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Versions: 00 02 03 04 RFC 2069

HTTP Working Group                                  Jeffery L. Hostetler
INTERNET-DRAFT                                               John Franks
<draft-ietf-http-digest-aa-02.txt>                   Philip Hallam-Baker
                                                            Ari Luotonen
                                                            Eric W. Sink
                                                     Lawrence C. Stewart
Expires SIX MONTHS FROM--->                            December 20, 1995

      A Proposed Extension to HTTP : Digest Access Authentication

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 groups may also distribute
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   Distribution of this document is unlimited. Please send comments
   to the proposed HTTP working group at <http-wg@cuckoo.hpl.hp.com>.
   Discussions of the working group are archived at
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   on the <www-talk@info.cern.ch> mailing list.


   The protocol referred to as "HTTP/1.0" includes specification
   for a Basic Access Authentication scheme.  This scheme is not
   considered to be a secure method of user authentication, as the
   user name and password are passed over the network in an
   unencrypted form.  A specification for a new authentication scheme
   is needed for future versions of the HTTP protocol.  This document
   provides specification for such a scheme, referred to as "Digest
   Access Authentication".  The encryption method used is the RSA Data
   Security, Inc. MD5 Message-Digest Algorithm [3].

Table of Contents

   1.  Introduction
       1.1  Purpose
       1.2  Overall Operation
       1.3  Representation of MD5 digest values
   2.  Basic Access Authentication Scheme
       2.1  Specification
       2.2  Security protocol negotiation
       2.3  Example
   3.  Acknowledgments
   4.  References
   5.  Authors Addresses

1. Introduction

1.1  Purpose

   The protocol referred to as "HTTP/1.0" includes specification
   for a Basic Access Authentication scheme[1].  This scheme is not
   considered to be a secure method of user authentication, as the
   user name and password are passed over the network in an
   unencrypted form.  A specification for a new authentication scheme
   is needed for future versions of the HTTP protocol.  This document
   provides specification for such a scheme, referred to as "Digest
   Access Authentication".

   The Digest Access Authentication scheme is not intended to be
   a complete answer to the need for security in the World Wide Web.
   This scheme provides no encryption of object content.  The intent
   is simply to facilitate secure access authentication.

   It is proposed that this access authentication scheme be included
   in the proposed HTTP/1.1 specification.

1.2  Overall Operation

   Like Basic Access Authentication, the Digest scheme is based on
   a simple challenge-response paradigm.  The Digest scheme challenges
   using a nonce value.  A valid response contains the MD5 checksum of
   the password and the given nonce value.  In this way, the password
   is never sent in the clear.  Just as with the Basic scheme, the
   username and password must be prearranged in some fashion.

1.3  Representation of MD5 digest values

   For the purposes of this document, an MD5 digest of 128 bits
   is represented as 32 ASCII printable characters.  The bits
   in the 128 bit digest are converted from most significant
   to least significant bit, four bits at a time to their
   ASCII presentation as follows.  Each four bits is
   represented by its familiar hexadecimal notation from the
   characters 0123456789abcdef.  That is binary 0000 gets
   represented by the character '0', 0001, by '1', and so on
   up to the representation of 1111 as 'f'.

2. Digest Access Authentication Scheme

2.1 Specification

   The Digest Access Authentication scheme is conceptually similar to the Basic
   scheme.  The formats of the modified WWW-Authenticate header line and the
   Authorization header line are specified below.  In addition, a new header,
   Digest-MessageDigest, is specified as well.

   Due to formatting constraints, all of the headers are depicted here
   on multiple lines.  In actual usage, they must follow the syntactic
   rules for HTTP/1.0 header lines [1].  Whitespace between the
   attribute-value pairs is allowed.

   If a server receives a request for an access-protected object, and an
   acceptable Authorizatation header is not sent, the server responds with:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest realm="<realm>",
                            stale="<TRUE | FALSE>"

   The meanings of the identifers used above are as follows:

         A name given to users so they know which username and password
         to send.

      <domain>  OPTIONAL
         A comma separated list of URIs, as specified for HTTP/1.0.  The
         intent is that the client could use this information to know the
         set of URIs for which the same authentication information should be
         sent.  The URIs in this list may exist on different servers.  If
         this keyword is omitted or empty, the client should assume that
         the domain consists of all URIs on the responding server.

         A server-specified integer value which may be uniquely generated each
         time a 401 response is made.  Servers may defend themselves against
         replay attacks by refusing to reuse nonce values.  The nonce should be
         considered opqaue by the client.

      <opaque>  OPTIONAL
         A string of data, specified by the server, which should returned by
         the client unchanged.  It is recommended that this string be
         base64 or hexadecimal data.  Specifically, since the string is passed
         in the header lines as a quoted string, the double-quote character
         is not allowed.

      <stale>   OPTIONAL
         A flag, indicating that the previous request from the client
         was rejected because the nonce value was stale.  If stale
         is TRUE, the client may wish to simply retry the request with
         a new encrypted response, without reprompting the user for a
         new username and password.

   The client is expected to retry the request, passing an Authorization header
   line as follows:

Authorization: Digest
           username="<username>",             -- required
           realm="<realm>",                   -- required
           nonce="<nonce>",                   -- required
           uri="<requested-uri>",             -- required
           response="<digest>",               -- required
           message="<message-digest>",        -- OPTIONAL
           opaque="<opaque>"                  -- required if provided by server

        where <digest> := H( H(A1) + ":" + N + ":" + H(A2) )
  and <message-digest> := H( H(A1) + ":" + N + ":" + H(<message-body>) )


                A1 := U + ':' + R + ':' + P
                A2 := <Method> + ':' + <requested-uri>

                        N -- nonce value
                        U -- username
                        R -- realm
                        P -- password
                        <Method> -- from header line 0
                        <requested-uri> -- uri sans proxy/routing

    When authorization succeeds, the Server may optionally provide the

HTTP/1.1 200 OK

        The Digest-MessageDigest header indicates that the server wants to
        communicate some info regarding the successful
        authentication (such as a message digest or a
        receipt of some kind).

        <message-digest> is computed as given above for
        the client.  this allows the client to verify that
        the message body has not been changed en-route.

        (The server would probably only send this when it
         has the document and can compute it (like the
         content-length field); the server would probably
         not bother generating this header for CGI output.)

   Upon receiving the Authorization information, the server may check its
   validity by looking up its known password which corresponds to the submitted
   <username>.  Then, the server must perform the same MD5 operation performed
   by the client, and compare the result to the given <response>.

   Note that the HTTP server does not actually need to know the user's
   clear text password.  As long as H(A1) is available to the server, the
   validity of an Authorization header may be verified.

   All keyword-value pairs must be expressed in characters from the
   US-ASCII character set, excluding control characters.

   A client may remember the username, password and nonce values, so that
   future requests within the specified <domain> may include the Authorization
   line preemptively.  The server may choose to accept the old Authorization
   information, even though the nonce value included might not be fresh.
   Alternatively, the server could return a 401 response with a new nonce
   value, causing the client to retry the request.  By specifying stale=TRUE
   with this response, the server hints to the client that the request should
   be retried with the new nonce, without reprompting the user for a new
   username and password.

   The <opaque> data is useful for transporting state information around.
   For example, a server could be responsible for authenticating content
   which actual sits on another server.  The first 401 response would include
   a <domain> which includes the URI on the second server, and the <opaque>
   for specifying state information.  The client will retry the request, at
   which time the server may respond with a 301/302 redirection, pointing
   to the URI on the second server.  The client will follow the redirection,
   and pass the same Authorization line, including the <opaque> data which
   the second server may require.

   As with the basic scheme, proxies must be completely transparent in
   the Digest access authentication scheme. That is, they must forward the
   WWW-Authenticate, Digest-MessageDigest and Authorization headers untouched.
   If a proxy wants to authenticate a client before a request is forwarded to
   the server, it can be done using the Proxy-Authenticate and
   Proxy-Authorization headers.

2.2 Security Protocol Negotiation

   It is useful for a server to be able to know which security schemes
   a client is capable of handling.  It is recommended that the HTTP extension
   mechanism proposed by Dave Kristol [2] be used.  If the client includes
   the following header line with the request, then a server can safely assume
   that the client can handle Digest authentication.

Extension: Security/Digest

   If this proposal is accepted as a required part of the HTTP/1.1
   specification, then a server may assume Digest support when a client
   identifies itself as HTTP/1.1 compliant.

   It is possible that a server may want to require Digest as its
   authentication method, even if the server does not know that the client
   supports it.  A client is encouraged to fail gracefully if the server
   specifies any authentication scheme it cannot handle.

2.3 Example

   The following example assumes that an access-protected document is being
   requested from the server.  The URI of the document is

   Both client and server know that the username for this document is "eric",
   and the password is "spyglass".

   The first time the client requests the document, no Authorization header
   is sent, so the server responds with:

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest    realm="testrealm",

   The client may prompt the user for the username and password, after which it
   will respond with a new request, including the following Authorization

Authorization: Digest       username="eric",

3. References

   [1]  T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen.
        "Hypertext Transfer Protocol -- HTTP/1.0"
        Internet-Draft (work in progress), UC Irvine,
        draft-ietf-http-v10-spec-00.txt>, March 1995.

   [2]  D. Kristol. "A Proposed Extension Mechanism for HTTP"
        December 1994.

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

4. Authors Addresses

   John Franks
   Professor of Mathematics
   Department of Mathematics
   Northwestern University
   Evanston, IL 60208-2730, USA

   Phillip M. Hallam-Baker
   European Union Fellow

   Jeffery L. Hostetler
   Senior Software Engineer
   Spyglass, Inc.
   3200 Farber Drive
   Champaign, IL  61821, USA

   Ari Luotonen
   Member of Technical Staff
   Netscape Communications Corporation
   501 East Middlefield Road
   Mountain View, CA 94043, USA

   Eric W. Sink
   Senior Software Engineer
   Spyglass, Inc.
   3200 Farber Drive
   Champaign, IL  61821, USA

   Lawrence C. Stewart
   Open Market, Inc.
   215 First Street
   Cambridge, MA  02142, USA

Eric W. Sink

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