HTTPAuth Working Group                               R. Shekh-Yusef, Ed.
Internet-Draft                                                 D. Ahrens
Obsoletes: 2617 (if approved)                                      Avaya
Intended Status: Standards Track                               S. Bremer
Expires: October 11, 28, 2014                                    Netzkonform
                                                          April 9, 26, 2014

                   HTTP Digest Access Authentication


   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. This document defines
   the HTTP Digest Authentication scheme that may be used with the
   authentication mechanism.

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Table of Contents

   1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1 Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4
   2 Syntax Convention  . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2 Algorithm Variants . . . . . . . . . . . . . . . . . . . . .  4
     2.3 ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3 Digest Access Authentication Scheme  . . . . . . . . . . . . . .  5
     3.1 Overall Operation  . . . . . . . . . . . . . . . . . . . . .  5
     3.2 Representation of Digest Values  . . . . . . . . . . . . . .  5
     3.3 The WWW-Authenticate Response Header . . . . . . . . . . . .  5
     3.4 The Authorization Request Header . . . . . . . . . . . . . .  8
       3.4.1 Response . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.4.2 A1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.4.3 A2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.4.4 Username Hashing . . . . . . . . . . . . . . . . . . . . 11
       3.4.5 Parameter Values and Quoted-String . . . . . . . . . . . 11
       3.4.6 Various Considerations . . . . . . . . . . . . . . . . . 12
     3.5 The Authentication-Info Header . . . . . . . . . . . . . . . 13
     3.6 Digest Operation . . . . . . . . . . . . . . . . . . . . . . 15
     3.7 Security Protocol Negotiation  . . . . . . . . . . . . . . . 16
     3.8 Proxy-Authenticate and Proxy-Authorization . . . . . . . . . 17
     3.9 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17
       3.9.1 Example with SHA-256 and MD5 . . . . . . . . . . . . . . 17
       3.9.2 Example with SHA-512-256, Charset, and Userhash  . . . . 18
   4 Internationalization . . . . . . . . . . . . . . . . . . . . . . 20
   5 Security Considerations  . . . . . . . . . . . . . . . . . . . . 20
     5.1 Limitations  . . . . . . . . . . . . . . . . . . . . . . . . 20
     5.2 Authentication of Clients using Digest Authentication  . . . 21
     5.3 Limited Use Nonce Values . . . . . . . . . . . . . . . . . . 21
     5.4 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . . 22
     5.5 Weakness Created by Multiple Authentication Schemes  . . . . 23
     5.6 Online dictionary attacks  . . . . . . . . . . . . . . . . . 23
     5.7 Man in the Middle  . . . . . . . . . . . . . . . . . . . . . 23
     5.8 Chosen plaintext attacks . . . . . . . . . . . . . . . . . . 24
     5.9 Precomputed dictionary attacks . . . . . . . . . . . . . . . 24
     5.10 Batch brute force attacks . . . . . . . . . . . . . . . . . 25
     5.11 Spoofing by Counterfeit Servers . . . . . . . . . . . . . . 25
     5.12 Storing passwords . . . . . . . . . . . . . . . . . . . . . 25
     5.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   6 IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 27
     6.1  HTTP Digest Hash Algorithms Registry  . . . . . . . . . . . 27
     6.2  Digest Scheme Registration  . . . . . . . . . . . . . . . . 27
     6.3  Authentication-Info Header Registration . . . . . . . . . . 27
   7 Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . 28
   8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     8.1 Normative References . . . . . . . . . . . . . . . . . . . . 29
     8.2 Informative References . . . . . . . . . . . . . . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30

1 Introduction

   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. This document defines
   the HTTP Digest Authentication scheme that may be used with the
   authentication mechanism.

   The details of the challenge-response authentication mechanism are
   specified in the [HTTP-P7] document.

   The combination of this document with Basic [BASIC] and [HTTP-P7]
   obsolete RFC2617.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2 Syntax Convention

2.1 Examples

   In the interest of clarity and readability, the extended parameters
   or the headers and parameters in the examples in this document might
   be broken into multiple lines. Any line that is indented in this
   document is a continuation of the preceding line.

2.2 Algorithm Variants

   When used with the Digest mechanism, each one of the algorithms has
   two variants: Session variant and non-Session variant.

   The non-Session variant is denoted by "<algorithm>", e.g. "SHA-256",
   and the Session variant is denoted by "<algorithm>-sess", e.g. "SHA-

2.3 ABNF

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

3 Digest Access Authentication Scheme

3.1 Overall Operation

   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 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. The username and password must
   be prearranged in some fashion not addressed by this document.

3.2 Representation of Digest Values

   An optional header allows the server to specify the algorithm used to
   create the checksum or digest. This documents adds SHA-256 and SHA-
   512/256 algorithms. To maintain backwards compatibility, the MD5
   algorithm is still supported but not recommended.

   The size of the digest depends on the algorithm used.  The bits in
   the digest are converted from the most significant to the least
   significant bit, four bits at a time to the ASCII representation as
   follows. Each four bits is represented by its familiar hexadecimal
   notation from the characters 0123456789abcdef, that is binary 0000 is
   represented by the character '0', 0001 by '1' and so on up to the
   representation of 1111 as 'f'. If the MD5 algorithm is used to
   calculate the digest, then the digest will be represented as 32
   hexadecimal characters, SHA-256 and SHA-512/256 by 64 hexadecimal

3.3 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 with
   Digest scheme as per the framework defined above, and include some or
   all of the following parameters:

      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 "". (See section 2.2 of
      [HTTP-P7] for more details).

      A quoted, space-separated list of URIs, as specified in RFC 3986
      [RFC3986], that define the protection space.  If a URI is an
      abs_path, it is relative to the canonical root URL of the server
      being accessed. An absolute-URI 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 parameter is
      omitted or its value is empty, the client should assume that the
      protection space consists of all URIs on the responding server.

      This parameter is not meaningful in Proxy-Authenticate headers,
      for which the protection space is always the entire proxy; if
      present it should be ignored.

      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.

      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

            time-stamp H(time-stamp ":" ETag ":" private-key)

      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.)

      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 section
      5 of this document.

      The nonce is opaque to the client.

      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.

      A case-insensitive flag, indicating that the previous request from
      the client was rejected because the nonce value was stale. If
      stale is TRUE, the client may wish to simply retry the request
      with a new encrypted response, without reprompting the user for a
      new username and password. The 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 parameter is not present,
      the username and/or password are invalid, and new values must be

      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).

      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.

           For "<algorithm>" and "<algorithm>-sess"

               H(data) = <algorithm>(data)


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

      For example:

           For the "SHA-256" and "SHA-256-sess" algorithms

               H(data) = SHA-256(data)

         i.e., the digest is the SHA-256 of the secret concatenated with
         a colon concatenated with the data. The "SHA-256-sess"
         algorithm is intended to allow efficient 3rd party
         authentication servers; for the difference in usage, see the
         description in section 3.4.2.

      This parameter MUST be used by all implementations compliant with
      this version of the Digest scheme. 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 parameter value for the application of this choice.
      Unrecognized options MUST be ignored.

      This is an optional parameter that is used by the server to
      indicate the encoding scheme it supports.

      This is an optional parameter that is used by the server to
      indicate that it supports username hashing. Valid value are:
      "true" or "false".

3.4 The Authorization Request Header

      The client is expected to retry the request, passing an
      Authorization header line with Digest scheme, which is defined
      according to the framework above. The values of the opaque and
      algorithm fields must be those supplied in the WWW-Authenticate
      response header for the entity being requested.

      The request includes some or all of the following parameters:

      A string of the hex digits computed as defined below, which proves
      that the user knows a password.

      The user's name in the specified realm.

      The URI from request-target of the Request-Line; duplicated here
      because proxies are allowed to change the Request-Line in transit.

      Indicates what "quality of protection" the client has applied to
      the message. 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 response. Note that this is a single
      token, not a quoted list of alternatives as in WWW-Authenticate.
      .in 3

      This MUST be specified if a qop parameter is sent (see above), and
      MUST NOT be specified if the server did not send a qop parameter
      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
      rspauth and response values.

      The "nc" parameter stands for "nonce count". This MUST be
      specified if a qop parameter is sent (see above), and MUST NOT be
      specified if the server did not send a qop parameter 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
      parameter 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 response value.

      This optional parameter is used by the client to indicate that the
      username has been hashed. Valid value are: "true" or "false".

   If a parameter or its value is improper, or required parameters 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.

   The definition of response above indicates the encoding for its
   value. The following definitions show how the value is computed.

3.4.1 Response

   If the "qop" value is "auth" or "auth-int":

         response = <"> < KD ( H(A1), unq(nonce)
                                      ":" nc
                                      ":" unq(cnonce)
                                      ":" unq(qop)
                                      ":" H(A2)
                             ) <">

   See below for the definitions for A1 and A2.

3.4.2 A1

   If the "algorithm" parameter's value is "<algorithm>", e.g. "SHA-
   256", then A1 is:

         A1       = unq(username) ":" unq(realm) ":" passwd


         passwd   = < user's password >

   If the "algorithm" parameter's value is "<algorithm>-sess", e.g.
   "SHA-256-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

         A1       = H( unq(username) ":" unq(realm)
                        ":" passwd )
                        ":" unq(nonce) ":" unq(cnonce)

   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
   section 3.6.) 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

3.4.3 A2

   If the "qop" parameter's value is "auth" or is unspecified, then A2

         A2       = Method ":" request-uri

      If the "qop" value is "auth-int", then A2 is:

         A2       = Method ":" request-uri ":" H(entity-body)

3.4.4 Username Hashing

   To protect the transport of the username from the client to the
   server, the server SHOULD set the "userhash" parameter with the value
   of "true" in the WWW-Authentication header.

   If the client supports the "userhash" parameter, and the "userhash"
   parameter value in the WWW-Authentication header is set to "true",
   then the client MUST calculate a hash of the username after any other
   hash calculation and include the "userhash" parameter with the value
   of "true" in the Authorization Request Header. If the client does not
   provide the "username" as a hash value or the "userhash" parameter
   with the value of "true", the server MAY reject the request.

   The following is the operation that the client will take to hash the

      username = H( unq(username) ":" unq(realm) )

3.4.5 Parameter Values and Quoted-String

   Note that the value of many of the parameters, 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


   and the user Mufasa has password "Circle Of Life" then H(A1) would be
   H( Of Life) 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 illustrated above must be

 Of Life

   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.

   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.

3.4.6 Various Considerations

   The "Method" value is the HTTP request method as specified in section
   3.1.1 of [HTTP-P1]. The "request-target" value is the request-target
   from the request line as specified in section 3.1.1 of [HTTP-P1].
   This may be "*", an "absolute-URI" or an "absolute-path" as specified
   in section 2.7 of [HTTP-P1], but it MUST agree with the request-
   target. In particular, it MUST be an "absolute-URI" if the request-
   target is an "absolute-URI". The "cnonce" value is an optional
   client-chosen value whose purpose is to foil chosen plaintext

   The authenticating server must assure that the resource designated by
   the "uri" parameter 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.

   Implementers should be aware of how authenticated transactions
   interact with shared caches. The HTTP/1.1 protocol specifies that
   when a shared cache (see [HTTP-P6]) 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 3.2 of [HTTP-P6]) directive 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.

3.5 The Authentication-Info Header

   The Authentication-Info header is used by the server to communicate
   some information regarding the successful authentication in the

           Authentication-Info = auth-info

           auth-info = *auth-param

   The request includes some or all of the following parameters:


      The value of the nextnonce parameter 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".

         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 parameter  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 "nc" parameter can retain most of the
         security advantages of a new server nonce without the
         deleterious affects on pipelining.

      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
      qop parameter in the response as was sent by the client in the
      corresponding request.


      The optional response digest in the "rspauth" parameter 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 "rspauth" value is
      calculated as for the response in the Authorization header, except
      that if "qop=auth" or is not specified in the Authorization header
      for the request, A2 is

            A2       = ":" request-uri

         and if "qop=auth-int", then A2 is

            A2       = ":" request-uri ":" H(entity-body)

   cnonce and nc

      The "cnonce" value and "nc" value MUST be the ones for the client
      request to which this message is the response. The "rspauth",
      "cnonce", and "nc" parameters MUST be present if "qop=auth" or
      "qop=auth-int" is specified.

   The Authentication-Info header is allowed in the trailer of an HTTP
   message transferred via chunked transfer-coding.

3.6 Digest Operation

   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 response value.

   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.

   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.

   Because the client is required to return the value of the opaque
   parameter 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 parameter whose value includes a
   URI on the second server, and an opaque parameter 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
   <opaque> data.

   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 3.6

3.7 Security Protocol Negotiation

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

   It is 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.

   When a server receives a request to access a resource, the server
   might challenge the client by responding with "401 Unauthorized"
   status code, and include one or more WWW-Authenticate headers. If the
   server challenges with multiple Digest headers, then each one of
   these headers MUST use a different digest algorithm. The server MUST
   add these Digest headers to the response in order of preference,
   starting with the most preferred header, followed by the less
   preferred headers.

   This specification defines the following preference list, starting
   with the most preferred algorithm: algorithms:

      * SHA2-256 (mandatory to implement)
      * SHA2-512/256 (as a backup algorithm)
      * MD5 (for backward compatibility).

   When the client receives the response it SHOULD use the topmost
   header that it supports, unless a local policy dictates otherwise.
   The client should ignore any challenge it does not understand.

3.8 Proxy-Authenticate and Proxy-Authorization

   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 4.2 and 4.3 of the
   HTTP/1.1 specification [HTTP-P7] 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 section 3.2.1.

   The client/proxy must then re-issue the request with a Proxy-
   Authorization header, with parameters as specified for the
   Authorization header in section 3.4 above.

   On subsequent responses, the server sends Proxy-Authenticate-Info
   with parameters the same as those for the Authentication-Info header

   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.

3.9 Examples

3.9.1 Example with SHA-256 and MD5

   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". 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).

   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 = "",
                qop="auth, auth-int",
        WWW-Authenticate: Digest
                qop="auth, auth-int",

   The client may prompt the user for their username and password, after
   which it will respond with a new request, including the following
   Authorization header if the client chooses MD5 digest:

        Authorization:Digest username="Mufasa",

   If the client chooses to use the SHA-256 algorithm for calculating
   the response, the client responds with a new request including the
   following Authorization header:

        Authorization:Digest username="Mufasa",

3.9.2 Example with SHA-512-256, Charset, and Userhash
   The following example assumes that an access protected document is
   being requested from the server via a GET request. The URI for the
   request is "". Both client and server
   know the userhash of the username, support the UTF-8 charset, and use
   the SHA-512-256 algorithm. The username for the request is "Jason
   Doe" and the password is "Secret, or not?".

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

        HTTP/2.0 401 Unauthorized
        WWW-Authenticate: Digest

   The client may prompt the user for the required credentials and send
   a new request with following Authorization header:

        Authorization: Digest

   If the client can not provide a hashed username for any reason, the
   client may try a request with this Authorization header:

        Authorization: Digest
                username="Jason Doe",

4 Internationalization

   In challenges, servers SHOULD use the "charset" authentication
   parameter (case-insensitive) to express the character encoding they
   expect the user agent to use when generating A1 (see section 3.4.2)
   and username hashing (see section 3.4.4).

   The only allowed value is "UTF-8", to be matched case-insensitively
   (see [RFC2978], Section 2.3).  It indicates that the server expects
   user name and password to be converted to Unicode Normalization Form
   C ("NFC", see Section 3 of [RFC5198]) and to be encoded into octets
   using the UTF-8 character encoding scheme ([RFC3629]).

   If the user agent does not support the encoding indicated by the
   server, it MUST fail the request.

5 Security Considerations

5.1 Limitations

   HTTP Digest authentication, when used with human-memorable passwords,
   is vulnerable to dictionary attacks. Such attacks are much easier
   than cryptographic attacks on any widely used algorithm, including
   those that are no longer considered secure. In other words, algorithm
   agility does not make this usage any more secure.

   As a result, Digest authentication SHOULD be used only with passwords
   that have a reasonable amount of entropy, e.g. 128-bit or more. Such
   passwords typically cannot be memorized by humans but can be used for
   automated web services.

   Digest authentication SHOULD be used over a secure channel like HTTPS

5.2 Authentication of Clients using Digest Authentication

   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 [RFC4513], POP and IMAP (see
   [RFC2195]).  It is intended to replace the much weaker and even more
   dangerous Basic mechanism.

   Digest Authentication offers no confidentiality protection beyond
   protecting the actual username and password. All of the rest of the
   request and response are available to an eavesdropper.

   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 parameter values
   (see section 3.2 above) are protected.  Most header fields and their
   values could be modified as a part of a man-in-the-middle attack.

   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

5.3 Limited Use Nonce Values

   The Digest scheme uses a server-specified nonce to seed the
   generation of the response value (as specified in section 3.4.1
   above).  As shown in the example nonce in section 3.2.1, 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 4.5).  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 parameter 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.

5.4 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 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

   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.

   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.

   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.

5.5 Weakness Created by Multiple Authentication Schemes

   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

      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.

   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 section 5.7 below for discussion of particular attack
   scenarios that exploit multiple authentication schemes.

5.6 Online dictionary attacks

   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.

   The server can mitigate this attack by not allowing users to select
   passwords that are in a dictionary.

5.7 Man in the Middle

   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.

   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.

   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.

   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.

   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

5.8 Chosen plaintext attacks

   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.

   However, no way to analyze the MD5 one-way function used by Digest
   using chosen plaintext is currently known.

   The countermeasure against this attack is for clients to be
   configured to require the use of the optional "cnonce" parameter;
   this allows the client to vary the input to the hash in a way not
   chosen by the attacker.

5.9 Precomputed dictionary attacks

   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.

   The countermeasure against this attack is to for clients to be
   configured to require the use of the optional "cnonce" parameter.

5.10 Batch brute force attacks

   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.

   The countermeasure against this attack is to for clients to be
   configured to require the use of the optional "cnonce" parameter.

5.11 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.  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.

5.12 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

   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.

   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.

5.13 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 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.

   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.

6 IANA Considerations

6.1  HTTP Digest Hash Algorithms Registry

   This specification creates a new IANA registry named "HTTP Digest
   Hash Algorithms". When registering a new hash algorithm, the
   following information MUST be provided:

   o  Hash Algorithm
      The textual name of the hash algorithm.

   o  Digest Size
      The size of the algorithm's output in hexadecimal characters. bits.

   o  Reference
      A reference to the specification that describes the new algorithm.

   The update policy for this registry shall be Specification Required.

   The initial registry will contain the following entries:

      Hash Algorithm   Digest Size   Reference
      --------------   -----------   ---------
      "MD5"                32                128       RFC XXXX
      "SHA-512-256"        64        256       RFC XXXX
      "SHA-256"            64            256       RFC XXXX

   Each one of the algorithms defined in the registry might have a -sess
   variant, e.g. MD5-sess, SHA-256-sess, etc.

6.2  Digest Scheme Registration

   This specification registers the Digest scheme with the
   Authentication Scheme Registry.

      Authentication Scheme Name: Digest

      Pointer to specification text: RFCXXX

6.3  Authentication-Info Header Registration

   This specification registers the Authentication-Info Header with the
   Message Header Field Registry.

   Header Field Name: Authentication-Info

   Protocol: http

   Status: standard

   Reference: RFCXXXX, Section 3.5

7 Acknowledgments

   The authors of this document would like to thank the authors of
   RFC2617, as this document heavily borrows text from their document to
   provide a complete description of the digest scheme and its

   The authors would like to thank Stephen Farrell, Yoav Nir, Phillip
   Hallam-Baker, Manu Sporny, Paul Hoffman, Julian Reschke, Yaron
   Sheffer, Sean Turner, Geoff Baskwill, Eric Cooper, Bjoern Hoehrmann,
   Martin Durst, Peter Saint-Andre, Michael Sweet, Daniel Stenberg,
   Brett Tate, Paul Leach, and Ilari Liusvaara for their careful review
   and comments.

   The authors would like to thank Jonathan Stoke, Nico Williams, Harry
   Halpin, and Phil Hunt for their comments on the mailing list when
   discussing various aspects of this document.

   The authors would like to thank Paul Kyzivat and Dale Worley for
   their careful review and feedback on some aspects of this document.

8 References

8.1 Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, October 2000.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4513]  Harrison, R., Ed., "Lightweight Directory Access Protocol
              (LDAP): Authentication Methods and Security Mechanisms",
              RFC 4513, June 2006.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234, January

   [HTTP-P1]  Fielding, R., Reschke, J., "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", draft-ietf-
              httpbis-p1-messaging (Work in Progress), November 2013.

   [HTTP-P6]  Fielding, R., Nottingham, M., Reschke, J., "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", draft-ietf-
              httpbis-p6-cache (Work in Progress), November 2013.

   [HTTP-P7]  Fielding, R., Reschke, J., "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth
              (Work in Progress), November 2013.

   [BASIC]    Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              draft-ietf-httpauth-basicauth-enc (Work in Progress),
              September 2013.

8.2 Informative References

   [RFC2195]  Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response",
              RFC 2195, September 1997.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

Authors' Addresses

   Rifaat Shekh-Yusef (Editor)
   250 Sydney Street
   Belleville, Ontario

   Phone: +1-613-967-5267

   David Ahrens


   Sophie Bremer