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Versions: 00 01 02 03 04 RFC 3230

Network Working Group                             Jeffrey Mogul, DECWRL,
Internet-Draft                                  Arthur van Hoff, Marimba
Expires: 1 July 1998                                      9 January 1998




                        Instance Digests in HTTP

                     draft-mogul-http-digest-00.txt


STATUS OF THIS MEMO

        This document is an Internet-Draft. Internet-Drafts are
        working documents of the Internet Engineering Task Force
        (IETF), its areas, and its working groups. Note that other
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        Distribution of this document is unlimited.  Please send
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ABSTRACT

        HTTP/1.1 defines a Content-MD5 header that allows a server
        to include a digest of the response body.  However, this is
        specifically defined to cover the body of the actual
        message, not the contents of the full file (which might be
        quite different, if the response is a Content-Range, or
        uses a delta encoding).  Also, the Content-MD5 is limited
        to one specific digest algorithm; other algorithms, such as
        SHA-1, may be more appropriate in some circumstances.
        Finally, HTTP/1.1 provides no explicit mechanism by which a
        client may request a digest.  This document proposes HTTP
        extensions that solve these problems.





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                           TABLE OF CONTENTS

1 Introduction                                                         2
     1.1 Other limitations of HTTP/1.1                                 4
2 Goals                                                                4
3 Terminology                                                          5
4 Specification                                                        6
     4.1 Protocol parameter specifications                             6
          4.1.1 Digest algorithms                                      6
     4.2 Instance digests                                              7
     4.3 Header specifications                                         7
          4.3.1 Want-Digest                                            7
          4.3.2 Digest                                                 8
5 Negotiation of Content-MD5                                           9
6 IANA Considerations                                                  9
7 Security Considerations                                              9
8 Acknowledgements                                                    10
9 References                                                          10
10 Authors' addresses                                                 11


1 Introduction

   Although HTTP is typically layered over a reliable transport
   protocol, such as TCP, this does not guarantee reliable transport of
   information from sender to receiver.  Various problems, including
   undetected transmission errors, programming errors, corruption of
   stored data, and malicious intervention can cause errors in the
   transmitted information.

   A common approach to the problem of data integrity in a network
   protocol or distributed system, such as HTTP, is the use of digests,
   checksums, or hash values.  The sender computes a digest and sends it
   with the data; the recipient computes a digest of the received data,
   and then verifies the integrity of this data by comparing the
   digests.

   Checksums are used at virtually all layers of the IP stack.  However,
   different digest algorithms might be used at each layer, for reasons
   of computational cost, because the size and nature of the data being
   protected varies, and because the possible threats to data integrity
   vary.  For example, Ethernet uses a Cyclic Redundancy Check (CRC).
   The IPv4 protocol uses a ones-complement checksum over the IP header
   (but not the rest of the packet).  TCP uses a ones-complement
   checksum over the TCP header and data, and includes a
   ``pseudo-header'' to detect certain kinds of programming errors.

   HTTP/1.1 [3] includes a mechanism for ensuring message integrity, the
   Content-MD5 header.  This header is actually defined for
   MIME-conformant messages in a standalone specification [9].
   According to the HTTP/1.1 specification,

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          The Content-MD5 entity-header field [...]  is an MD5
       digest of the entity-body for the purpose of providing an
       end-to-end message integrity check (MIC) of the entity-body.

   HTTP/1.1 borrowed Content-MD5 from the MIME world based on an analogy
   between MIME messages (e.g., electronic mail messages) and HTTP
   messages (requests to or responses from an HTTP server).

   As discussed in more detail in section 3, this analogy between MIME
   messages and HTTP messages has resulted in some confusion.  In
   particular, while a MIME message is self-contained, an HTTP message
   might not contain the entire representation of the current state of a
   resource.  (More precisely, an HTTP response might not contain an
   entire ``instance''; see section 3 for a definition of this term.)

   There are at least two situations where this distinction is an issue:

      1. When an HTTP server sends a 206 (Partial Content)
         response, as defined in HTTP/1.1.  The client may form its
         view of an instance (e.g., an HTML document) by combining
         a cache entry with the partial content in the message.

      2. When an HTTP server uses a ``delta encoding'', as proposed
         in a separate document [8].  A delta encoding represents
         the changes between the current instance of a resource and
         a previous instance, and is an efficient way of reducing
         the bandwidth required for cache updates.  The client
         forms its view of an instance by applying the delta in the
         message to one of its cache entries.

   In each of these cases, the server might use a Content-MD5 header to
   protect the integrity of the response message.  However, because the
   MIC in a Content-MD5 header field applies only to the entity in that
   message, and not to the entire instance being reassembled, it cannot
   protect against errors due to data corruption (e.g., of cache
   entries), programming errors (e.g., improper application of a partial
   content or delta), certain malicious attacks [8], or corruption of
   certain HTTP headers in transit.

   Thus, the Content-MD5 header, while useful and sufficient in many
   cases, is not sufficient for verifying instance integrity in all uses
   of HTTP.

   The Digest Authentication mechanism [4] provides (in addition to its
   other goals) a message-digest function similar to Content-MD5, except
   that it includes certain header fields.  Like Content-MD5, it covers
   a specific message, not an entire instance.





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1.1 Other limitations of HTTP/1.1
   Checksums are not free.  Computing a digest takes CPU resources, and
   might add latency to the generation of a message.  (Some of these
   costs can be avoided by careful caching at the sender's end, but in
   many cases such a cache would not have a useful hit ratio.)
   Transmitting a digest consumes HTTP header space (and therefore
   increases latency and network bandwidth requirements.)  If the
   message recipient does not intend to use the digest, why should the
   message sender waste resources computing and sending it?

   The Content-MD5 header, of course, implies the use of the MD5
   algorithm [13].  Other algorithms, however, might be more appropriate
   for some purposes.  These include the SHA-1 algorithm [10] and
   various ``fingerprinting'' algorithms [6].  HTTP currently provides
   no standardized support for the use of these algorithms.

   HTTP/1.1 apparently assumes that the choice to generate a digest is
   up to the sender, and provides no mechanism for the recipient to
   indicate whether a checksum would be useful, or what checksum
   algorithms it would understand.


2 Goals

   The goals of this proposal are:

      1. Digest coverage for entire instances communicated via
         HTTP.

      2. Support for multiple digest algorithms.

      3. Negotiation of the use of digests.

   The goals do not include:

      - header integrity
        The digest mechanisms described here cover only the bodies
        of instances, and do not protect the integrity of
        associated ``entity headers'' or other message headers.

      - authentication
        The digest mechanisms described here are not meant to
        support authentication of the source of a digest or of a
        message or instance.  These mechanisms, therefore, are not
        sufficient defense against many kinds of malicious attacks.

      - privacy
        Digest mechanisms do not provide message privacy.

      - authorization
        The digest mechanisms described here are not meant to
        support authorization or other kinds of access controls.
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   The Digest Access Authentication mechanism [4] can provide some
   integrity for certain HTTP headers, and does provide authentication.


3 Terminology

   HTTP/1.1 [3] defines the following terms:

   resource         A network data object or service that can be
                   identified by a URI, as defined in section 3.2.
                   Resources may be available in multiple
                   representations (e.g. multiple languages, data
                   formats, size, resolutions) or vary in other ways.

   entity           The information transferred as the payload of a
                   request or response.  An entity consists of
                   metainformation in the form of entity-header fields
                   and content in the form of an entity-body, as
                   described in section 7.

   variant          A resource may have one, or more than one,
                   representation(s) associated with it at any given
                   instant. Each of these representations is termed a
                   `variant.' Use of the term `variant' does not
                   necessarily imply that the resource is subject to
                   content negotiation.

   The dictionary definition for ``entity'' is ``something that has
   separate and distinct existence and objective or conceptual
   reality'' [7].  Unfortunately, the definition for ``entity'' in
   HTTP/1.1 is similar to that used in MIME [5], based on an entirely
   false analogy between MIME and HTTP.

   In MIME, electronic mail messages do have distinct and separate
   existences, so the MIME definite ``entity'' as something that
   ``refers specifically to the MIME-defined header fields and contents
   of either a message or one of the parts in the body of a multipart
   entity'' make sense.

   In HTTP, however, a response message to a GET does not have a
   distinct and separate existence.  Rather, it is describing the
   current state of a resource (or a variant, subject to a set of
   constraints).  The HTTP/1.1 specification provides no term to
   describe ``the value that would be returned in response to a GET
   request at the current time for the selected variant of the specified
   resource.''  This leads to awkward wordings in the HTTP/1.1
   specification in places where this concept is necessary.

   It is too late to fix the terminological failure in the HTTP/1.1
   specification, so we instead define a new term, for use in this
   document:

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   instance         The entity that would be returned in a status-200
                   response to a GET request, at the current time, for
                   the selected variant of the specified resource, but
                   without the application of any content-coding or
                   transfer-coding.

   One can think of an instance as a snapshot in the life of a resource.

   It is convenient to think of an entity tag, in HTTP/1.1, as being
   associated with an instance, rather than an entity.  That is, for a
   given resource, two different response messages might include the
   same entity tag, but two different instances of the resource should
   never be associated with the same (strong) entity tag.


4 Specification

4.1 Protocol parameter specifications

4.1.1 Digest algorithms
   Digest algorithm values are used to indicate a specific digest
   computation.  For some algorithms, one or more parameters may be
   supplied.

       digest-algorithm = token

   The BNF for "parameter" is as is used in RFC2068 [3].  All
   digest-algorithm values are case-insensitive.

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   digest-algorithm values.  Initially, the registry contains the
   following tokens:

   MD5             The MD5 algorithm, as specified in RFC 1321 [13].
                   The output of this algorithm is encoded using the
                   base64 encoding [1].

   SHA             The SHA-1 algorithm [10].  The output of this
                   algorithm is encoded using the base64 encoding [1].

   UNIXsum         The algorithm computed by the UNIX ``sum'' command,
                   as defined by the Single UNIX Specification, Version
                   2 [11].  The output of this algorithm is an ASCII
                   decimal-digit string representing the 16-bit
                   checksum, which is the first word of the output of
                   the UNIX ``sum'' command.

   UNIXcksum       The algorithm computed by the UNIX ``cksum'' command,
                   as defined by the Single UNIX Specification, Version
                   2 [11].  The output of this algorithm is an ASCII
                   digit string representing the 32-bit CRC, which is

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                   the first word of the output of the UNIX ``cksum''
                   command.

   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 content-coding 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.

      ---------
      Note: the digest is computed before the application of any
      content-coding, because if a delta-content-coding [8] is used,
      the computation of the digest after the computation of the
      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])


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   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

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.

      ---------
      Is it useful to allow a Digest to contain *no* instance-digest
      values?
      ---------

   Examples:

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





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5 Negotiation of Content-MD5

      ---------
      This proposal (for negotiating the use of Content-MD5) might be
      controversial, and can be removed from the digest proposal
      without altering any other details of the digest proposal.
      ---------

   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.


6 IANA Considerations

      ---------
      This section will provide the necessary IANA considerations
      information, for the entering of delta encoding algorithms into
      the registries for HTTP content-codings and transfer-codings,
      and for any parameters on the transfer-codings.  (Will reflect
      rules in draft-iesg-iana-considerations-01.)
      ---------


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
   spoofing attack [8].  However, it is not intended as general
   protection against malicious tampering with HTTP messages.

   The HTTP Digest Access Authentication mechanism [4] provides some
   protection against malicious tampering.




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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 [2].  Dave Raggett
   suggested the use of the term ``digest'' instead of
   ``checksum'' [12].


9 References

   NOTE TO RFC EDITOR: many of the references here might be out of date.
   Please verify these with the primary author of this Internet-Draft
   before issuing this document as an RFC.

   1.  N. Borenstein and N. Freed.  MIME (Multipurpose Internet Mail
   Extensions) Part One:  Mechanisms for Specifying and Describing the
   Format of Internet Message Bodies.  RFC 1521, IETF, September, 1993.

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

   3.  Roy T. Fielding, Jim Gettys, Jeffrey C. Mogul, Henrik Frystyk
   Nielsen, and Tim Berners-Lee.  Hypertext Transfer Protocol --
   HTTP/1.1.  RFC 2068, HTTP Working Group, January, 1997.

   4.  J. Franks, P. Hallam-Baker, J. Hostetler, P. Leach, A. Luotonen,
   E. Sink, L. Stewart.  An Extension to HTTP: Digest Access
   Authentication.  RFC 2069, HTTP Working Group, January, 1997.

   5.  N. Freed and N. Borenstein.  Multipurpose Internet Mail
   Extensions (MIME) Part One:  Format of Internet Message Bodies.  RFC
   2045, Network Working Group, November, 1996.

   6.  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.

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

   8.  Jeffrey C. Mogul, Fred Douglis, Anja Feldmann, Balachander
   Krishnamurthy, Yaron Goland, and Arthur van Hoff.  Delta encoding in
   HTTP.  Internet-Draft draft-mogul-http-delta-00, HTTP Working Group,
   January, 1998. This is a work in progress.



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   9.  J. Myers.  The Content-MD5 Header Field.  RFC 1864, Network
   Working Group, October, 1995.

   10.  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.

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

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

   13.  R. Rivest.  The MD5 Message-Digest Algorithm.  RFC 1321, Network
   Working Group, April, 1992.


10 Authors' addresses

   Jeffrey C. Mogul
   Western Research Laboratory
   Digital Equipment Corporation
   250 University Avenue
   Palo Alto, California, 94305, U.S.A.
   Email: mogul@wrl.dec.com
   Phone: 1 650 617 3304 (email preferred)



   Arthur van Hoff
   Marimba, Inc.
   440 Clyde Avenue
   Mountain View, CA 94043
   1 (650) 930 5283
   avh@marimba.com















Mogul, van Hoff                                                [Page 11]


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