source: draft-ietf-httpbis/orig/rfc2617.xml @ 2496

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<rfc number="2617" category="std" obsoletes="2069" xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:grddl='http://www.w3.org/2003/g/data-view#' grddl:transformation='rfc2629grddl.xslt'>
  <front>
    <title abbrev="HTTP Authentication">HTTP Authentication: Basic and Digest Access Authentication</title>
    <author initials="J." surname="Franks" fullname="John Franks">
      <organization>Northwestern University, Department of Mathematics</organization>
      <address>
        <postal>
          <street>Northwestern University</street>
          <city>Evanston</city>
          <region>IL</region>
          <code>60208-2730</code>
          <country>USA</country>
        </postal>
        <email>john@math.nwu.edu</email>
      </address>
    </author>
    <author initials="P.M." surname="Hallam-Baker" fullname="Phillip M. Hallam-Baker">
      <organization>Verisign Inc.</organization>
      <address>
        <postal>
          <street>301 Edgewater Place</street>
          <street>Suite 210</street>
          <city>Wakefield</city>
          <region>MA</region>
          <code>01880</code>
          <country>USA</country>
        </postal>
        <email>pbaker@verisign.com</email>
      </address>
    </author>
    <author initials="J.L." surname="Hostetler" fullname="Jeffery L. Hostetler">
    <organization>AbiSource, Inc.</organization>
      <address>
        <postal>
          <street>6 Dunlap Court</street>
          <city>Savoy</city>
          <region>IL</region>
          <code>61874</code>
          <country>USA</country>
        </postal>
        <email>jeff@AbiSource.com</email>
      </address>
    </author>
    <author initials="S.D." surname="Lawrence" fullname="Scott D. Lawrence">
      <organization>Agranat Systems, Inc.</organization>
      <address>
        <postal>
          <street>5 Clocktower Place</street>
          <street>Suite 400</street>
          <city>Maynard</city>
          <region>MA</region>
          <code>01754</code>
          <country>USA</country>
        </postal>
        <email>lawrence@agranat.com</email>
      </address>
    </author>
    <author initials="P.J." surname="Leach" fullname="Paul J. Leach">
      <organization>Microsoft Corporation</organization>
      <address>
        <postal>
          <street>1 Microsoft Way</street>
          <city>Redmond</city>
          <region>WA</region>
          <code>98052</code>
          <country>USA</country>
        </postal>
        <email>paulle@microsoft.com</email>
      </address>
    </author>
    <author initials="A." surname="Luotonen" fullname="Ari Luotonen">
      <organization>Netscape Communications Corporation</organization>
      <address>
        <postal>
          <street>501 East Middlefield Road</street>
          <city>Mountain View</city>
          <region>CA</region>
          <code>94043</code>
          <country>USA</country>
        </postal>
      </address>
    </author>
    <author initials="L." surname="Stewart" fullname="Lawrence C. Stewart">
      <organization>Open Market, Inc.</organization>
      <address>
        <postal>
          <street>215 First Street</street>
          <city>Cambridge</city>
          <region>MA</region>
          <code>02142</code>
          <country>USA</country>
        </postal>
        <email>stewart@OpenMarket.com</email>
      </address>
    </author>
    <date month="June" year="1999"/>
        
    <abstract>
      <t>
   "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 (unless used in conjunction with some
   external secure system such as SSL <xref target="RFC2246"/>), as the user name and
   password are passed over the network as cleartext.
      </t><t>
   This document also provides the specification for HTTP's
   authentication framework, the original Basic authentication scheme
   and a scheme based on cryptographic hashes, referred to as "Digest
   Access Authentication".  It is therefore also intended to serve as a
   replacement for RFC 2069 <xref target="RFC2069"/>.  Some optional elements specified by
   RFC 2069 have been removed from this specification due to problems
   found since its publication; other new elements have been added for
   compatibility, those new elements have been made optional, but are
   strongly recommended.
      </t><t>
   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.
    </t>
    </abstract>
  </front>
  <middle>
  
<section title="Access Authentication">

<section title="Reliance on the HTTP/1.1 Specification">
<t>
   This specification is a companion to the HTTP/1.1 specification <xref target="RFC2616"/>.
   It uses the augmented BNF section <xref target="RFC2616" x:fmt="number" x:rel="#notation.abnf"/> of that document, and relies on
   both the non-terminals defined in that document and other aspects of
   the HTTP/1.1 specification.
</t>
</section>

<section title="Access Authentication Framework" anchor="access.authentication.framework">
<t>
   HTTP provides a simple challenge-response authentication mechanism
   that &MAY; be used by a server to challenge a client request and by a
   client to provide authentication information. It uses an extensible,
   case-insensitive token to identify the authentication scheme,
   followed by a comma-separated list of attribute-value pairs which
   carry the parameters necessary for achieving authentication via that
   scheme.
</t>
<figure><artwork type="abnf2616"><iref item="auth-scheme" primary="true"
/>      auth-scheme    = token
<iref item="auth-param" primary="true"
/>      auth-param     = token "=" ( token | quoted-string )
</artwork></figure>
<t>
   The 401 (Unauthorized) response message is used by an origin server
   to challenge the authorization of a user agent. This response &MUST;
   include a WWW-Authenticate header field containing at least one
   challenge applicable to the requested resource. The 407 (Proxy
   Authentication Required) response message is used by a proxy to
   challenge the authorization of a client and &MUST; include a Proxy-Authenticate
   header field containing at least one challenge
   applicable to the proxy for the requested resource.
</t>
<figure><artwork type="abnf2616"><iref item="challenge" primary="true"
/>      challenge   = auth-scheme 1*SP 1#auth-param
</artwork></figure>
<t>
   Note: User agents will need to take special care in parsing the WWW-Authenticate
   or Proxy-Authenticate header field value if it contains
   more than one challenge, or if more than one WWW-Authenticate header
   field is provided, since the contents of a challenge may itself
   contain a comma-separated list of authentication parameters.
</t>
<t>
   The authentication parameter realm is defined for all authentication
   schemes:
</t>
<figure><artwork type="abnf2616"><iref item="realm" primary="true"
/>      realm       = "realm" "=" realm-value
<iref item="realm-value" primary="true"
/>      realm-value = quoted-string
</artwork></figure>
<t>
   The realm directive (case-insensitive) is required for all
   authentication schemes that issue a challenge. The realm value
   (case-sensitive), in combination with the canonical root URL (the
   absoluteURI for the server whose abs_path is empty; see section <xref target="RFC2616" x:fmt="number" x:rel="#request-uri"/>
   of <xref target="RFC2616"/>) of the server being accessed, defines the protection space.
   These realms allow the protected resources on a server to be
   partitioned into a set of protection spaces, each with its own
   authentication scheme and/or authorization database. The realm value
   is a string, generally assigned by the origin server, which may have
   additional semantics specific to the authentication scheme. Note that
   there may be multiple challenges with the same auth-scheme but
   different realms.
</t>
<t>
   A user agent that wishes to authenticate itself with an origin
   server--usually, but not necessarily, after receiving a 401
   (Unauthorized)--MAY do so by including an Authorization header field
   with the request. A client that wishes to authenticate itself with a
   proxy--usually, but not necessarily, after receiving a 407 (Proxy
   Authentication Required)--MAY do so by including a Proxy-Authorization
   header field with the request.  Both the Authorization
   field value and the Proxy-Authorization field value consist of
   credentials containing the authentication information of the client
   for the realm of the resource being requested. The user agent &MUST;
   choose to use one of the challenges with the strongest auth-scheme it
   understands and request credentials from the user based upon that
   challenge.
</t>
<figure><artwork type="abnf2616"><iref item="credentials" primary="true"
/>   credentials = auth-scheme #auth-param
</artwork></figure>
<t>
  <list><t>
      Note that many browsers will only recognize Basic and will require
      that it be the first auth-scheme presented. Servers should only
      include Basic if it is minimally acceptable.
  </t></list>
</t>
<t>
   The protection space determines the domain over which credentials can
   be automatically applied. If a prior request has been authorized, the
   same credentials &MAY; be reused for all other requests within that
   protection space for a period of time determined by the
   authentication scheme, parameters, and/or user preference. Unless
   otherwise defined by the authentication scheme, a single protection
   space cannot extend outside the scope of its server.
</t>
<t>
   If the origin server does not wish to accept the credentials sent
   with a request, it &SHOULD; return a 401 (Unauthorized) response. The
   response &MUST; include a WWW-Authenticate header field containing at
   least one (possibly new) challenge applicable to the requested
   resource. If a proxy does not accept the credentials sent with a
   request, it &SHOULD; return a 407 (Proxy Authentication Required). The
   response &MUST; include a Proxy-Authenticate header field containing a
   (possibly new) challenge applicable to the proxy for the requested
   resource.
</t>
<t>
   The HTTP protocol does not restrict applications to this simple
   challenge-response mechanism for access authentication. Additional
   mechanisms &MAY; be used, such as encryption at the transport level or
   via message encapsulation, and with additional header fields
   specifying authentication information. However, these additional
   mechanisms are not defined by this specification.
</t>
<t>
   Proxies &MUST; be completely transparent regarding user agent
   authentication by origin servers. That is, they must forward the
   WWW-Authenticate and Authorization headers untouched, and follow the
   rules found in section <xref target="RFC2616" x:fmt="number" x:rel="#header.authorization"/> of <xref target="RFC2616"/>. Both the Proxy-Authenticate and
   the Proxy-Authorization header fields are hop-by-hop headers (see
   section <xref target="RFC2616" x:fmt="number" x:rel="#end-to-end.and.hop-by-hop.headers"/> of <xref target="RFC2616"/>).
</t>
</section>
</section>
   
<section title="Basic Authentication Scheme">
<t>
   The "basic" authentication scheme is based on the model that the
   client must authenticate itself with a user-ID and a password for
   each realm.  The realm value should be considered an opaque string
   which can only be compared for equality with other realms on that
   server. The server will service the request only if it can validate
   the user-ID and password for the protection space of the Request-URI.
   There are no optional authentication parameters.
</t>
<t>
   For Basic, the framework above is utilized as follows:
</t>
<figure><artwork type="abnf2616"><iref item="challenge"
/>      challenge   = "Basic" realm
<iref item="credentials"
/>      credentials = "Basic" basic-credentials
</artwork></figure>
<t>
   Upon receipt of an unauthorized request for a URI within the
   protection space, the origin server &MAY; respond with a challenge like
   the following:
</t>
<figure><artwork type="example">
      WWW-Authenticate: Basic realm="WallyWorld"
</artwork></figure>
<t>
   where "WallyWorld" is the string assigned by the server to identify
   the protection space of the Request-URI. A proxy may respond with the
   same challenge using the Proxy-Authenticate header field.
</t>
<t>
   To receive authorization, the client sends the userid and password,
   separated by a single colon (":") character, within a base64 <xref target="RFC2396"/>
   encoded string in the credentials.
</t>
<figure><artwork type="abnf2616"><iref item="basic-credentials" primary="true"
/>      basic-credentials = base64-user-pass
<iref item="base64-user-pass" primary="true"
/>      base64-user-pass  = &lt;base64 [4] encoding of user-pass,
                       except not limited to 76 char/line>
<iref item="user-pass" primary="true"
/>      user-pass   = userid ":" password
<iref item="userid" primary="true"
/>      userid      = *&lt;TEXT excluding ":">
<iref item="password" primary="true"
/>      password    = *TEXT
</artwork></figure>
<t>
   Userids might be case sensitive.
</t>
<t>
   If the user agent wishes to send the userid "Aladdin" and password
   "open sesame", it would use the following header field:
</t>
<figure><artwork type="example">
      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
</artwork></figure>
<t>
   A client &SHOULD; assume that all paths at or deeper than the depth of
   the last symbolic element in the path field of the Request-URI also
   are within the protection space specified by the Basic realm value of
   the current challenge. A client &MAY; preemptively send the
   corresponding Authorization header with requests for resources in
   that space without receipt of another challenge from the server.
   Similarly, when a client sends a request to a proxy, it may reuse a
   userid and password in the Proxy-Authorization header field without
   receiving another challenge from the proxy server. See <xref target="security.considerations"/> for
   security considerations associated with Basic authentication.
</t>
</section>
   
<section title="Digest Access Authentication Scheme">

<section title="Introduction">

<section title="Purpose">
<t>
   The protocol referred to as "HTTP/1.0" includes the specification for
   a Basic Access Authentication scheme<xref target="RFC1945"/>. That 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.
   This section provides the specification for a scheme that does not
   send the password in cleartext,  referred to as "Digest Access
   Authentication".
</t>
<t>
   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 message content. The intent is
   simply to create an access authentication method that avoids the most
   serious flaws of Basic authentication.
</t>
</section>

<section title="Overall Operation">
<t>
   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 not
   addressed by this document.
</t>
</section>

<section title="Representation of digest values">
<t>
   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.
</t>
<t>
   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'.
</t>
</section>

<section title="Limitations">
<t>
   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.
</t>
<t>
   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.
</t>
</section>
</section>

<section title="Specification of Digest Headers" anchor="specification.of.digest.headers">
<t>
   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, Authentication-Info, is specified.
</t>

<section title="The WWW-Authenticate Response Header" anchor="the.www-authenticate.response.header">
<iref item="Headers" subitem="WWW-Authenticate" primary="true"/>
<iref item="WWW-Authenticate header" primary="true"/>
<t>
   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 as
   per the framework defined above, which for the digest scheme is
   utilized as follows:
</t>
<figure><artwork type="abnf2616">
<iref item="challenge"
/>      challenge        =  "Digest" digest-challenge

<iref item="digest-challenge" primary="true"
/>      digest-challenge  = 1#( realm | [ domain ] | nonce |
                          [ opaque ] |[ stale ] | [ algorithm ] |
                          [ qop-options ] | [auth-param] )


<iref item="domain" primary="true"
/>      domain            = "domain" "=" &lt;"> URI ( 1*SP URI ) &lt;">
<iref item="URI" primary="true"
/>      URI               = absoluteURI | abs_path
<iref item="nonce" primary="true"
/>      nonce             = "nonce" "=" nonce-value
<iref item="nonce-value" primary="true"
/>      nonce-value       = quoted-string
<iref item="opaque" primary="true"
/>      opaque            = "opaque" "=" quoted-string
<iref item="stale" primary="true"
/>      stale             = "stale" "=" ( "true" | "false" )
<iref item="algorithm" primary="true"
/>      algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
                           token )
<iref item="qop-options" primary="true"
/>      qop-options       = "qop" "=" &lt;"> 1#qop-value &lt;">
<iref item="qop-value" primary="true"
/>      qop-value         = "auth" | "auth-int" | token
</artwork></figure>
<t>
   The meanings of the values of the directives used above are as
   follows:
</t>
<t>
   realm
   <list><t>
     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".
   </t></list>
</t>
<t>
   domain
   <list><t>
     A quoted, space-separated list of URIs, as specified in RFC XURI
     <xref target="RFC2396"/>, that define the protection space.  If a URI is an abs_path, it
     is relative to the canonical root URL (see <xref target="access.authentication.framework"/> above) of
     the server being accessed. An absoluteURI in this list may refer to
     a different server than the one being accessed. The client can use
     this list to determine the set of URIs for which the same
     authentication information may be sent: any URI that has a URI in
     this list as a prefix (after both have been made absolute) may be
     assumed to be in the same protection space. If this directive is
     omitted or its value is empty, the client should assume that the
     protection space consists of all URIs on the responding server.
     This directive is not meaningful in Proxy-Authenticate headers, for
     which the protection space is always the entire proxy; if present
     it should be ignored.
   </t></list>
</t>
<t>
   nonce
   <list><t>
     A server-specified data string which should 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.
  </t>
  <t>
     The contents of the nonce are implementation dependent. The quality
     of the implementation depends on a good choice. A nonce might, for
     example, be constructed as the base 64 encoding of
  </t>
  <t><figure><artwork type="code">
         time-stamp H(time-stamp ":" ETag ":" private-key)
  </artwork></figure></t>
  <t>
     where time-stamp is a server-generated time or other non-repeating
     value, ETag is the value of the HTTP ETag header associated with
     the requested entity, and private-key is data known only to the
     server.  With a nonce of this form a server would recalculate the
     hash portion after receiving the client authentication header and
     reject the request if it did not match the nonce from that header
     or if the time-stamp value is not recent enough. In this way the
     server can limit the time of the nonce's validity. The inclusion of
     the ETag prevents a replay request for an updated version of the
     resource.  (Note: including the IP address of the client in the
     nonce would appear to offer the server the ability to limit the
     reuse of the nonce to the same client that originally got it.
     However, that would break proxy farms, where requests from a single
     user often go through different proxies in the farm. Also, IP
     address spoofing is not that hard.)
  </t>
  <t>
     An implementation might choose not to accept a previously used
     nonce or a previously used digest, in order 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 <xref target="security.considerations"/>
     of this document.
  </t>
  <t>
     The nonce is opaque to the client.
   </t></list></t>
<t>
   opaque
   <list><t>
     A string of data, specified by the server, which should be returned
     by the client unchanged in the Authorization header of subsequent
     requests with URIs in the same protection space. It is recommended
     that this string be base64 or hexadecimal data.
   </t></list>
</t>
<t>
   stale
   <list><t>
     A flag, indicating that the previous request from the client was
     rejected because the nonce value was stale. If stale is TRUE
     (case-insensitive), 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). If stale is FALSE, or anything other
     than TRUE, or the stale directive is not present, the username
     and/or password are invalid, and new values must be obtained.
   </t></list>
</t>
<t>
   algorithm
   <list><t>
     A string indicating a pair of algorithms used to produce the digest
     and a checksum. If this is not present it is assumed to be "MD5".
     If the algorithm is not understood, the challenge should be ignored
     (and a different one used, if there is more than one).
    </t>
    <t>
     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). The
     notation unq(X) means the value of the quoted-string X without the
     surrounding quotes.
    </t>
    <t>
     For the "MD5" and "MD5-sess" algorithms
    </t>
    <t><figure><artwork type="code">
         H(data) = MD5(data)
    </artwork></figure></t>
    <t>
     and
    </t>
    <t><figure><artwork type="code">
         KD(secret, data) = H(concat(secret, ":", data))
    </artwork></figure></t>
    <t>
     i.e., the digest is the MD5 of the secret concatenated with a colon
     concatenated with the data. The "MD5-sess" algorithm is intended to
     allow efficient 3rd party authentication servers; for the
     difference in usage, see the description in <xref target="A1"/>.
   </t></list>
</t>
<t>
   qop-options
   <list><t>
     This directive is optional, but is made so only for backward
     compatibility with RFC 2069 <xref target="RFC2069"/>; it &SHOULD; be used by all
     implementations compliant with this version of the Digest scheme.
     If present, it is a quoted string of one or more tokens indicating
     the "quality of protection" values supported by the server.  The
     value "auth" indicates authentication; the value "auth-int"
     indicates authentication with integrity protection; see the
     descriptions below for calculating the response directive value for
     the application of this choice. Unrecognized options &MUST; be
     ignored.
   </t></list>
</t>
<t>
   auth-param
   <list><t>
     This directive allows for future extensions. Any unrecognized
     directive &MUST; be ignored.
   </t></list>
</t>
</section>

<section title="The Authorization Request Header" anchor="the.authorization.request.header">
<iref item="Headers" subitem="Authorization" primary="true"/>
<iref item="Authorization header" primary="true"/>
<t>
   The client is expected to retry the request, passing an Authorization
   header line, which is defined according to the framework above,
   utilized as follows.
</t>
<figure><artwork type="abnf2616">
<iref item="credentials"
/>       credentials      = "Digest" digest-response
<iref item="digest-response" primary="true"
/>       digest-response  = 1#( username | realm | nonce | digest-uri
                       | response | [ algorithm ] | [cnonce] |
                       [opaque] | [message-qop] |
                           [nonce-count]  | [auth-param] )

<iref item="username" primary="true"
/>       username         = "username" "=" username-value
<iref item="username-value" primary="true"
/>       username-value   = quoted-string
<iref item="digest-uri" primary="true"
/>       digest-uri       = "uri" "=" digest-uri-value
<iref item="digest-uri-value" primary="true"
/>       digest-uri-value = request-uri   ; As specified by HTTP/1.1
<iref item="message-qop" primary="true"
/>       message-qop      = "qop" "=" qop-value
<iref item="cnonce" primary="true"
/>       cnonce           = "cnonce" "=" cnonce-value
<iref item="cnonce-value" primary="true"
/>       cnonce-value     = nonce-value
<iref item="nonce-count" primary="true"
/>       nonce-count      = "nc" "=" nc-value
<iref item="nc-value" primary="true"
/>       nc-value         = 8LHEX
<iref item="response" primary="true"
/>       response         = "response" "=" request-digest
<iref item="request-digest" primary="true"
/>       request-digest = &lt;"> 32LHEX &lt;">
<iref item="LHEX" primary="true"
/>       LHEX             =  "0" | "1" | "2" | "3" |
                           "4" | "5" | "6" | "7" |
                           "8" | "9" | "a" | "b" |
                           "c" | "d" | "e" | "f"
</artwork></figure>
<t>
   The values of the opaque and algorithm fields must be those supplied
   in the WWW-Authenticate response header for the entity being
   requested.
</t>
<t>
   response
   <list><t>
     A string of 32 hex digits computed as defined below, which proves
     that the user knows a password
   </t></list>
</t>
<t>
   username
   <list><t>
     The user's name in the specified realm.
   </t></list>
</t>
<t>
   digest-uri
   <list><t>
     The URI from Request-URI of the Request-Line; duplicated here
     because proxies are allowed to change the Request-Line in transit.
   </t></list>
</t>
<t>
   qop
   <list><t>
     Indicates what "quality of protection" the client has applied to
     the message. If present, its value &MUST; be one of the alternatives
     the server indicated it supports in the WWW-Authenticate header.
     These values affect the computation of the request-digest. Note
     that this is a single token, not a quoted list of alternatives as
     in WWW-Authenticate.  This directive is optional in order to
     preserve backward compatibility with a minimal implementation of
     RFC 2069 <xref target="RFC2069"/>, but &SHOULD; be used if the server indicated that qop
     is supported by providing a qop directive in the WWW-Authenticate
     header field.
   </t></list>
</t>
<t>
   cnonce
   <list><t>
     This &MUST; be specified if a qop directive is sent (see above), and
     &MUST-NOT; be specified if the server did not send a qop directive in
     the WWW-Authenticate header field.  The cnonce-value is an opaque
     quoted string value provided by the client and used by both client
     and server to avoid chosen plaintext attacks, to provide mutual
     authentication, and to provide some message integrity protection.
     See the descriptions below of the calculation of the response-digest
     and request-digest values.
   </t></list>
</t>
<t>
   nonce-count
   <list><t>
     This &MUST; be specified if a qop directive is sent (see above), and
     &MUST-NOT; be specified if the server did not send a qop directive in
     the WWW-Authenticate header field.  The nc-value is the hexadecimal
     count of the number of requests (including the current request)
     that the client has sent with the nonce value in this request.  For
     example, in the first request sent in response to a given nonce
     value, the client sends "nc=00000001".  The purpose of this
     directive is to allow the server to detect request replays by
     maintaining its own copy of this count - if the same nc-value is
     seen twice, then the request is a replay.   See the description
     below of the construction of the request-digest value.
   </t></list>
</t>
<t>
   auth-param
   <list><t>
     This directive allows for future extensions. Any unrecognized
     directive &MUST; be ignored.
   </t></list>
</t>
<t>
   If a directive or its value is improper, or required directives are
   missing, the proper response is 400 Bad Request. If the request-digest
   is invalid, then a login failure should be logged, since
   repeated login failures from a single client may indicate an attacker
   attempting to guess passwords.
</t>
<t>
   The definition of request-digest above indicates the encoding for its
   value. The following definitions show how the value is computed.
</t>

<section title="Request-Digest" anchor="request-digest">
<t>
   If the "qop" value is "auth" or "auth-int":
</t>
<figure><artwork type="abnf2616">
      request-digest  = &lt;"> &lt; KD ( H(A1),     unq(nonce-value)
                                          ":" nc-value
                                          ":" unq(cnonce-value)
                                          ":" unq(qop-value)
                                          ":" H(A2)
                                  ) &lt;">
</artwork></figure>
<t>
   If the "qop" directive is not present (this construction is for
   compatibility with RFC 2069):
</t>
<figure><artwork type="abnf2616">
      request-digest  =
                 &lt;"> &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) >
   &lt;">
</artwork></figure>
<t>
   See below for the definitions for A1 and A2.
</t>
</section>

<section title="A1" anchor="A1">
<t>
   If the "algorithm" directive's value is "MD5" or is unspecified, then
   A1 is:
</t>
<figure><artwork type="abnf2616">
      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
</artwork></figure>
<t>
   where
</t>
<figure><artwork type="abnf2616">
      passwd   = &lt; user's password >
</artwork></figure>
<t>
   If the "algorithm" directive's value is "MD5-sess", then A1 is
   calculated only once - on the first request by the client following
   receipt of a WWW-Authenticate challenge from the server.  It uses the
   server nonce from that challenge, and the first client nonce value to
   construct A1 as follows:
</t>
<figure><artwork type="abnf2616">
      A1       = H( unq(username-value) ":" unq(realm-value)
                     ":" passwd )
                     ":" unq(nonce-value) ":" unq(cnonce-value)
</artwork></figure>
<t>
   This creates a 'session key' for the authentication of subsequent
   requests and responses which is different for each "authentication
   session", thus limiting the amount of material hashed with any one
   key.  (Note: see further discussion of the authentication session in
   <xref target="digest.operation"/>) Because the server need only use the hash of the user
   credentials in order to create the A1 value, this construction could
   be used in conjunction with a third party authentication service so
   that the web server would not need the actual password value.  The
   specification of such a protocol is beyond the scope of this
   specification.
</t>
</section>

<section title="A2">
<t>
   If the "qop" directive's value is "auth" or is unspecified, then A2
   is:
</t>
<figure><artwork type="abnf2616">
      A2       = Method ":" digest-uri-value
</artwork></figure>
<t>
   If the "qop" value is "auth-int", then A2 is:
</t>
<figure><artwork type="abnf2616">
      A2       = Method ":" digest-uri-value ":" H(entity-body)
</artwork></figure>
</section>


<section title="Directive values and quoted-string">
<t>
   Note that the value of many of the directives, such as "username-value",
   are defined as a "quoted-string". However, the "unq" notation
   indicates that surrounding quotation marks are removed in forming the
   string A1. Thus if the Authorization header includes the fields
</t>
<figure><artwork type="example">
     username="Mufasa", realm=myhost@testrealm.com
</artwork></figure>
<t>
   and the user Mufasa has password "Circle Of Life" then H(A1) would be
   H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks
   in the digested string.
</t>
<t>
   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 illustrated above must be
</t>
<figure><artwork type="example">
        Mufasa:myhost@testrealm.com:Circle Of Life
</artwork></figure>
<t>
   with no white space on either side of the colons, but with the white
   space between the words used in the password value.  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.
</t>
<t>
   Also note that if integrity protection is applied (qop=auth-int), the
   H(entity-body) is the hash of the entity body, not the message body -
   it is computed before any transfer encoding is applied by the sender
   and after it has been removed by the recipient. Note that this
   includes multipart boundaries and embedded headers in each part of
   any multipart content-type.
</t>
</section>

<section title="Various considerations">
<t>
   The "Method" value is the HTTP request method as specified in section
   <xref target="RFC2616" x:fmt="number" x:rel="#method"/> of <xref target="RFC2616"/>. The "request-uri" value is the Request-URI from the
   request line as specified in section <xref target="RFC2616" x:fmt="number" x:rel="#request-uri"/> of <xref target="RFC2616"/>. This may be "*",
   an "absoluteURL" or an "abs_path" as specified in section <xref target="RFC2616" x:fmt="number" x:rel="#request-uri"/> of
   <xref target="RFC2616"/>, but it &MUST; agree with the Request-URI. In particular, it &MUST;
   be an "absoluteURL" if the Request-URI is an "absoluteURL". The
   "cnonce-value" is an optional  client-chosen value whose purpose is
   to foil chosen plaintext attacks.
</t>
<t>
   The authenticating server must assure that the resource designated by
   the "uri" directive is the same as the resource specified in the
   Request-Line; if they are not, the server &SHOULD; return a 400 Bad
   Request error. (Since this may be a symptom of an attack, server
   implementers may want to consider logging such errors.) 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-Line. This altered (but presumably semantically
   equivalent) request would not result in the same digest as that
   calculated by the client.
</t>
<t>
   Implementers should be aware of how authenticated transactions
   interact with shared caches. The HTTP/1.1 protocol specifies that
   when a shared cache (see section <xref target="RFC2616" x:fmt="number" x:rel="#shared.and.non-shared.caches"/> of <xref target="RFC2616"/>) 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 <xref target="RFC2616" x:fmt="number" x:rel="#header.cache-control"/> of <xref target="RFC2616"/>)
   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.
</t>
</section>
</section>

<section title="The Authentication-Info Header">
<iref item="Headers" subitem="Authentication-Info" primary="true"/>
<iref item="Authentication-Info header" primary="true"/>
<t>
   The Authentication-Info header is used by the server to communicate
   some information regarding the successful authentication in the
   response.
</t>
<figure><artwork type="abnf2616"><iref item="Authentication-Info" primary="true"
/>        AuthenticationInfo = "Authentication-Info" ":" auth-info
<iref item="auth-info" primary="true"
/>        auth-info          = 1#(nextnonce | [ message-qop ]
                               | [ response-auth ] | [ cnonce ]
                               | [nonce-count] )
<iref item="nextnonce" primary="true"
/>        nextnonce          = "nextnonce" "=" nonce-value
<iref item="response-auth" primary="true"
/>        response-auth      = "rspauth" "=" response-digest
<iref item="response-digest" primary="true"
/>        response-digest    = &lt;"> *LHEX &lt;">
</artwork></figure>
<t>
   The value of the nextnonce directive is the nonce the server wishes
   the client to use for a future authentication response.  The server
   may send the Authentication-Info header with a nextnonce field as a
   means of implementing one-time or otherwise changing  nonces. If the
   nextnonce field is present the client &SHOULD; use it when constructing
   the Authorization header for its next request. Failure of the client
   to do so may result in a request to re-authenticate from the server
   with the "stale=TRUE".
</t>
<t>
  <list><t>
     Server implementations should carefully consider the performance
     implications of the use of this mechanism; pipelined requests will
     not be possible if every response includes a nextnonce directive
     that must be used on the next request received by the server.
     Consideration should be given to the performance vs. security
     tradeoffs of allowing an old nonce value to be used for a limited
     time to permit request pipelining.  Use of the nonce-count can
     retain most of the security advantages of a new server nonce
     without the deleterious affects on pipelining.
  </t></list>
</t>
<t>
   message-qop
</t>
<t><list><t>
     Indicates the "quality of protection" options applied to the
     response by the server.  The value "auth" indicates authentication;
     the value "auth-int" indicates authentication with integrity
     protection. The server &SHOULD; use the same value for the message-qop
     directive in the response as was sent by the client in the
     corresponding request.
</t></list></t>
<t>
   The optional response digest in the "response-auth" directive
   supports mutual authentication -- the server proves that it knows the
   user's secret, and with qop=auth-int also provides limited integrity
   protection of the response. The "response-digest" value is calculated
   as for the "request-digest" in the Authorization header, except that
   if "qop=auth" or is not specified in the Authorization header for the
   request, A2 is
</t>
<figure><artwork type="abnf2616">
      A2       = ":" digest-uri-value
</artwork></figure>
<t>
   and if "qop=auth-int", then A2 is
</t>
<figure><artwork type="abnf2616">
      A2       = ":" digest-uri-value ":" H(entity-body)
</artwork></figure>
<t>
   where "digest-uri-value" is the value of the "uri" directive on the
   Authorization header in the request. The "cnonce-value" and "nc-value"
   &MUST; be the ones for the client request to which this message
   is the response. The "response-auth", "cnonce", and "nonce-count"
   directives &MUST; BE present if "qop=auth" or "qop=auth-int" is
   specified.
</t>
<t>
   The Authentication-Info header is allowed in the trailer of an HTTP
   message transferred via chunked transfer-coding.
</t>
</section>
</section>

<section title="Digest Operation" anchor="digest.operation">
<t>
   Upon receiving the Authorization header, the server may check its
   validity by looking up the password that corresponds to the submitted
   username. Then, the server must perform the same digest operation
   (e.g., MD5) performed by the client, and compare the result to the
   given request-digest value.
</t>
<t>
   Note that the HTTP server does not actually need to know the user's
   cleartext password. As long as H(A1) is available to the server, the
   validity of an Authorization header may be verified.
</t>
<t>
   The client response to a WWW-Authenticate challenge for a protection
   space starts an authentication session with that protection space.
   The authentication session lasts until the client receives another
   WWW-Authenticate challenge from any server in the protection space. A
   client should remember the username, password, nonce, nonce count and
   opaque values associated with an authentication session to use to
   construct the Authorization header in future requests within that
   protection space. The Authorization header may be included
   preemptively; doing so improves server efficiency and avoids extra
   round trips for authentication challenges. The server may choose to
   accept the old Authorization header information, even though the
   nonce value included might not be fresh. Alternatively, the server
   may 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 tells the client to retry with the new nonce, but without
   prompting for a new username and password.
</t>
<t>
   Because the client is required to return the value of the opaque
   directive given to it by the server for the duration of a session,
   the opaque data may be used to transport authentication session state
   information. (Note that any such use can also be accomplished more
   easily and safely by including the state in the nonce.) For example,
   a server could be responsible for authenticating content that
   actually sits on another server. It would achieve this by having the
   first 401 response include a domain directive whose value includes a
   URI on the second server, and an opaque directive whose value
   contains the state information. The client will retry the request, at
   which time the server might respond with a 301/302 redirection,
   pointing to the URI on the second server. The client will follow the
   redirection, and pass an Authorization header , including the
   &lt;opaque> data.
</t>
<t>
   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 <xref target="proxy-authentication.and.proxy-authorization"/>
   below.
</t>
</section>

<section title="Security Protocol Negotiation">
<t>
   It is useful for a server to be able to know which security schemes a
   client is capable of handling.
</t>
<t>
   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 only authentication schemes it cannot handle.
</t>
</section>

<section title="Example" anchor="specification.of.digest.headers.example">
<t>
   The following example assumes that an access-protected document is
   being requested from the server via a GET request. 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 "Circle Of Life" (with one space between each of the
   three words).
</t>
<t>
   The first time the client requests the document, no Authorization
   header is sent, so the server responds with:
</t>
<figure><artwork type='message/http; msgytpe="response"'>
         HTTP/1.1 401 Unauthorized
         WWW-Authenticate: Digest
                 realm="testrealm@host.com",
                 qop="auth,auth-int",
                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
</artwork></figure>
<t>
   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:
</t>
<figure><artwork type="example">
         Authorization: Digest username="Mufasa",
                 realm="testrealm@host.com",
                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
                 uri="/dir/index.html",
                 qop=auth,
                 nc=00000001,
                 cnonce="0a4f113b",
                 response="6629fae49393a05397450978507c4ef1",
                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
</artwork></figure>
</section>

<section title="Proxy-Authentication and Proxy-Authorization" anchor="proxy-authentication.and.proxy-authorization">
<t>
   The digest authentication scheme may also be used for authenticating
   users to proxies, proxies to proxies, or proxies to origin servers by
   use of the Proxy-Authenticate and Proxy-Authorization headers. These
   headers are instances of the Proxy-Authenticate and Proxy-Authorization
   headers specified in sections <xref target="RFC2616" x:fmt="number" x:sec="10.33"/> and <xref target="RFC2616" x:fmt="number" x:sec="10.34"/> of the
   HTTP/1.1 specification <xref target="RFC2616"/> 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 "407 Proxy Authentication Required" response with a
   "Proxy-Authenticate" header.  The digest-challenge used in the
   Proxy-Authenticate header is the same as that for the WWW-Authenticate
   header as defined above in <xref target="the.www-authenticate.response.header"/>.
</t>
<t>
   The client/proxy must then re-issue the request with a Proxy-Authorization
   header, with directives as specified for the
   Authorization header in <xref target="the.authorization.request.header"/> above.
</t>
<t>
   On subsequent responses, the server sends Proxy-Authentication-Info
   with directives the same as those for the Authentication-Info header
   field.
</t>
<t>
   Note that in principle a client could be asked to authenticate itself
   to both a proxy and an end-server, but never in the same response.
</t>
</section>
</section>
   
<section title="Security Considerations" anchor="security.considerations">

<section title="Authentication of Clients using Basic Authentication">
<t>
   The Basic authentication scheme is not a secure method of user
   authentication, nor does it in any way protect the entity, which is
   transmitted in cleartext across the physical network used as the
   carrier. HTTP does not prevent additional authentication schemes and
   encryption mechanisms from being employed to increase security or the
   addition of enhancements (such as schemes to use one-time passwords)
   to Basic authentication.
</t>
<t>
   The most serious flaw in Basic authentication is that it results in
   the essentially cleartext transmission of the user's password over
   the physical network. It is this problem which Digest Authentication
   attempts to address.
</t>
<t>
   Because Basic authentication involves the cleartext transmission of
   passwords it &SHOULD-NOT; be used (without enhancements) to protect
   sensitive or valuable information.
</t>
<t>
   A common use of Basic authentication is for identification purposes
   -- requiring the user to provide a user name and password as a means
   of identification, for example, for purposes of gathering accurate
   usage statistics on a server. When used in this way it is tempting to
   think that there is no danger in its use if illicit access to the
   protected documents is not a major concern. This is only correct if
   the server issues both user name and password to the users and in
   particular does not allow the user to choose his or her own password.
   The danger arises because naive users frequently reuse a single
   password to avoid the task of maintaining multiple passwords.
</t>
<t>
   If a server permits users to select their own passwords, then the
   threat is not only unauthorized access to documents on the server but
   also unauthorized access to any other resources on other systems that
   the user protects with the same password. Furthermore, in the
   server's password database, many of the passwords may also be users'
   passwords for other sites. The owner or administrator of such a
   system could therefore expose all users of the system to the risk of
   unauthorized access to all those sites if this information is not
   maintained in a secure fashion.
</t>
<t>
   Basic Authentication is also vulnerable to spoofing by counterfeit
   servers. If a user can be led to believe that he is connecting to a
   host containing information protected by Basic authentication when,
   in fact, he is connecting to a hostile server or gateway, then the
   attacker can request a password, store it for later use, and feign an
   error. This type of attack is not possible with Digest
   Authentication. Server implementers &SHOULD; guard against the
   possibility of this sort of counterfeiting by gateways or CGI
   scripts. In particular it is very dangerous for a server to simply
   turn over a connection to a gateway.  That gateway can then use the
   persistent connection mechanism to engage in multiple transactions
   with the client while impersonating the original server in a way that
   is not detectable by the client.
</t>
</section>

<section title="Authentication of Clients using Digest Authentication">
<t>
   Digest Authentication does not provide a strong authentication
   mechanism, when compared to public key based mechanisms, for example.
   However, it is significantly stronger than (e.g.) CRAM-MD5, which has
   been proposed for use with LDAP <xref target="ref10"/>, POP and IMAP (see RFC 2195
   <xref target="RFC2195"/>).  It is intended to replace the much weaker and even more
   dangerous Basic mechanism.
</t>
<t>
   Digest Authentication offers no confidentiality protection beyond
   protecting the actual password. All of the rest of the request and
   response are available to an eavesdropper.
</t>
<t>
   Digest Authentication offers only limited integrity protection for
   the messages in either direction. If  qop=auth-int mechanism is used,
   those parts of the message used in the calculation of the WWW-Authenticate
   and Authorization header field response directive values
   (see <xref target="specification.of.digest.headers"/> above) are  protected.  Most header fields and their
   values could be modified as a part of a man-in-the-middle attack.
</t>
<t>
   Many needs for secure HTTP transactions cannot be met by Digest
   Authentication. For those needs TLS or SHTTP are more appropriate
   protocols. In particular Digest authentication cannot be used for any
   transaction requiring confidentiality protection.  Nevertheless many
   functions remain for which Digest authentication is both useful and
   appropriate.  Any service in present use that uses Basic should be
   switched to Digest as soon as practical.
</t>
</section>

<section title="Limited Use Nonce Values">
<t>
   The Digest scheme uses a server-specified nonce to seed the
   generation of the request-digest value (as specified in <xref target="request-digest"/>
   above).  As shown in the example nonce in <xref target="the.www-authenticate.response.header"/>, the
   server is free to construct the nonce such that it may only be used
   from a particular client, for a particular resource, for a limited
   period of time or number of uses, or any other restrictions.  Doing
   so strengthens the protection provided against, for example, replay
   attacks (see <xref target="replay.attacks" format="counter"/>).  However, it should be noted that the method
   chosen for generating and checking the nonce also has performance and
   resource implications.  For example, a server may choose to allow
   each nonce value to be used only once by maintaining a record of
   whether or not each recently issued nonce has been returned and
   sending a next-nonce directive in the Authentication-Info header
   field of every response. This protects against even an immediate
   replay attack, but has a high cost checking nonce values, and perhaps
   more important will cause authentication failures for any pipelined
   requests (presumably returning a stale nonce indication).  Similarly,
   incorporating a request-specific element such as the Etag value for a
   resource limits the use of the nonce to that version of the resource
   and also defeats pipelining. Thus it may be useful to do so for
   methods with side effects but have unacceptable performance for those
   that do not.
</t>
</section>

<section title="Comparison of Digest with Basic Authentication">
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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
   may be limited by the server's choice of nonce.
</t>
</section>

<section title="Replay Attacks" anchor="replay.attacks">
<t>
   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 request 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.
</t>
<t>
   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, the resource
   ETag, 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.
</t>
<t>
   For applications where no possibility of replay attack can be
   tolerated the server can use one-time nonce values which will not be
   honored for a second use. This requires the overhead of the server
   remembering which nonce values have been used until the nonce time-stamp
   (and hence the digest built with it) has expired, but it
   effectively protects against replay attacks.
</t>
<t>
   An implementation must give special attention to the possibility of
   replay attacks with POST and PUT requests. Unless the server employs
   one-time or otherwise limited-use nonces and/or insists on the use of
   the integrity protection of qop=auth-int, an attacker could replay
   valid credentials from a successful request with counterfeit form
   data or other message body. Even with the use of integrity protection
   most metadata in header fields is not protected. Proper nonce
   generation and checking provides some protection against replay of
   previously used valid credentials, but see 4.8.
</t>
</section>

<section title="Weakness Created by Multiple Authentication Schemes">
<t>
   An HTTP/1.1 server may return multiple challenges with a 401
   (Authenticate) response, and each challenge may use a different
   auth-scheme. A user agent &MUST; choose to use the strongest auth-scheme
   it understands and request credentials from the user based
   upon that challenge.
</t>
<t>
  <list><t>
      Note that many browsers will only recognize Basic and will require
      that it be the first auth-scheme presented. Servers should only
      include Basic if it is minimally acceptable.
  </t></list>
</t>
<t>
   When the server offers choices of authentication schemes using the
   WWW-Authenticate header, the strength of the resulting authentication
   is only as good as that of the of the weakest of the authentication
   schemes. See <xref target="man.in.the.middle"/> below for discussion of particular attack
   scenarios that exploit multiple authentication schemes.
</t>
</section>

<section title="Online dictionary attacks">
<t>
   If the attacker can eavesdrop, then it can test any overheard
   nonce/response pairs against a list of common words. Such a list is
   usually much smaller than the total number of possible passwords. The
   cost of computing the response for each password on the list is paid
   once for each challenge.
</t>
<t>
   The server can mitigate this attack by not allowing users to select
   passwords that are in a dictionary.
</t>
</section>

<section title="Man in the Middle" anchor="man.in.the.middle">
<t>
   Both Basic and Digest authentication are vulnerable to "man in the
   middle" (MITM) attacks, for example, from a hostile or compromised
   proxy. Clearly, this would present all the problems of eavesdropping.
   But it also offers some additional opportunities to the attacker.
</t>
<t>
   A possible man-in-the-middle attack would be to add a weak
   authentication scheme to the set of choices, hoping that the client
   will use one that exposes the user's credentials (e.g. password). For
   this reason, the client should always use the strongest scheme that
   it understands from the choices offered.
</t>
<t>
   An even better MITM attack would be to remove all offered choices,
   replacing them with a challenge that requests only Basic
   authentication, then uses the cleartext credentials from the Basic
   authentication to authenticate to the origin server using the
   stronger scheme it requested. A particularly insidious way to mount
   such a MITM attack would be to offer a "free" proxy caching service
   to gullible users.
</t>
<t>
   User agents should consider measures such as presenting a visual
   indication at the time of the credentials request of what
   authentication scheme is to be used, or remembering the strongest
   authentication scheme ever requested by a server and produce a
   warning message before using a weaker one. It might also be a good
   idea for the user agent to be configured to demand Digest
   authentication in general, or from specific sites.
</t>
<t>
   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.
</t>
</section>

<section title="Chosen plaintext attacks">
<t>
   With Digest authentication, a MITM or a malicious server can
   arbitrarily choose the nonce that the client will use to compute the
   response. This is called a "chosen plaintext" attack. The ability to
   choose the nonce is known to make cryptanalysis much easier <xref target="ref8"/>.
</t>
<t>
   However, no way to analyze the MD5 one-way function used by Digest
   using chosen plaintext is currently known.
</t>
<t>
   The countermeasure against this attack is for clients to be
   configured to require the use of the optional "cnonce" directive;
   this allows the client to vary the input to the hash in a way not
   chosen by the attacker.
</t>
</section>

<section title="Precomputed dictionary attacks">
<t>
   With Digest authentication, if the attacker can execute a chosen
   plaintext attack, the attacker can precompute the response for many
   common words to a nonce of its choice, and store a dictionary of
   (response, password) pairs. Such precomputation can often be done in
   parallel on many machines. It can then use the chosen plaintext
   attack to acquire a response corresponding to that challenge, and
   just look up the password in the dictionary. Even if most passwords
   are not in the dictionary, some might be. Since the attacker gets to
   pick the challenge, the cost of computing the response for each
   password on the list can be amortized over finding many passwords. A
   dictionary with 100 million password/response pairs would take about
   3.2 gigabytes of disk storage.
</t>
<t>
   The countermeasure against this attack is to for clients to be
   configured to require the use of the optional "cnonce" directive.
</t>
</section>

<section title="Batch brute force attacks">
<t>
   With Digest authentication, a MITM can execute a chosen plaintext
   attack, and can gather responses from many users to the same nonce.
   It can then find all the passwords within any subset of password
   space that would generate one of the nonce/response pairs in a single
   pass over that space. It also reduces the time to find the first
   password by a factor equal to the number of nonce/response pairs
   gathered. This search of the password space can often be done in
   parallel on many machines, and even a single machine can search large
   subsets of the password space very quickly -- reports exist of
   searching all passwords with six or fewer letters in a few hours.
</t>
<t>
   The countermeasure against this attack is to for clients to be
   configured to require the use of the optional "cnonce" directive.
</t>
</section>

<section title="Spoofing by Counterfeit Servers">
<t>
   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.  Again, the
   user can be helped in detecting this attack by a visual indication of
   the authentication mechanism in use with appropriate guidance in
   interpreting the implications of each scheme.
</t>
</section>

<section title="Storing passwords">
<t>
   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.
</t>
<t>
   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).
</t>
<t>
   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.
</t>
<t>
   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.
</t>
</section>

<section title="Summary">
<t>
   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 some, 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. Others may satisfied with a nonce like the one
   recommended above restricted to a single IP address and a single ETag
   or with a limited lifetime.
</t>
<t>
   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.
</t>
</section>
</section>
   
<section title="Sample implementation">
<t>
   The following code implements the calculations of H(A1), H(A2),
   request-digest and response-digest, and a test program which computes
   the values used in the example of <xref target="specification.of.digest.headers.example"/>. It uses the MD5
   implementation from RFC 1321.
</t>
<figure><preamble>
   File "digcalc.h":
</preamble><artwork type="code" name="digcalc.h">
#define HASHLEN 16
typedef char HASH[HASHLEN];
#define HASHHEXLEN 32
typedef char HASHHEX[HASHHEXLEN+1];
#define IN
#define OUT

/* calculate H(A1) as per HTTP Digest spec */
void DigestCalcHA1(
    IN char * pszAlg,
    IN char * pszUserName,
    IN char * pszRealm,
    IN char * pszPassword,
    IN char * pszNonce,
    IN char * pszCNonce,
    OUT HASHHEX SessionKey
    );

/* calculate request-digest/response-digest as per HTTP Digest spec */
void DigestCalcResponse(
    IN HASHHEX HA1,           /* H(A1) */
    IN char * pszNonce,       /* nonce from server */
    IN char * pszNonceCount,  /* 8 hex digits */
    IN char * pszCNonce,      /* client nonce */
    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
    IN char * pszMethod,      /* method from the request */
    IN char * pszDigestUri,   /* requested URL */
    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
    OUT HASHHEX Response      /* request-digest or response-digest */
    );
</artwork></figure>
<figure><preamble>
File "digcalc.c":
</preamble><artwork type="example" name="digcalc.c">
#include &lt;global.h>
#include &lt;md5.h>
#include &lt;string.h>
#include "digcalc.h"

void CvtHex(
    IN HASH Bin,
    OUT HASHHEX Hex
    )
{
    unsigned short i;
    unsigned char j;

    for (i = 0; i &lt; HASHLEN; i++) {
        j = (Bin[i] >> 4) &amp; 0xf;
        if (j &lt;= 9)
            Hex[i*2] = (j + '0');
         else
            Hex[i*2] = (j + 'a' - 10);
        j = Bin[i] &amp; 0xf;
        if (j &lt;= 9)
            Hex[i*2+1] = (j + '0');
         else
            Hex[i*2+1] = (j + 'a' - 10);
    };
    Hex[HASHHEXLEN] = '\0';
};

/* calculate H(A1) as per spec */
void DigestCalcHA1(
    IN char * pszAlg,
    IN char * pszUserName,
    IN char * pszRealm,
    IN char * pszPassword,
    IN char * pszNonce,
    IN char * pszCNonce,
    OUT HASHHEX SessionKey
    )
{
      MD5_CTX Md5Ctx;
      HASH HA1;

      MD5Init(&amp;Md5Ctx);
      MD5Update(&amp;Md5Ctx, pszUserName, strlen(pszUserName));
      MD5Update(&amp;Md5Ctx, ":", 1);
      MD5Update(&amp;Md5Ctx, pszRealm, strlen(pszRealm));
      MD5Update(&amp;Md5Ctx, ":", 1);
      MD5Update(&amp;Md5Ctx, pszPassword, strlen(pszPassword));
      MD5Final(HA1, &amp;Md5Ctx);
      if (stricmp(pszAlg, "md5-sess") == 0) {
            MD5Init(&amp;Md5Ctx);
            MD5Update(&amp;Md5Ctx, HA1, HASHLEN);
            MD5Update(&amp;Md5Ctx, ":", 1);
            MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
            MD5Update(&amp;Md5Ctx, ":", 1);
            MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
            MD5Final(HA1, &amp;Md5Ctx);
      };
      CvtHex(HA1, SessionKey);
};

/* calculate request-digest/response-digest as per HTTP Digest spec */
void DigestCalcResponse(
    IN HASHHEX HA1,           /* H(A1) */
    IN char * pszNonce,       /* nonce from server */
    IN char * pszNonceCount,  /* 8 hex digits */
    IN char * pszCNonce,      /* client nonce */
    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
    IN char * pszMethod,      /* method from the request */
    IN char * pszDigestUri,   /* requested URL */
    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
    OUT HASHHEX Response      /* request-digest or response-digest */
    )
{
      MD5_CTX Md5Ctx;
      HASH HA2;
      HASH RespHash;
       HASHHEX HA2Hex;

      // calculate H(A2)
      MD5Init(&amp;Md5Ctx);
      MD5Update(&amp;Md5Ctx, pszMethod, strlen(pszMethod));
      MD5Update(&amp;Md5Ctx, ":", 1);
      MD5Update(&amp;Md5Ctx, pszDigestUri, strlen(pszDigestUri));
      if (stricmp(pszQop, "auth-int") == 0) {
            MD5Update(&amp;Md5Ctx, ":", 1);
            MD5Update(&amp;Md5Ctx, HEntity, HASHHEXLEN);
      };
      MD5Final(HA2, &amp;Md5Ctx);
       CvtHex(HA2, HA2Hex);

      // calculate response
      MD5Init(&amp;Md5Ctx);
      MD5Update(&amp;Md5Ctx, HA1, HASHHEXLEN);
      MD5Update(&amp;Md5Ctx, ":", 1);
      MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
      MD5Update(&amp;Md5Ctx, ":", 1);
      if (*pszQop) {
          MD5Update(&amp;Md5Ctx, pszNonceCount, strlen(pszNonceCount));
          MD5Update(&amp;Md5Ctx, ":", 1);
          MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
          MD5Update(&amp;Md5Ctx, ":", 1);
          MD5Update(&amp;Md5Ctx, pszQop, strlen(pszQop));
          MD5Update(&amp;Md5Ctx, ":", 1);
      };
      MD5Update(&amp;Md5Ctx, HA2Hex, HASHHEXLEN);
      MD5Final(RespHash, &amp;Md5Ctx);
      CvtHex(RespHash, Response);
};
</artwork></figure>
<figure><preamble>
File "digtest.c":
</preamble><artwork type="example" name="digtest.c">
#include &lt;stdio.h>
#include "digcalc.h"

void main(int argc, char ** argv) {

      char * pszNonce = "dcd98b7102dd2f0e8b11d0f600bfb0c093";
      char * pszCNonce = "0a4f113b";
      char * pszUser = "Mufasa";
      char * pszRealm = "testrealm@host.com";
      char * pszPass = "Circle Of Life";
      char * pszAlg = "md5";
      char szNonceCount[9] = "00000001";
      char * pszMethod = "GET";
      char * pszQop = "auth";
      char * pszURI = "/dir/index.html";
      HASHHEX HA1;
      HASHHEX HA2 = "";
      HASHHEX Response;

      DigestCalcHA1(pszAlg, pszUser, pszRealm, pszPass, pszNonce,
pszCNonce, HA1);
      DigestCalcResponse(HA1, pszNonce, szNonceCount, pszCNonce, pszQop,
       pszMethod, pszURI, HA2, Response);
      printf("Response = %s\n", Response);
};
</artwork></figure>
</section>
   
<section title="Acknowledgments">
<t>
   Eric W. Sink, of AbiSource, Inc., was one of the original authors
   before the specification underwent substantial revision.
</t>
<t>
   In addition to the authors, valuable discussion instrumental in
   creating this document has come from Peter J. Churchyard, Ned Freed,
   and David M.  Kristol.
</t>
<t>
   Jim Gettys and Larry Masinter edited this document for update.
</t>
</section>
  </middle>
  <back>
   
<references>

<reference anchor='RFC1945'>
<front>
<title abbrev='HTTP/1.0'>Hypertext Transfer Protocol -- HTTP/1.0</title>
<author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
<organization>MIT, Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139</code>
<country>US</country></postal>
<facsimile>+1 617 258 8682</facsimile>
<email>timbl@w3.org</email></address></author>
<author initials='R.T.' surname='Fielding' fullname='Roy T. Fielding'>
<organization>University of California, Irvine, Department of Information and Computer Science</organization>
<address>
<postal>
<street />
<city>Irvine</city>
<region>CA</region>
<code>92717-3425</code>
<country>US</country></postal>
<facsimile>+1 714 824 4056</facsimile>
<email>fielding@ics.uci.edu</email></address></author>
<author initials='H.F.' surname='Nielsen' fullname='Henrik Frystyk Nielsen'>
<organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139</code>
<country>US</country></postal>
<facsimile>+1 617 258 8682</facsimile>
<email>frystyk@w3.org</email></address></author>
<date year='1996' month='May' />
</front>
<seriesInfo name='RFC' value='1945' />
</reference>

<reference anchor="RFC2616">

<front>
<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
<organization>University of California, Irvine, Information and Computer Science</organization>
<address>
<postal>
<street/>
<city>Irvine</city>
<region>CA</region>
<code>92697-3425</code>
<country>US</country></postal>
<phone>+1 949 824 1715</phone>
<email>fielding@ics.uci.edu</email></address></author>
<author initials="J." surname="Gettys" fullname="James Gettys">
<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139</code>
<country>US</country></postal>
<phone/>
<facsimile>+1 617 258 8682</facsimile>
<email>jg@w3.org</email></address></author>
<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
<organization>Compaq Computer Corporation, Western Research Laboratory</organization>
<address>
<postal>
<street>250 University Avenue</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94301</code>
<country>US</country></postal>
<phone/>
<email>mogul@wrl.dec.com</email></address></author>
<author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139</code>
<country>US</country></postal>
<phone/>
<facsimile>+1 617 258 8682</facsimile>
<email>frystyk@w3.org</email></address></author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization>Xerox Corporation</organization>
<address>
<postal>
<street>3333 Coyote Hill Road</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94034</code>
<country>US</country></postal>
<phone/>
<email>masinter@parc.xerox.com</email></address></author>
<author initials="P.J." surname="Leach" fullname="Paul J. Leach">
<organization>Microsoft Corporation</organization>
<address>
<postal>
<street>1 Microsoft Way</street>
<city>Redmond</city>
<region>WA</region>
<code>98052</code>
<country>US</country></postal>
<phone/>
<email>paulle@microsoft.com</email></address></author>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139</code>
<country>US</country></postal>
<phone>+1 617 258 8682</phone>
<facsimile/>
<email>timbl@w3.org</email></address></author>
<date month="June" year="1999"/>
</front>
<seriesInfo name="RFC" value="2616"/>

  <x:source href="rfc2616.xml"/>
</reference>

<reference anchor='RFC1321'>
<front>
<title abbrev='MD5 Message-Digest Algorithm'>The MD5 Message-Digest Algorithm</title>
<author initials='R.' surname='Rivest' fullname='Ronald L. Rivest'>
<organization>Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science</organization>
<address>
<postal>
<street>545 Technology Square</street>
<street>NE43-324</street>
<city>Cambridge</city>
<region>MA</region>
<code>02139-1986</code>
<country>US</country></postal>
<phone>+1 617 253 5880</phone>
<email>rivest@theory.lcs.mit.edu</email></address></author>
<date year='1992' month='April' /></front>
<seriesInfo name='RFC' value='1321' />
</reference>

<reference anchor='RFC2045'>
<front>
<title abbrev='Internet Message Bodies'>Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
<author initials='N.' surname='Freed' fullname='Ned Freed'>
<organization>Innosoft International, Inc.</organization>
<address>
<postal>
<street>1050 East Garvey Avenue South</street>
<city>West Covina</city>
<region>CA</region>
<code>91790</code>
<country>US</country></postal>
<phone>+1 818 919 3600</phone>
<facsimile>+1 818 919 3614</facsimile>
<email>ned@innosoft.com</email></address></author>
<author initials='N.S.' surname='Borenstein' fullname='Nathaniel S. Borenstein'>
<organization>First Virtual Holdings</organization>
<address>
<postal>
<street>25 Washington Avenue</street>
<city>Morristown</city>
<region>NJ</region>
<code>07960</code>
<country>US</country></postal>
<phone>+1 201 540 8967</phone>
<facsimile>+1 201 993 3032</facsimile>
<email>nsb@nsb.fv.com</email></address></author>
<date year='1996' month='November' />
</front>
<seriesInfo name='RFC' value='2045' />
</reference>

<reference anchor='RFC2246'>
<front>
<title>The TLS Protocol Version 1.0</title>
<author initials='T.' surname='Dierks' fullname='Tim Dierks'>
<organization>Certicom</organization>
<address>
<email>tdierks@certicom.com</email></address></author>
<author initials='C.' surname='Allen' fullname='Christopher Allen'>
<organization>Certicom</organization>
<address>
<email>callen@certicom.com</email></address></author>
<date year='1999' month='January' />
</front>
<seriesInfo name='RFC' value='2246' />
<format type='TXT' octets='170401' target='ftp://ftp.isi.edu/in-notes/rfc2246.txt' />
</reference>


<reference anchor='RFC2069'>
<front>
<title abbrev='Digest Access Authentication'>An Extension to HTTP : Digest Access Authentication</title>
<author initials='J.' surname='Franks' fullname='John Franks'>
<organization>Northwestern University,  Department of Mathematics</organization>
<address>
<postal>
<street />
<city>Evanston</city>
<region>IL</region>
<code>60208-2730</code>
<country>US</country></postal>
<email>john@math.nwu.edu</email></address></author>
<author initials='P.' surname='Hallam-Baker' fullname='Phillip M. Hallam-Baker'>
<organization>CERN</organization>
<address>
<postal>
<street />
<city>Geneva</city>
<country>CH</country></postal>
<email>hallam@w3.org</email></address></author>
<author initials='J.' surname='Hostetler' fullname='Jeffery L. Hostetler'>
<organization>Spyglass, Inc.</organization>
<address>
<postal>
<street>3200 Farber Drive</street>
<city>Champaign</city>
<region>IL</region>
<code>61821</code>
<country>US</country></postal>
<email>jeff@spyglass.com</email></address></author>
<author initials='P.' surname='Leach' fullname='Paul J. Leach'>
<organization>Microsoft Corporation</organization>
<address>
<postal>
<street>1 Microsoft Way</street>
<city>Redmond</city>
<region>WA</region>
<code>98052</code>
<country>US</country></postal>
<email>paulle@microsoft.com</email></address></author>
<author initials='A.' surname='Luotonen' fullname='Ari Luotonen'>
<organization>Netscape Communications Corporation</organization>
<address>
<postal>
<street>501 East Middlefield Road</street>
<city>Mountain View</city>
<region>CA</region>
<code>94043</code>
<country>US</country></postal>
<email>luotonen@netscape.com</email></address></author>
<author initials='E.' surname='Sink' fullname='Eric W. Sink'>
<organization>Spyglass, Inc.</organization>
<address>
<postal>
<street>3200 Farber Drive</street>
<city>Champaign</city>
<region>IL</region>
<code>61821</code>
<country>US</country></postal>
<email>eric@spyglass.com</email></address></author>
<author initials='L.' surname='Stewart' fullname='Lawrence C. Stewart'>
<organization>Open Market, Inc.</organization>
<address>
<postal>
<street>215 First Street</street>
<city>Cambridge</city>
<region>MA</region>
<code>02142</code>
<country>US</country></postal>
<email>stewart@OpenMarket.com</email></address></author>
<date year='1997' month='January' />
</front>
<seriesInfo name='RFC' value='2069' />
</reference>

<reference anchor="RFC2396">
  <front>
    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization>World Wide Web Consortium</organization>
      <address>
      <email>timbl@w3.org</email></address>
    </author>
    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
      <organization>Department of Information and Computer Science</organization>
      <address>
        <email>fielding@ics.uci.edu</email>
      </address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization>Xerox PARC</organization>
      <address>
        <email>masinter@parc.xerox.com</email>
      </address>
    </author>
    <date month="August" year="1998"/>
  </front>
  <seriesInfo name="RFC" value="2396"/>
</reference>

<reference anchor="ref8" target="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">
  <front>
    <title>Message Authentication with MD5</title>
    <author initials="B." surname="Kaliski">
      <organization/>
    </author>
    <author initials="M." surname="Robshaw">
      <organization/>
    </author>
    <date year="1995"/>
  </front>
  <annotation>CryptoBytes, Spring 1995</annotation>
</reference>

<reference anchor='RFC2195'>
<front>
<title abbrev='IMAP/POP AUTHorize Extension'>IMAP/POP AUTHorize Extension for Simple Challenge/Response</title>
<author initials='J.C.' surname='Klensin' fullname='John C. Klensin'>
<organization>MCI</organization>
<address>
<postal>
<street>800 Boylston</street>
<street>7th floor</street>
<street>Boston</street>
<street>MA 02199</street>
<country>USA</country></postal>
<phone>+1 617 960 1011</phone>
<email>klensin@mci.net</email></address></author>
<author initials='R.' surname='Catoe' fullname='Randy Catoe'>
<organization>MCI</organization>
<address>
<postal>
<street>2100 Reston Parkway</street>
<street>Reston</street>
<street>VA 22091</street>
<country>USA</country></postal>
<phone>+1 703 715 7366</phone>
<email>randy@mci.net</email></address></author>
<author initials='P.' surname='Krumviede' fullname='Paul Krumviede'>
<organization>MCI</organization>
<address>
<postal>
<street>2100 Reston Parkway</street>
<street>Reston</street>
<street>VA 22091</street>
<country>USA</country></postal>
<phone>+1 703 715 7251</phone>
<email>paul@mci.net</email></address></author>
<date year='1997' month='September' />
<area>Applications</area>
<keyword>IMAP</keyword>
<keyword>authentication</keyword>
<keyword>internet message access protocol</keyword>
<keyword>post office protocol</keyword>
<keyword>security</keyword>
</front>
<seriesInfo name='RFC' value='2195' />
</reference>

<reference anchor="ref10">
  <front>
    <title>Authentication Methods for LDAP</title>
    <author initials="B." surname="Morgan">
      <organization/>
    </author>
    <author initials="H." surname="Alvestrand">
      <organization/>
    </author>
    <author initials="J." surname="Hodges">
      <organization/>
    </author>
    <author initials="M." surname="Wahl">
      <organization/>
    </author>
    <date/>
  </front>
  <annotation>Work in progress.</annotation>
</reference>
</references>  
  
</back>
</rfc>