2004/id/draft-ietf-http-digest-aa-rev-00.txt


HTTP Working Group                                             J. Franks
INTERNET-DRAFT                                   Northwestern University
<draft-ietf-http-digest-aa-rev-00>                   P. Hallam-Baker
                                                                  M.I.T.
                                                            J. Hostetler
                                                          Spyglass, Inc.
                                                                P. Leach
                                                   Microsoft Corporation
                                                             A. Luotonen
                                     Netscape Communications Corporation
                                                                 E. Sink
                                                          Spyglass, Inc.
                                                              L. Stewart
                                                       Open Market, Inc.
                                                           July 30, 1977


          An 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
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     Distribution of this document is unlimited. Please send comments
     to the HTTP working group at <http-wg@cuckoo.hpl.hp.com>. 
     Discussions of the working group are archived at 
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Abstract

   The protocol referred to as "HTTP/1.0" includes the 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 as clear text.  A specification
   for a different authentication scheme is needed to address this
   severe limitation.  This document provides specification for such a
   scheme, referred to as "Digest Access Authentication".  Like Basic,
   Digest access authentication verifies that both parties to a
   communication know a shared secret (a password); unlike Basic, this
   verification can be done without sending the password in the clear,
   which is Basic's biggest weakness. As with most other authentication
   protocols, the greatest sources of risks are usually found not in the
   core protocol itself but in policies and procedures surrounding its
   use.

   This is the final draft of any document under this name.  Digest and
   Basic Authentication from the HTTP/1.1 specification will be combined
   and issued as a document titled "Authentication in HTTP".  Our intent
   is that RFC 2068 and RFC 2069 will go to draft standard as a pair of
   documents, but with all authentication schemes (Digest and Basic)
   documented together in a single place.  This transition has not yet
   taken place.

   Changes since RFC 2069 was issued:

   Inclusion of a missing ')' in the BNF production for "response-digest" in section
   2.1.2, the replacement of "digest-opaque" by "opaque" in the
   production for "digest-challenge" in the same section, and the
   replacement the value of the "response" field in the example in
   section 2.4.  This last change was done to make the this value be an
   accurate computation of the digest of the example data.

   An optional "digest-required" field was added to both the 
   "WWW-Authenticate" response header (section 2.1.1) and the 
   "Authorization" request header (section 2.1.2).  (Issue 
   DIGEST-REQUIRED.

   The PROXY-LENGTH, PROXY-MAXAGE issues have NOT yet been incorporated.


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Table of Contents

   INTRODUCTION......................................................  2
    1.1  PURPOSE ....................................................  2
    1.2  OVERALL OPERATION ..........................................  3
    1.3  REPRESENTATION OF DIGEST VALUES ............................  3
    1.4  LIMITATIONS ................................................  3
   2. DIGEST ACCESS AUTHENTICATION SCHEME............................  3
    2.1 SPECIFICATION OF DIGEST HEADERS .............................  3
     2.1.1 THE WWW-AUTHENTICATE RESPONSE HEADER .....................  4
     2.1.2 THE AUTHORIZATION REQUEST HEADER .........................  6
     2.1.3 THE AUTHENTICATION-INFO HEADER ...........................  9
    2.2 DIGEST OPERATION ............................................ 10
    2.3 SECURITY PROTOCOL NEGOTIATION ............................... 10
    2.4 EXAMPLE ..................................................... 11
    2.5 PROXY-AUTHENTICATION AND PROXY-AUTHORIZATION ................ 11
   3. SECURITY CONSIDERATIONS........................................ 12
    3.1 COMPARISON WITH BASIC AUTHENTICATION ........................ 13
    3.2 REPLAY ATTACKS .............................................. 13
    3.3 MAN IN THE MIDDLE ........................................... 14
    3.4 SPOOFING BY COUNTERFEIT SERVERS ............................. 15
    3.5 STORING PASSWORDS ........................................... 15
    3.6 SUMMARY ..................................................... 16
   4.  ACKNOWLEDGMENTS............................................... 16
   5. REFERENCES..................................................... 16
   6. AUTHORS' ADDRESSES............................................. 17

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 create a weak access authentication method which avoids the most
   serious flaws of Basic authentication.

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


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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 a checksum (by
   default the MD5 checksum) of the username, the password, the given
   nonce value, the HTTP method, and the requested URI.  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
   which is not addressed by this document.

1.3  Representation of digest values

   An optional header allows the server to specify the algorithm used to
   create the checksum or digest.  By default the MD5 algorithm is used
   and that is the only algorithm described in this document.

   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'.

1.4  Limitations

   The digest authentication scheme described in this document suffers
   from many known limitations.  It is intended as a replacement for
   basic authentication and nothing more.  It is a password-based system
   and (on the server side) suffers from all the same problems of any
   password system.  In particular, no provision is made in this
   protocol for the initial secure arrangement between user and server
   to establish the user's password.

   Users and implementors should be aware that this protocol is not as
   secure as kerberos, and not as secure as any client-side private-key
   scheme.  Nevertheless it is better than nothing, better than what is
   commonly used with telnet and ftp, and better than Basic
   authentication.

2. Digest Access Authentication Scheme

2.1 Specification of Digest Headers

   The Digest Access Authentication scheme is conceptually similar to
   the Basic scheme.  The formats of the modified WWW-Authenticate


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   header line and the Authorization header line are specified below,
   using the extended BNF defined in the HTTP/1.1 specification, section
   2.1.  In addition, a new header, Authentication-info, is specified.

2.1.1 The WWW-Authenticate Response Header

   If a server receives a request for an access-protected object, and an
   acceptable Authorization header is not sent, the server responds with
   a "401 Unauthorized" status code, and a WWW-Authenticate header,
   which is defined as follows:

     WWW-Authenticate    = "WWW-Authenticate" ":" "Digest"
                              digest-challenge

     digest-challenge    = 1#( realm | [ domain ] | nonce |
                          [ opaque ] |[ stale ] | [ algorithm ] |
                          [ digest-required ] )

     realm               = "realm" "=" realm-value
     realm-value         = quoted-string
     domain              = "domain" "=" <"> 1#URI <">
     nonce               = "nonce" "=" nonce-value
     nonce-value         = quoted-string
     opaque              = "opaque" "=" quoted-string
     stale               = "stale" "=" ( "true" | "false" )
     algorithm           = "algorithm" "=" ( "MD5" | token )
     digest-required     = "digest-required"

   The meanings of the values of the parameters used above are as
   follows:

     realm
     A string to be displayed to users so they know which username and
     password to use.  This string should contain at least the name of
     the host performing the authentication and might additionally
     indicate the collection of users who might have access.  An example
     might be "registered_users@gotham.news.com".  The realm is a
     "quoted-string" as specified in section 2.2 of the HTTP/1.1
     specification [2].

     domain
     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.


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     nonce
     A server-specified data string which may be uniquely generated each
     time a 401 response is made.  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.

     The contents of the nonce are implementation dependent.  The
     quality of the implementation depends on a good choice.  A
     recommended nonce would include

             H(client-IP ":" time-stamp ":" private-key )

     Where client-IP is the dotted quad IP address of the client making
     the request, time-stamp is a server-generated time value,  private-
     key is data known only to the server.  With a nonce of this form a
     server would normally recalculate the nonce after receiving the
     client authentication header and reject the request if it did not
     match the nonce from that header. In this way the server can limit
     the reuse of a nonce to the IP address to which it was issued and
     limit the time of the nonce's validity.  Further discussion of the
     rationale for nonce construction is in section 3.2 below.

     An implementation might choose not to accept a previously used
     nonce or a previously used digest to protect against a replay
     attack.  Or, an implementation might choose to use one-time nonces
     or digests for POST or PUT requests and a time-stamp for GET
     requests.  For more details on the issues involved see section 3.
     of this document.

     The nonce is opaque to the client.

     opaque
     A string of data, specified by the server, which should be
     returned by the client unchanged.  It is recommended that this
     string be base64 or hexadecimal data.  This field is a
     "quoted-string" as specified in section 2.2 of the HTTP/1.1
     specification [2].

     stale
     A flag, indicating that the previous request from the client was
     rejected because the nonce value was stale.  If stale is TRUE (in
     upper or lower case), 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 server should only set
     stale to true if it receives a request for which the nonce is
     invalid but with a valid digest for that nonce (indicating that
     the client knows the correct username/password).


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     algorithm
     A string indicating a pair of algorithms used to produce the
     digest and a checksum.  If this not present it is assumed to be
     "MD5". In this document the string obtained by applying the
     digest algorithm to the data "data" with secret "secret" will be
     denoted by KD(secret, data), and the string obtained by applying
     the checksum algorithm to the data "data" will be denoted
     H(data).

     For the "MD5" algorithm

        H(data) = MD5(data)

     and

        KD(secret, data) = H(concat(secret, ":", data))

     i.e., the digest is the MD5 of the secret concatenated with a colon
     concatenated with the data.

     digest-required
     A flag, indicating that any request with an entity-body (such as a PUT
     or a POST) for the resource to which this response applies MUST
     include the 'digest' attribute in its Authorization header.  If the
     request has no entity-body (such as a GET) then the digest-required
     field can be ignored.  If a request with an entity-body is made without
     the digest field in response to an authentication header with the 
     digest-required field, then this request is an error and the
     request MUST not be honored.


2.1.2 The Authorization Request Header

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

Authorization       = "Authorization" ":" "Digest" digest-response

digest-response     = 1#( username | realm | nonce | digest-uri |
                         response | [ digest ] | [ algorithm ] |
                         opaque | [digest-required] )

username            = "username" "=" username-value
username-value      = quoted-string
digest-uri          = "uri" "=" digest-uri-value
digest-uri-value    = request-uri         ; As specified by HTTP/1.1
response            = "response" "=" response-digest
digest             = "digest" "=" entity-digest
opaque              = "opaque" "=" quoted-string
algorithm           = "algorithm" "=" ( "MD5" | token )
digest-required     = "digest-required"

response-digest     = <"> *LHEX <">
entity-digest      = <"> *LHEX <">
LHEX                = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                      "8" | "9" | "a" | "b" | "c" | "d" | "e" | "f"

   The values of the opaque and algorithm fields must be those supplied
   in the WWW-Authenticate response header for the entity being
   requested.

   The digest-required field is a flag, indicating that the response to
   this request MUST either include the  'digest' field in its
   Authentication-Info header or the response should be an error
   message indicating the server is unable or unwilling to supply
   this field.  In the latter case the requested entity MUST not be returned
   as part of the response.

   The definitions of response-digest and entity-digest above indicate
   the encoding for their values. The following definitions show how the
   value is computed:


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     response-digest     =
          <"> < KD ( H(A1), unquoted nonce-value ":" H(A2) ) > <">

     A1             = unquoted username-value ":" unquoted realm-value
                                                ":" password
     password       = < user's password >
     A2             = Method ":" digest-uri-value

   The "username-value" field is a "quoted-string" as specified in
   section 2.2 of the HTTP/1.1 specification [2].  However, the
   surrounding quotation marks are removed in forming the string A1.
   Thus if the Authorization header includes the fields

    username="Mufasa", realm="myhost@testrealm.com"

   and the user Mufasa has password "CircleOfLife" then H(A1) would be
   H(Mufasa:myhost@testrealm.com:CircleOfLife) with no quotation marks
   in the digested string.

   No white space is allowed in any of the strings to which the digest
   function H() is applied unless that white space exists in the quoted
   strings or entity body whose contents make up the string to be
   digested.  For example, the string A1 in the illustrated above must
   be Mufasa:myhost@testrealm.com:CircleOfLife with no white space on
   either side of the colons.  Likewise, the other strings digested by
   H() must not have white space on either side of the colons which
   delimit their fields unless that white space was in the quoted
   strings or entity body being digested.

   "Method" is the HTTP request method as specified in section 5.1 of
   [2].  The "request-uri" value is the Request-URI from the request
   line as specified in section 5.1 of [2].  This may be "*", an
   "absoluteURL" or an "abs_path" as specified in section 5.1.2 of [2],
   but it MUST agree with the Request-URI. In particular, it MUST be an
   "absoluteURL" if the Request-URI is an "absoluteURL".

   The authenticating server must assure that the document designated by
   the "uri" parameter is the same as the document served.  The purpose
   of duplicating information from the request URL in this field is to
   deal with the possibility that an intermediate proxy may alter the
   client's request.  This altered (but presumably semantically
   equivalent) request would not result in the same digest as that
   calculated by the client.

   The optional "digest" field contains a digest of the entity body and
   some of the associated entity headers.  This digest can be useful in
   both request and response transactions.  In a request it can insure
   the integrity of POST data or data being PUT to the server.  In a


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   response it insures the integrity of the served document.  The value
   of the "digest" field is an <entity-digest> which is defined as
   follows.

entity-digest = <"> KD (H(A1), unquoted nonce-value ":" Method ":"
                           date ":" entity-info ":" H(entity-body)) <">
       ; format is <"> *LHEX <">

date = = rfc1123-date            ; see section 3.3.1 of [2]
entity-info = H(
          digest-uri-value ":"
          media-type ":"         ; Content-type, see section 3.7 of [2]
          *DIGIT ":"             ; Content length, see 10.12 of [2]
          content-coding ":"     ; Content-encoding, see 3.5 of [2]
          last-modified ":"      ; last modified date, see 10.25 of [2]
          expires                ; expiration date; see 10.19 of [2]
          )

last-modified   = rfc1123-date  ; see section 3.3.1 of [2]
expires         = rfc1123-date

   The entity-info elements incorporate the values of the URI used to
   request the entity as well as the associated entity headers Content-
   type, Content-length, Content-encoding, Last-modified, and Expires.
   These headers are all end-to-end headers (see section 13.5.1 of [2])
   which must not be modified by proxy caches.  The "entity-body" is as
   specified by section 10.13 of [2] or RFC 1864.

   Note that not all entities will have an associated URI or all of
   these headers.  For example, an entity which is the data of a POST
   request will typically not have a digest-uri-value or Last-modified
   or Expires headers.  If an entity does not have a digest-uri-value or
   a header corresponding to one of the entity-info fields, then that
   field is left empty in the computation of entity-info.  All the
   colons specified above are present, however.  For example the value
   of the entity-info associated with POST data which has content-type
   "text/plain", no content-encoding and a length of 255 bytes would be
   H(:text/plain:255:::).  Similarly a request may not have a "Date"
   header.  In this case the date field of the entity-digest should be
   empty.

   In the entity-info and entity-digest computations, except for the
   blank after the comma in "rfc1123-date", there must be no white space
   between "words" and "tspecials", and exactly one blank between
   "words" (see section 2.2 of [2]).


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   Implementors should be aware of how authenticated transactions
   interact with proxy caches.  The HTTP/1.1 protocol specifies that
   when a shared cache (see section 13.10 of [2]) has received a request
   containing an Authorization header and a response from relaying that
   request, it MUST NOT return that response as a reply to any other
   request, unless one of two Cache-control (see section 14.9 of [2])
   directives was present in the response.  If the original response
   included the "must-revalidate" Cache-control directive, the cache MAY
   use the entity of that response in replying to a subsequent request,
   but MUST first revalidate it with the origin server, using the
   request headers from the new request to allow the origin server to
   authenticate the new request.  Alternatively, if the original
   response included the "public" Cache-control directive, the response
   entity MAY be returned in reply to any subsequent request.

2.1.3 The AuthenticationInfo Header

   When authentication succeeds, the Server may optionally provide a
   Authentication-info header indicating that the server wants to
   communicate some information regarding the successful authentication
   (such as an entity digest or a new nonce to be used for the next
   transaction).  It has two fields, digest and nextnonce.  Both are
   optional.

    AuthenticationInfo = "Authentication-info" ":"
                                      1#( digest | nextnonce )

    nextnonce      = "nextnonce" "=" nonce-value

    digest = "digest" "=" entity-digest

   The optional digest allows the client to verify that the body of the
   response has not been changed en-route.  The server would probably
   only send this when it has the document and can compute it.  The
   server would probably not bother generating this header for CGI
   output.  The value of the "digest" is an <entity-digest> which is
   computed as described above.

   The value of the nextnonce parameter is the nonce the server wishes
   the client to use for the next authentication response.  Note that
   either field is optional.  In particular the server may send the
   Authentication-info header with only the nextnonce field as a means
   of implementing one-time nonces.  If the nextnonce field is present
   the client is strongly encouraged to use it for the next WWW-
   Authenticate header.  Failure of the client to do so may result in a
   request to re-authenticate from the server with the "stale=TRUE."


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2.2 Digest Operation

   Upon receiving the Authorization header, 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-digest.

   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.

   A client may remember the username, password and nonce values, so
   that future requests within the specified <domain> may include the
   Authorization header preemptively.  The server may choose to accept
   the old Authorization header 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 actually sits on another server.  The first 401 response would
   include a domain field which includes the URI on the second server,
   and the opaque field 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
   header, 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, Authentication-info 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 described in section 2.5
   below.

2.3 Security Protocol Negotiation

   It is useful for a server to be able to know which security schemes a
   client is capable of handling.

   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


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   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.

2.4 Example

   The following example assumes that an access-protected document is
   being requested from the server.  The URI of the document is
   "http://www.nowhere.org/dir/index.html".  Both client and server know
   that the username for this document is "Mufasa", and the password is
   "CircleOfLife".

   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@host.com",
                            nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
                            opaque="5ccc069c403ebaf9f0171e9517f40e41"

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

Authorization: Digest       username="Mufasa",
                            realm="testrealm@host.com",
                            nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
                            uri="/dir/index.html",
                            response="1949323746fe6a43ef61f9606e7febea",
                            opaque="5ccc069c403ebaf9f0171e9517f40e41"

2.5 Proxy-Authentication and Proxy-Authorization

   The digest authentication scheme may also be used for authenticating
   users to proxies, proxies to proxies, or proxies to end servers by
   use of the Proxy-Authenticate and Proxy-Authorization headers. These
   headers are instances of the general Proxy-Authenticate and Proxy-
   Authorization headers specified in sections 10.30 and 10.31 of the
   HTTP/1.1 specification [2] and their behavior is subject to
   restrictions described there.  The transactions for proxy
   authentication are very similar to those already described.  Upon
   receiving a request which requires authentication, the proxy/server
   must issue the "HTTP/1.1 401 Unauthorized" header followed by a
   "Proxy-Authenticate" header of the form


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     Proxy-Authentication     = "Proxy-Authentication" ":" "Digest"
                                   digest-challenge

   where digest-challenge is as defined above in section 2.1. The
   client/proxy must then re-issue the request with a Proxy-Authenticate
   header of the form

     Proxy-Authorization      = "Proxy-Authorization" ":"
                                   digest-response

   where digest-response is as defined above in section 2.1. When
   authentication succeeds, the Server may optionally provide a Proxy-
   Authentication-info header of the form

Proxy-Authentication-info = "Proxy-Authentication-info" ":" nextnonce

   where nextnonce has the same semantics as the nextnonce field in the
   Authentication-info header described above in section 2.1.

   Note that in principle a client could be asked to authenticate itself
   to both a proxy and an end-server.  It might receive an "HTTP/1.1 401
   Unauthorized" header followed by both a WWW-Authenticate and a
   Proxy-Authenticate header.  However, it can never receive more than
   one Proxy-Authenticate header since such headers are only for
   immediate connections and must not be passed on by proxies.  If the
   client receives both headers, it must respond with both the
   Authorization and Proxy-Authorization headers as described above,
   which will likely involve different combinations of username,
   password, nonce, etc.

3. Security Considerations

   Digest Authentication does not provide a strong authentication
   mechanism.  That is not its intent.  It is intended solely to replace
   a much weaker and even more dangerous authentication mechanism: Basic
   Authentication.  An important design constraint is that the new
   authentication scheme be free of patent and export restrictions.

   Most needs for secure HTTP transactions cannot be met by Digest
   Authentication.  For those needs SSL or SHTTP are more appropriate
   protocols.  In particular digest authentication cannot be used for
   any transaction requiring encrypted content.  Nevertheless many
   functions remain for which digest authentication is both useful and
   appropriate.


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3.1 Comparison with Basic Authentication

   Both Digest and Basic Authentication are very much on the weak end of
   the security strength spectrum. But a comparison between the two
   points out the utility, even necessity, of replacing Basic by Digest.

   The greatest threat to the type of transactions for which these
   protocols are used is network snooping.  This kind of transaction
   might involve, for example, online access to a database whose use is
   restricted to paying subscribers.  With Basic authentication an
   eavesdropper can obtain the password of the user.  This not only
   permits him to access anything in the database, but, often worse,
   will permit access to anything else the user protects with the same
   password.

   By contrast, with Digest Authentication the eavesdropper only gets
   access to the transaction in question and not to the user's password.
   The information gained by the eavesdropper would permit a replay
   attack, but only with a request for the same document, and even that
   might be difficult.

3.2 Replay Attacks

   A replay attack against digest authentication would usually be
   pointless for a simple GET request since an eavesdropper would
   already have seen the only document he could obtain with a replay.
   This is because the URI of the requested document is digested in the
   client response and the server will only deliver that document. By
   contrast under Basic Authentication once the eavesdropper has the
   user's password, any document protected by that password is open to
   him.  A GET request containing form data could only be "replayed"
   with the identical data.  However, this could be problematic if it
   caused a CGI script to take some action on the server.

   Thus, for some purposes, it is necessary to protect against replay
   attacks.  A good digest implementation can do this in various ways.
   The server created "nonce" value is implementation dependent, but if
   it contains a digest of the client IP, a time-stamp, and a private
   server key (as recommended above) then a replay attack is not simple.
   An attacker must convince the server that the request is coming from
   a false IP address and must cause the server to deliver the document
   to an IP address different from the address to which it believes it
   is sending the document.  An attack can only succeed in the period
   before the time-stamp expires.  Digesting the client IP and time-
   stamp in the nonce permits an implementation which does not maintain
   state between transactions.


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   For applications where no possibility of replay attack can be
   tolerated the server can use one-time response digests which will not
   be honored for a second use.  This requires the overhead of the
   server remembering which digests have been used until the nonce
   time-stamp (and hence the digest built with it) has expired, but it
   effectively protects against replay attacks. Instead of maintaining a
   list of the values of used digests, a server would hash these values
   and require re-authentication whenever a hash collision occurs.

   An implementation must give special attention to the possibility of
   replay attacks with POST and PUT requests.  A successful replay
   attack could result in counterfeit form data or a counterfeit version
   of a PUT file.  The use of one-time digests or one-time nonces is
   recommended.  It is also recommended that the optional <digest> be
   implemented for use with POST or PUT requests to assure the integrity
   of the posted data.  Alternatively, a server may choose to allow
   digest authentication only with GET requests. Responsible server
   implementors will document the risks described here as they pertain
   to a given implementation.

3.3 Man in the Middle

   Both Basic and Digest authentication are vulnerable to "man in the
   middle" attacks, for example, from a hostile or compromised proxy.
   Clearly, this would present all the problems of eavesdropping.  But
   it could also offer some additional threats.

   A simple but effective attack would be to replace the Digest
   challenge with a Basic challenge, to spoof the client into revealing
   their password. To protect against this attack, clients should

   remember if a site has used Digest authentication in the past, and
   warn the user if the site stops using it. It might also be a good
   idea for the browser to be configured to demand Digest authentication
   in general, or from specific sites.

   Or, a hostile proxy might spoof the client into making a request the
   attacker wanted rather than one the client wanted.  Of course, this
   is still much harder than a comparable attack against Basic
   Authentication.

   There are several attacks on the "digest" field in the
   Authentication-info header. The attacker can alter any of the
   entity-headers not incorporated in the computation of the digest, The
   attacker can alter most of the request headers in the client's


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   request, and can alter any response header in the origin-server's
   reply, except those headers whose values are incorporated into the
   "digest" field.

   Alteration of Accept* or User-Agent request headers can only result
   in a denial of service attack that returns content in an unacceptable
   media type or language. Alteration of cache control headers also can
   only result in denial of service. Alteration of Host will be
   detected, if the full URL is in the response-digest. Alteration of
   Referer or From is not important, as these are only hints.

3.4 Spoofing by Counterfeit Servers

   Basic Authentication is vulnerable to spoofing by counterfeit
   servers. If a user can be led to believe that she is connecting to a
   host containing information protected by a password she knows, when
   in fact she is connecting to a hostile server, then the hostile
   server can request a password, store it away for later use, and feign
   an error.  This type of attack is more difficult with Digest
   Authentication -- but the client must know to demand that Digest
   authentication be used, perhaps using some of the techniques
   described above to counter "man-in-the-middle" attacks.

3.5 Storing passwords

   Digest authentication requires that the authenticating agent (usually
   the server) store some data derived from the user's name and password
   in a "password file" associated with a given realm.  Normally this
   might contain pairs consisting of username and H(A1), where H(A1) is
   the digested value of the username, realm, and password as described
   above.

   The security implications of this are that if this password file is
   compromised, then an attacker gains immediate access to documents on
   the server using this realm.  Unlike, say a standard UNIX password
   file, this information need not be decrypted in order to access
   documents in the server realm associated with this file.  On the
   other hand, decryption, or more likely a brute force attack, would be
   necessary to obtain the user's password.  This is the reason that the
   realm is part of the digested data stored in the password file.  It
   means that if one digest authentication password file is compromised,
   it does not automatically compromise others with the same username
   and password (though it does expose them to brute force attack).

   There are two important security consequences of this.  First the
   password file must be protected as if it contained unencrypted
   passwords, because for the purpose of accessing documents in its
   realm, it effectively does.


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   A second consequence of this is that the realm string should be
   unique among all realms which any single user is likely to use.  In
   particular a realm string should include the name of the host doing
   the authentication.  The inability of the client to authenticate the
   server is a weakness of Digest Authentication.

3.6 Summary

   By modern cryptographic standards Digest Authentication is weak.  But
   for a large range of purposes it is valuable as a replacement for
   Basic Authentication.  It remedies many, but not all, weaknesses of
   Basic Authentication.  Its strength may vary depending on the
   implementation.  In particular the structure of the nonce (which is
   dependent on the server implementation) may affect the ease of
   mounting a replay attack.  A range of server options is appropriate
   since, for example, some implementations may be willing to accept the
   server overhead of one-time nonces or digests to eliminate the
   possibility of replay while others may satisfied with a nonce like
   the one recommended above restricted to a single IP address and with
   a limited lifetime.

   The bottom line is that *any* compliant implementation will be
   relatively weak by cryptographic standards, but *any* compliant
   implementation will be far superior to Basic Authentication.

4.  Acknowledgments

   In addition to the authors, valuable discussion instrumental in
   creating this document has come from Peter J. Churchyard, Ned Freed,
   and David M. Kristol.

5. References

   [1]  Berners-Lee, T.,  Fielding, R., and H. Frystyk,
        "Hypertext Transfer Protocol -- HTTP/1.0",
        RFC 1945, May 1996.

   [2]  Berners-Lee, T., Fielding, R., and H. Frystyk,
        "Hypertext Transfer Protocol -- HTTP/1.1"
        RFC 2068,July 30, 1977.

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


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6. Authors' Addresses

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

   EMail: john@math.nwu.edu


   Phillip M. Hallam-Baker
   European Union Fellow
   CERN
   Geneva
   Switzerland

   EMail: hallam@w3.org


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

   EMail: jeff@spyglass.com


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052, USA

   EMail: paulle@microsoft.com


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

   EMail: luotonen@netscape.com


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

   EMail: eric@spyglass.com


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

   EMail: stewart@OpenMarket.com


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