used to compute the digest is the
   transmission byte order defined for the content-type of the instance.








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      Note: the digest is computed before the application of any
      instance manipulation.  If a range or a delta-coding [9] is used,
      the computation of the digest after the computation of the range
      or delta would not provide a digest useful for checking the
      integrity of the reassembled instance.

   The encoded digest output uses the encoding format defined for the
   specific digest-algorithm.  For example, if the digest-algorithm is
   "MD5", the encoding is base64; if the digest-algorithm is "UNIXsum",
   the encoding is an ASCII string of decimal digits.

   Examples:

      MD5=HUXZLQLMuI/KZ5KDcJPcOA==
      sha=thvDyvhfIqlvFe+A9MYgxAfm1q5=
      UNIXsum=30637

4.3 Header specifications

   The following headers are defined.

4.3.1 Want-Digest

   The Want-Digest message header field indicates the sender's desire to
   receive an instance digest on messages associated with the Request-
   URI.

       Want-Digest = "Want-Digest" ":"
                        #(digest-algorithm [ ";" "q" "=" qvalue])

   If a digest-algorithm is not accompanied by a qvalue, it is treated
   as if its associated qvalue were 1.0.

   The sender is willing to accept a digest-algorithm if and only if it
   is listed in a Want-Digest header field of a message, and its qvalue
   is non-zero.

   If multiple acceptable digest-algorithm values are given, the
   sender's preferred digest-algorithm is the one (or ones) with the
   highest qvalue.

   Examples:

      Want-Digest: md5
      Want-Digest: MD5;q=0.3, sha;q=1






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4.3.2 Digest

   The Digest message header field provides a message digest of the
   instance described by the message.

      Digest = "Digest" ":" #(instance-digest)

   The instance described by a message might be fully contained in the
   message-body, partially-contained in the message-body, or not at all
   contained in the message-body.  The instance is specified by the
   Request-URI and any cache-validator contained in the message.

   A Digest header field MAY contain multiple instance-digest values.
   This could be useful for responses expected to reside in caches
   shared by users with different browsers, for example.

   A recipient MAY ignore any or all of the instance-digests in a Digest
   header field.

   A sender MAY send an instance-digest using a digest-algorithm without
   knowing whether the recipient supports the digest-algorithm, or even
   knowing that the recipient will ignore it.

   Examples:

      Digest: md5=HUXZLQLMuI/KZ5KDcJPcOA==
      Digest: SHA=thvDyvhfIqlvFe+A9MYgxAfm1q5=,unixsum=30637

5 Negotiation of Content-MD5

   HTTP/1.1 provides a Content-MD5 header field, but does not provide
   any mechanism for requesting its use (or non-use).  The Want-Digest
   header field defined in this document provides the basis for such a
   mechanism.

   First, we add to the set of digest-algorithm values (in section
   4.1.1) the token "contentMD5", with the provision that this digest-
   algorithm MUST NOT be used in a Digest header field.

   The presence of the "contentMD5" digest-algorithm with a non-zero
   qvalue in a Want-Digest header field indicates that the sender wishes
   to receive a Content-MD5 header on messages associated with the
   Request-URI.

   The presence of the "contentMD5" digest-algorithm with a zero qvalue
   in a Want-Digest header field indicates that the sender will ignore
   Content-MD5 headers on messages associated with the Request-URI.




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6 IANA Considerations

   The Internet Assigned Numbers Authority (IANA) administers the name
   space for digest-algorithm values.  Values and their meaning must be
   documented in an RFC or other peer-reviewed, permanent, and readily
   available reference, in sufficient detail so that interoperability
   between independent implementations is possible.  Subject to these
   constraints, name assignments are First Come, First Served (see RFC
   2434 [11]).

7 Security Considerations

   This document specifies a data integrity mechanism that protects HTTP
   instance data, but not HTTP entity headers, from certain kinds of
   accidental corruption.  It is also useful in detecting at least one
   spoofing attack [9].  However, it is not intended as general
   protection against malicious tampering with HTTP messages.

   The HTTP Digest Access Authentication mechanism [5] provides some
   protection against malicious tampering.

8 Acknowledgements

   It is not clear who first realized that the Content-MD5 header field
   is not sufficient to provide data integrity when ranges or deltas are
   used.

   Laurent Demailly may have been the first to suggest an algorithm-
   independent checksum header for HTTP [3].  Dave Raggett suggested the
   use of the term "digest" instead of "checksum" [14].

9 References

   [1]  Freed, N. and N. Borenstein, N., "MIME (Multipurpose Internet
        Mail Extensions) Part One:  Mechanisms for Specifying and
        Describing the Format of Internet Message Bodies", RFC 2049,
        November 1996.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Laurent Demailly.  Re: Revised Charter.
        http://www.ics.uci.edu/pub/ietf/http/hypermail/1995q4/0165.html.

   [4]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
        Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1.", RFC 2616, June 1999.




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   [5]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
        Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
        Basic and Digest Access Authentication", RFC 2617, June 1999.

   [6]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [7]  Nevin Heintze.  Scalable Document Fingerprinting.  Proc. Second
        USENIX Workshop on Electronic Commerce, USENIX, Oakland, CA,
        November, 1996, pp. 191-200.
        http://www.cs.cmu.edu/afs/cs/user/nch/www/koala/main.html.

   [8]  Merriam-Webster.  Webster's Seventh New Collegiate Dictionary.
        G. & C. Merriam Co., Springfield, MA, 1963.

   [9]  Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland,
        Y. and A. van Hoff, "Delta encoding in HTTP", RFC 3229, December
        2001.

   [10] Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864,
        October 1995.

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

   [12] National Institute of Standards and Technology.  Secure Hash
        Standard.  FEDERAL INFORMATION PROCESSING STANDARDS PUBLICATION
        180-1, U.S. Department of Commerce, April, 1995.
        http://csrc.nist.gov/fips/fip180-1.txt.

   [13] The Open Group.  The Single UNIX Specification, Version 2 - 6
        Vol Set for UNIX 98.  Document number T912, The Open Group,
        February, 1997.

   [14] Dave Raggett.  Re: Revised Charter.
        http://www.ics.uci.edu/pub/ietf/http/hypermail/1995q4/0182.html.

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











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10 Authors' Addresses

   Jeffrey C. Mogul
   Western Research Laboratory
   Compaq Computer Corporation
   250 University Avenue
   Palo Alto, California, 94305, U.S.A.

   EMail: [e-mail]
   Phone: [phone] (email preferred)

   Arthur van Hoff
   Marimba, Inc.
   440 Clyde Avenue
   Mountain View, CA 94043

   EMail: [e-mail]
   Phone: [phone]

































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11 Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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