HTTPAuth                                             R. Shekh-Yusef, Ed.
Internet-Draft                                                     Avaya
Obsoletes: 2617 (if approved)                                  D. Ahrens
Intended status: Standards Track                             Independent
Expires: July 14, 24, 2015                                         S. Bremer
                                                        January 10, 20, 2015

                   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 can be used with the HTTP
   authentication mechanism.

Editorial Note (To be removed by RFC Editor before publication)

   Discussion of this draft takes place on the HTTPAuth working group
   mailing list (, which is archived at [1].

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Syntax Convention . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Digest Access Authentication Scheme . . . . . . . . . . . . .   4
     3.1.  Overall Operation . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Representation of Digest Values . . . . . . . . . . . . .   4
     3.3.  The WWW-Authenticate Response Header  . . . . . . . . . .   5
     3.4.  The Authorization Request Header Field  . . . . . . . . .   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
       3.5.1.  Digest Usage of Authentication-Info . . . . . . . . .  13
     3.6.  Digest Operation  . . . . . . . . . . . . . . . . . . . .  15
     3.7.  Security Protocol Negotiation . . . . . . . . . . . . . .  16
     3.8.  Proxy-Authenticate and Proxy-Authorization  . . . . . . .  16
     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 . . .  19

   4.  Internationalization  . . . . . . . . . . . . . . . . . . . .  20
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
     5.1.  Limitations . . . . . . . . . . . . . . . . . . . . . . .  20
     5.2.  Storing passwords . . . . . . . . . . . . . . . . . . . .  21
     5.3.  Authentication of Clients using Digest Authentication . .  21
     5.4.  Limited Use Nonce Values  . . . . . . . . . . . . . . . .  22
     5.5.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  23
     5.6.  Weakness Created by Multiple Authentication Schemes . . .  23
     5.7.  Online dictionary attacks . . . . . . . . . . . . . . . .  24
     5.8.  Man in the Middle . . . . . . . . . . . . . . . . . . . .  24
     5.9.  Chosen plaintext attacks  . . . . . . . . . . . . . . . .  25
     5.10. Precomputed dictionary attacks  . . . . . . . . . . . . .  25
     5.11. Batch brute force attacks . . . . . . . . . . . . . . . .  25
     5.12. Summary . . . . . . . . . . . . . . . . . . . . . . . . .  26
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     6.1.  HTTP Digest Hash Algorithms Registry  . . . . . . . . . .  26
     6.2.  Digest Scheme Registration  . . . . . . . . . . . . . . .  27
     6.3.  Authentication-Info Header Registration . . . . . . . . .  27
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  27
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  29

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 can be used with the HTTP
   authentication mechanism.

   The details of the challenge-response authentication mechanism are
   specified in the "Hypertext Transfer Protocol (HTTP/1.1):
   Authentication" [RFC7235].

   The combination of this document with the definition of the "Basic"
   authentication scheme [BASIC] and [RFC7235] obsolete RFC 2617. [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 header fields 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.  ABNF

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234], and the ABNF List Extension of [RFC7230].

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, and might indicate
   that username hashing is supported.  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, and the username can be hashed, depending on
   the indication received from the server.  The username and password
   must be prearranged in some fashion not addressed by this document.

   The security of this protocol is critically dependent on the
   randomness of the randomly chosen parameters, such as client and
   server nonces.  These should be generated by a strong random or
   properly seeded pseudorandom source (see [RFC4086]).

   Some or all of

   For the parameters used in following parameters, the various headers fields used
   by this document value can be sent encoded using the
   [RFC5987] encoding. encoding: username.

3.2.  Representation of Digest Values

   An optional header field 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 with
   [RFC2617], 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 MD5 digest will be represented as 32
   hexadecimal characters, while SHA-256 and SHA-512/256 are represented
   as 64 hexadecimal characters.

3.3.  The WWW-Authenticate Response Header

   If a server receives a request for an access-protected object, and an
   acceptable Authorization header field is not sent, the server
   responds with a "401 Unauthorized" status code and a WWW-Authenticate
   header field with Digest scheme as per the framework defined above.
   The value of the header field can include parameters from the
   following list:


      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 [RFC7235] 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 (See Section 2.2 of [RFC7235])
      of the web-origin [RFC6454].  An absolute-URI in this list may
      refer to a different server than the web-origin.  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 web-origin.  All URIs
      in this list SHOULD use the same scheme (https or http); mixing
      them is a bad idea.

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


      A server-specified data string which should be uniquely generated
      each time a 401 response is made.  It is advised that this string
      be base64 or hexadecimal data.  Specifically, since the string is
      passed in the header field lines as a quoted string, the double-
      quote character is not allowed, unless suitably escaped.

      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 ":" secret-data)

      where time-stamp is a server-generated time, which preferably
      includes micro or nano seconds, or other non-repeating values,
      ETag is the value of the HTTP ETag header field associated with
      the requested entity, and secret-data 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
      field and reject the request if it did not match the nonce from
      that header field 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.  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 when requests from a single user often
      go through different proxies.  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 field
      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 re-prompting 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.  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 obtained.


      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).

      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.

      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 and with quoting slashes removed.

      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 "<algorithm>" of the secret concatenated
      with a colon concatenated with the data.  The "<algorithm>-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.  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 values are:
      "true" or "false".  Default value is "false".

   For historical reasons, a sender MUST only generate the quoted-string
   syntax values for the following parameters: realm, domain, nonce,
   opaque, and qop.

   For historical reasons, a sender MUST NOT generate the quoted-string
   syntax values for the following parameters: stale and algorithm.

3.4.  The Authorization Request Header Field

   The client is expected to retry the request, passing an Authorization
   header field 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 field
   for the entity being requested.

   The request can include parameters from the following list:


      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 Effective Request URI [RFC7230] 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 field.  These
      values affect the computation of the response.  Note that this is
      a single token, not a quoted list of alternatives as in WWW-


      This parameter MUST be used by all implementations.  The cnonce
      value is an opaque quoted ASCII-only 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.


      This parameter MUST be used by all implementations.  The "nc"
      parameter stands for "nonce count".  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 values are: "true" or "false".
      Default value is "false".

   For historical reasons, a sender MUST only generate the quoted-string
   syntax for the following parameters: username, realm, nonce, uri,
   response, cnonce, and opaque.

   For historical reasons, a sender MUST NOT generate the quoted-string
   syntax for the following parameters: algorithm, qop, and nc.

   If a parameter or its value is improper, or required parameters are
   missing, the proper response is a 4xx error code.  If the response 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 server implementation SHOULD be careful with
   the information being logged so that it won't put a cleartext
   password (e.g. entered into the username field) into the log.

   The definition of the 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 using the nonce value provided
   in the challenge from the server, and cnounce value from the request
   by the client following receipt of a WWW-Authenticate challenge from
   the server.  It uses the server nonce from that challenge, herein
   called nonce-prime, and the client nonce value from the response,
   herein called cnonce-prime, to construct A1 as follows:

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

   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 field.

   If the client supports the "userhash" parameter, and the "userhash"
   parameter value in the WWW-Authentication header field 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 field.  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, using the same algorithm used to hash the credentials:

      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 field includes the


   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 header fields in each part
   of any multipart content-type.

3.4.6.  Various Considerations

   The "Method" value is the HTTP request method, in US-ASCII letters,
   as specified in section 3.1.1 of [RFC7230].  The "request-target"
   value is the request-target from the request line as specified in
   section 3.1.1 of [RFC7230].  This MAY be "*", an "absolute-URI" or an
   "absolute-path" as specified in section 2.7 of [RFC7230], 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 a client-chosen value whose purpose is to foil
   chosen plaintext attacks.

   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 need
   to interact with shared caches.  The HTTP protocol specifies that
   when a shared cache (see [RFC7234]) has received a request containing
   an Authorization header field 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
   [RFC7234]) 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 header fields 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 field is a generic field that MAY be
   used by a server to communicate some information regarding the
   successful authentication of a client response.  The following is the
   syntax of the header:

           Authentication-Info = auth-info

           auth-info = #auth-param

   The auth-param is defined in [RFC7235].

3.5.1.  Digest Usage of Authentication-Info

   The Digest authentication scheme MAY add the Authentication-Info
   header field in the confirmation request and include parameters from
   the following list:


      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 field 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 field 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 field,
      except that if "qop=auth" or is not specified in the Authorization
      header field 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 field is allowed in the trailer of an
   HTTP message transferred via chunked transfer-coding.

   For historical reasons, a sender MUST only generate the quoted-string
   syntax for the following parameters: nextnonce, rspauth, and cnonce.

   For historical reasons, a sender MUST NOT generate the quoted-string
   syntax for the following parameters: qop and nc.

   For historical reasons, the nc value MUST be exactly 8 hexadecimal

3.6.  Digest Operation

   Upon receiving the Authorization header field, 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, SHA-256) 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 field can 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 field in future requests within
   that protection space.  The Authorization header field 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 field 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 can 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 "HTTP Redirection
   (Section 6.4 of [RFC7231]), pointing to the URI on the second server.

   The client will follow the redirection, and pass an Authorization
   header field, including the <opaque> data.

   Proxies MUST be completely transparent in the Digest access
   authentication scheme.  That is, they MUST forward the WWW-
   Authenticate, Authentication-Info and Authorization header fields
   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 header fields described in
   section 3.8 below.

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 wants 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"
   response, and include one or more WWW-Authenticate header fields.  If
   the server responds with multiple challenges, then each one of these
   challenges MUST use a different digest algorithm.  The server MUST
   add these challenges to the response in order of preference, starting
   with the most preferred algorithm, followed by the less preferred

   This specification defines the following algorithms:

   o  SHA2-256 (mandatory to implement)

   o  SHA2-512/256 (as a backup algorithm)

   o  MD5 (for backward compatibility).

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

3.8.  Proxy-Authenticate and Proxy-Authorization

   The digest authentication scheme can 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 header fields.

   These header fields are instances of the Proxy-Authenticate and
   Proxy-Authorization header fields specified in sections 4.2 and 4.3
   of the HTTP/1.1 specification [RFC7235] 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 field.  The digest-challenge used in the
   Proxy-Authenticate header field is the same as that for the WWW-
   Authenticate header field as defined above in Section 3.3.

   The client/proxy MUST then re-issue the request with a Proxy-
   Authorization header field, with parameters as specified for the
   Authorization header field 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 field is sent, so the server responds with:

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Digest
       qop="auth, auth-int",
   WWW-Authenticate: Digest
       qop="auth, auth-int",

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

   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 character
   encoding scheme, and use the SHA-512-256 algorithm.  The username for
   the request is "Jaesoen a variation of "Jason Doe" and has the byte sequence
   4A C3 A4 73 C3 B8 6E 20 44 6F 65.  The password is "Secret, or not?".

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

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Digest

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

   Authorization: Digest

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

   Authorization: Digest

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]), ), and percent
   escaped in extended notation ([RFC5987]). [RFC3629].

   For the username, recipients MUST support all characters defined in
   the "UsernameCasePreserved" profile defined in in Section 3.3 of
   [PRECIS], with the exception of the colon (":") character.

   For the password, recipients MUST support all characters defined in
   the "OpaqueString" profile defined in in Section 4.2 of [PRECIS].

   If the user agent does not support the encoding indicated by the
   server, it can 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.  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.

5.3.  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 was 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 is more appropriate protocol.
   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

5.4.  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 3.3, 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.

5.5.  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.

5.6.  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 field, 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

5.7.  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.

5.8.  Man in the Middle

   Digest authentication is 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

5.9.  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 one-way functions used by Digest using
   chosen plaintext is currently known.

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

5.10.  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 use the
   "cnonce" parameter.

5.11.  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 use of
   the "cnonce" parameter.

5.12.  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:

   Hash Algorithm

      The textual name of the hash algorithm.

   Digest Size

      The size of the algorithm's output in bits.


      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"          | 128         | RFC XXXX  |
               | "SHA-512-256"  | 256         | RFC XXXX  |
               | "SHA-256"      | 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: this specification

6.3.  Authentication-Info Header Registration

   This specification registers the Authentication-Info Header field
   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

   Special thanks to Julian Reschke for his reviews, comments,
   suggestions, and text provided to various areas in this document.

   The authors would like to thank Stephen Farrell, Yoav Nir, Phillip
   Hallam-Baker, Manu Sporny, Paul Hoffman, Yaron Sheffer, Sean Turner,
   Geoff Baskwill, Eric Cooper, Bjoern Hoehrmann, Martin Durst, Peter
   Saint-Andre, Michael Sweet, Daniel Stenberg, Brett Tate, Paul Leach,
   Ilari Liusvaara, and Gary Mort, Alexey Melnikov, and Benjamin Kaduk
   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

   [PRECIS]   Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", draft-ietf-precis-
              saslprepbis-12 (work in progress), December 2014.

   [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.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [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 2008.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, August 2010.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454, December

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing", RFC
              7230, June 2014.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              June 2014.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, June 2014.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.

8.2.  Informative References

   [BASIC]    Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              draft-ietf-httpauth-basicauth-update-04 (work in
              progress), December 2014.

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

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

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

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

Authors' Addresses

   Rifaat Shekh-Yusef (editor)
   250 Sidney Street
   Belleville, Ontario

   Phone: +1-613-967-5267
   David Ahrens


   Sophie Bremer