





Internet Engineering Task Force (IETF)               R. Shekh-Yusef, Ed.
Request for Comments: 7616                                         Avaya
Obsoletes: 2617                                                D. Ahrens
Category: Standards Track                                    Independent
ISSN: 2070-1721                                                S. Bremer
                                                             Netzkonform
                                                          September 2015


                   HTTP Digest Access Authentication

Abstract

   The Hypertext Transfer Protocol (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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7616.



















Shekh-Yusef, et al.          Standards Track                    [Page 1]

RFC 7616            HTTP Digest Access Authentication     September 2015


Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























Shekh-Yusef, et al.          Standards Track                    [Page 2]

RFC 7616            HTTP Digest Access Authentication     September 2015


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Syntax Convention . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  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 Field  . . . . . . .   5
     3.4.  The Authorization Header Field  . . . . . . . . . . . . .   9
       3.4.1.  Response  . . . . . . . . . . . . . . . . . . . . . .  11
       3.4.2.  A1  . . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.4.3.  A2  . . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.4.4.  Username Hashing  . . . . . . . . . . . . . . . . . .  12
       3.4.5.  Parameter Values and Quoted-String  . . . . . . . . .  12
       3.4.6.  Various Considerations  . . . . . . . . . . . . . . .  13
     3.5.  The Authentication-Info and Proxy-Authentication-Info
           Header Fields . . . . . . . . . . . . . . . . . . . . . .  14
     3.6.  Digest Operation  . . . . . . . . . . . . . . . . . . . .  15
     3.7.  Security Protocol Negotiation . . . . . . . . . . . . . .  16
     3.8.  Proxy-Authenticate and Proxy-Authorization  . . . . . . .  17
     3.9.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  18
       3.9.1.  Example with SHA-256 and MD5  . . . . . . . . . . . .  18
       3.9.2.  Example with SHA-512-256, Charset, and Userhash . . .  19
   4.  Internationalization Considerations . . . . . . . . . . . . .  20
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
     5.1.  Limitations . . . . . . . . . . . . . . . . . . . . . . .  21
     5.2.  Storing Passwords . . . . . . . . . . . . . . . . . . . .  21
     5.3.  Authentication of Clients Using Digest Authentication . .  22
     5.4.  Limited-Use Nonce Values  . . . . . . . . . . . . . . . .  23
     5.5.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  23
     5.6.  Weakness Created by Multiple Authentication Schemes . . .  24
     5.7.  Online Dictionary Attacks . . . . . . . . . . . . . . . .  24
     5.8.  Man-in-the-Middle Attacks . . . . . . . . . . . . . . . .  25
     5.9.  Chosen Plaintext Attacks  . . . . . . . . . . . . . . . .  25
     5.10. Precomputed Dictionary Attacks  . . . . . . . . . . . . .  26
     5.11. Batch Brute-Force Attacks . . . . . . . . . . . . . . . .  26
     5.12. Parameter Randomness  . . . . . . . . . . . . . . . . . .  26
     5.13. Summary . . . . . . . . . . . . . . . . . . . . . . . . .  26
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     6.1.  Hash Algorithms for HTTP Digest Authentication  . . . . .  27
     6.2.  Digest Scheme Registration  . . . . . . . . . . . . . . .  28
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Appendix A.  Changes from RFC 2617  . . . . . . . . . . . . . . .  31



Shekh-Yusef, et al.          Standards Track                    [Page 3]

RFC 7616            HTTP Digest Access Authentication     September 2015


   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

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.

   This document extends but is generally backward compatible with
   [RFC2617].  See Appendix A for the new capabilities introduced by
   this specification.

   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 [RFC7617], "HTTP Authentication-Info and Proxy-
   Authentication-Info Response Header Fields" [RFC7615], and "Hypertext
   Transfer Protocol (HTTP/1.1): Authentication" [RFC7235] obsolete
   [RFC2617].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [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].






Shekh-Yusef, et al.          Standards Track                    [Page 4]

RFC 7616            HTTP Digest Access Authentication     September 2015


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 an
   unkeyed digest 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.

3.2.  Representation of Digest Values

   An optional header field allows the server to specify the algorithm
   used to create the unkeyed digest or digest.  This document 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 sequence of 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 Field

   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:

   realm

      A string to be displayed to users so they know which username and
      password to use.  This string should contain at least the name of
      the host performing the authentication and might additionally
      indicate the collection of users who might have access.  An




Shekh-Yusef, et al.          Standards Track                    [Page 5]

RFC 7616            HTTP Digest Access Authentication     September 2015


      example is "registered_users@example.com".  (See Section 2.2 of
      [RFC7235] for more details.)

   domain

      A quoted, space-separated list of URIs, as specified in [RFC3986],
      that define the protection space.  If a URI is a path-absolute, it
      is relative to the canonical root URL.  (See Section 2.2 of
      [RFC7235].)  An absolute-URI in this list may refer to a different
      server than the web-origin [RFC6454].  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.

      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.

   nonce

      A server-specified 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 Base64 encoding of

            timestamp H(timestamp ":" ETag ":" secret-data)

      where timestamp is a server-generated time, which preferably
      includes micro- or nanoseconds, 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 timestamp 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



Shekh-Yusef, et al.          Standards Track                    [Page 6]

RFC 7616            HTTP Digest Access Authentication     September 2015


      originally got it.  However, that would break because 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 timestamp for GET
      requests.  For more details on the issues involved, see Section 5
      of this document.

      The nonce is opaque to the client.

   opaque

      A string of data, specified by the server, that 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.

   stale

      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.

   algorithm

      A string indicating an algorithm used to produce the digest and an
      unkeyed digest.  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.,
      "SHA-256-sess".

      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



Shekh-Yusef, et al.          Standards Track                    [Page 7]

RFC 7616            HTTP Digest Access Authentication     September 2015


      unkeyed digest algorithm to the data "data" will be denoted
      H(data).  KD stands for Keyed Digest, and 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)

        and

            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"
      is intended to allow efficient third-party authentication servers;
      for the difference in usage, see the description in Section 3.4.2.

   qop

      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.

   charset

      This is an OPTIONAL parameter that is used by the server to
      indicate the encoding scheme it supports.  The only allowed value
      is "UTF-8".

   userhash

      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".






Shekh-Yusef, et al.          Standards Track                    [Page 8]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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 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:

   response

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

   username

      The user's name in the specified realm.  The quoted string
      contains the name in plaintext or the hash code in hexadecimal
      notation.  If the username contains characters not allowed inside
      the ABNF quoted-string production, the username* parameter can be
      used.  Sending both username and username* in the same header
      option MUST be treated as an error.

   username*

      If the userhash parameter value is set "false" and the username
      contains characters not allowed inside the ABNF quoted-string
      production, the user's name can be sent with this parameter, using
      the extended notation defined in [RFC5987].

   realm

      See "realm" definition in Section 3.3.

   uri

      The Effective Request URI (Section 5.5 of [RFC7230]) of the HTTP
      request; duplicated here because proxies are allowed to change the
      request target ("request-target", Section 3.1.1 of [RFC7230]) in
      transit.



Shekh-Yusef, et al.          Standards Track                    [Page 9]

RFC 7616            HTTP Digest Access Authentication     September 2015


   qop

      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-
      Authenticate.

   cnonce

      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.

   nc

      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.

   userhash

      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



Shekh-Yusef, et al.          Standards Track                   [Page 10]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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

   where

         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 cnonce 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 that 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 needs only use the hash of the user
   credentials in order to create the A1 value, this construction could



Shekh-Yusef, et al.          Standards Track                   [Page 11]

RFC 7616            HTTP Digest Access Authentication     September 2015


   be used in conjunction with a third-party authentication service so
   that the web server would not need the actual password value.  The
   specification of such a protocol is beyond the scope of this
   specification.

3.4.3.  A2

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

         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 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 perform 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
   fields

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

   and the user Mufasa has password "Circle Of Life", then H(A1) would
   be H(Mufasa:myhost@example.com:Circle Of Life) with no quotation
   marks in the digested string.



Shekh-Yusef, et al.          Standards Track                   [Page 12]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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

      Mufasa:myhost@example.com:Circle 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 that 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 (see [RFC7234]).






Shekh-Yusef, et al.          Standards Track                   [Page 13]

RFC 7616            HTTP Digest Access Authentication     September 2015


3.5.  The Authentication-Info and Proxy-Authentication-Info Header
      Fields

   The Authentication-Info header field and the Proxy-Authentication-
   Info header field [RFC7615] are generic fields that MAY be used by a
   server to communicate some information regarding the successful
   authentication of a client response.

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

   nextnonce

      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 nonces 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 effects on pipelining.

   qop

      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.








Shekh-Yusef, et al.          Standards Track                   [Page 14]

RFC 7616            HTTP Digest Access Authentication     September 2015


   rspauth

      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 is set to "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
   digits.

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.





Shekh-Yusef, et al.          Standards Track                   [Page 15]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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 in the
   WWW-Authenticate header field, 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



Shekh-Yusef, et al.          Standards Track                   [Page 16]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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
   algorithm.

   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 first
   challenge it supports, unless a local policy dictates otherwise.

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 4.3 and 4.4
   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 that 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 reissue 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-Authentication-Info
   with parameters the same as those for the Authentication-Info header
   field.





Shekh-Yusef, et al.          Standards Track                   [Page 17]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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 "http://www.example.org/dir/index.html".  Both client and
   server know that the username for this document is "Mufasa" and the
   password is "Circle of Life" (with one space between each of the
   three words).

   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
       realm="http-auth@example.org",
       qop="auth, auth-int",
       algorithm=SHA-256,
       nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
       opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"
   WWW-Authenticate: Digest
       realm="http-auth@example.org",
       qop="auth, auth-int",
       algorithm=MD5,
       nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
       opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"

   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",
       realm="http-auth@example.org",
       uri="/dir/index.html",
       algorithm=MD5,
       nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
       nc=00000001,
       cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ",
       qop=auth,
       response="8ca523f5e9506fed4657c9700eebdbec",
       opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"





Shekh-Yusef, et al.          Standards Track                   [Page 18]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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",
       realm="http-auth@example.org",
       uri="/dir/index.html",
       algorithm=SHA-256,
       nonce="7ypf/xlj9XXwfDPEoM4URrv/xwf94BcCAzFZH4GiTo0v",
       nc=00000001,
       cnonce="f2/wE4q74E6zIJEtWaHKaf5wv/H5QzzpXusqGemxURZJ",
       qop=auth,
       response="753927fa0e85d155564e2e272a28d1802ca10daf449
          6794697cf8db5856cb6c1",
       opaque="FQhe/qaU925kfnzjCev0ciny7QMkPqMAFRtzCUYo5tdS"

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 "http://api.example.org/doe.json".  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 a variation of "Jason Doe", where the 'a' actually is
   Unicode code point U+00E4 ("LATIN SMALL LETTER A WITH DIAERESIS"),
   and the first 'o' is Unicode code point U+00F8 ("LATIN SMALL LETTER O
   WITH STROKE"), leading to the octet sequence using the UTF-8 encoding
   scheme:

      J  U+00E4 s  U+00F8 n      D  o  e
      4A C3A4   73 C3B8   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
       realm="api@example.org",
       qop="auth",
       algorithm=SHA-512-256,
       nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK",
       opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS",
       charset=UTF-8,
       userhash=true





Shekh-Yusef, et al.          Standards Track                   [Page 19]

RFC 7616            HTTP Digest Access Authentication     September 2015


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

   Authorization: Digest
       username="488869477bf257147b804c45308cd62ac4e25eb717
          b12b298c79e62dcea254ec",
       realm="api@example.org",
       uri="/doe.json",
       algorithm=SHA-512-256,
       nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK",
       nc=00000001,
       cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v",
       qop=auth,
       response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d
          6c861229025f607a79dd",
       opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS",
       userhash=true

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

   Authorization: Digest
       username*=UTF-8''J%C3%A4s%C3%B8n%20Doe,
       realm="api@example.org",
       uri="/doe.json",
       algorithm=SHA-512-256,
       nonce="5TsQWLVdgBdmrQ0XsxbDODV+57QdFR34I9HAbC/RVvkK",
       nc=00000001,
       cnonce="NTg6RKcb9boFIAS3KrFK9BGeh+iDa/sm6jUMp2wds69v",
       qop=auth,
       response="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d
          6c861229025f607a79dd",
       opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS",
       userhash=false

4.  Internationalization Considerations

   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 Section 2.3 in [RFC2978]).  It indicates that the server expects
   the username 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].




Shekh-Yusef, et al.          Standards Track                   [Page 20]

RFC 7616            HTTP Digest Access Authentication     September 2015


   For the username, recipients MUST support all characters defined in
   the "UsernameCasePreserved" profile defined in Section 3.3 of
   [RFC7613], with the exception of the colon (":") character.

   For the password, recipients MUST support all characters defined in
   the "OpaqueString" profile defined in Section 4.2 of [RFC7613].

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

   When usernames cannot be sent hashed and include non-ASCII
   characters, clients can include the username* parameter instead
   (using the value encoding defined in [RFC5987]).

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.

   If Digest Authentication is being used, it SHOULD be over a secure
   channel like HTTPS [RFC2818].

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
   above.

   The security implications of this are that if this password file is
   compromised, then an attacker gains immediate access to documents on
   the server using this realm.  Unlike, say, a standard UNIX password
   file, this information needs 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



Shekh-Yusef, et al.          Standards Track                   [Page 21]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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 that 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 Lightweight Directory Access Protocol
   (LDAP) [RFC4513] and IMAP/POP (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 the "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 a 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
   appropriate.








Shekh-Yusef, et al.          Standards Track                   [Page 22]

RFC 7616            HTTP Digest Access Authentication     September 2015


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 Section 5.5).  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 it has a high cost due to checking nonce
   values; perhaps more important, it 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
   him.

   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 timestamp, 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 timestamp expires.  Digesting the
   client IP and timestamp in the nonce permits an implementation that
   does not maintain state between transactions.



Shekh-Yusef, et al.          Standards Track                   [Page 23]

RFC 7616            HTTP Digest Access Authentication     September 2015


   For applications where no possibility of replay attack can be
   tolerated, the server can use one-time nonce values that 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
   timestamp (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 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 Section 5.8.

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
   challenge.

   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
   schemes.

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.









Shekh-Yusef, et al.          Standards Track                   [Page 24]

RFC 7616            HTTP Digest Access Authentication     September 2015


5.8.  Man-in-the-Middle Attacks

   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 producing 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
   Authentication.

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, a method to analyze the one-way functions used by Digest
   using chosen plaintext is not 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.




Shekh-Yusef, et al.          Standards Track                   [Page 25]

RFC 7616            HTTP Digest Access Authentication     September 2015


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 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 for clients to use the
   cnonce parameter.

5.12.  Parameter Randomness

   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]).

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



Shekh-Yusef, et al.          Standards Track                   [Page 26]

RFC 7616            HTTP Digest Access Authentication     September 2015


   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 be satisfied with a nonce like the
   one recommended above, i.e., 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.  Hash Algorithms for HTTP Digest Authentication

   This specification creates a new IANA registry named "Hash Algorithms
   for HTTP Digest Authentication" under the existing "Hypertext
   Transfer Protocol (HTTP) Digest Algorithm Values" category.  This
   registry lists the hash algorithms that can be used in HTTP Digest
   Authentication.

   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.

   Reference

      A reference to the specification adding the algorithm to this
      registry.

   The update policy for this registry shall be Specification Required
   [RFC5226].











Shekh-Yusef, et al.          Standards Track                   [Page 27]

RFC 7616            HTTP Digest Access Authentication     September 2015


   The initial registry contains the following entries:

               +----------------+-------------+-----------+
               | Hash Algorithm | Digest Size | Reference |
               +----------------+-------------+-----------+
               | "MD5"          | 128         | RFC 7616  |
               | "SHA-512-256"  | 256         | RFC 7616  |
               | "SHA-256"      | 256         | RFC 7616  |
               +----------------+-------------+-----------+

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

   To clarify the purpose of the existing "HTTP Digest Algorithm Values"
   registry and to avoid confusion between the two registries, IANA has
   added the following description to the existing "HTTP Digest
   Algorithm Values" registry:

      This registry lists the algorithms that can be used when creating
      digests of an HTTP message body, as specified in RFC 3230.

6.2.  Digest Scheme Registration

   This specification updates the existing entry of the Digest scheme in
   the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
   Registry" and adds a new reference to this specification.

      Authentication Scheme Name: Digest

      Pointer to specification text: RFC 7616

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <http://www.rfc-editor.org/info/rfc2978>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.




Shekh-Yusef, et al.          Standards Track                   [Page 28]

RFC 7616            HTTP Digest Access Authentication     September 2015


   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <http://www.rfc-editor.org/info/rfc5198>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
              <http://www.rfc-editor.org/info/rfc5987>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <http://www.rfc-editor.org/info/rfc6454>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <http://www.rfc-editor.org/info/rfc7234>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.




Shekh-Yusef, et al.          Standards Track                   [Page 29]

RFC 7616            HTTP Digest Access Authentication     September 2015


   [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 7613,
              DOI 10.17487/RFC7613, August 2015,
              <http://www.rfc-editor.org/info/rfc7613>.

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <http://www.rfc-editor.org/info/rfc7615>.

7.2.  Informative References

   [RFC2195]  Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response",
              RFC 2195, DOI 10.17487/RFC2195, September 1997,
              <http://www.rfc-editor.org/info/rfc2195>.

   [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, DOI 10.17487/RFC2617, June 1999,
              <http://www.rfc-editor.org/info/rfc2617>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC4513]  Harrison, R., Ed., "Lightweight Directory Access Protocol
              (LDAP): Authentication Methods and Security Mechanisms",
              RFC 4513, DOI 10.17487/RFC4513, June 2006,
              <http://www.rfc-editor.org/info/rfc4513>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,
              <http://www.rfc-editor.org/info/rfc7617>.










Shekh-Yusef, et al.          Standards Track                   [Page 30]

RFC 7616            HTTP Digest Access Authentication     September 2015


Appendix A.  Changes from RFC 2617

   This document introduces the following changes:

   o  Adds support for two new algorithms, SHA2-256 as mandatory and
      SHA2-512/256 as a backup, and defines the proper algorithm
      negotiation.  The document keeps the MD5 algorithm support but
      only for backward compatibility.

   o  Introduces the username hashing capability and the parameter
      associated with that, mainly for privacy reasons.

   o  Adds various internationalization considerations that impact the
      A1 calculation and username and password encoding.

   o  Introduces a new IANA registry, "Hash Algorithms for HTTP Digest
      Authentication", that lists the hash algorithms that can be used
      in HTTP Digest Authentication.

   o  Deprecates backward compatibility with RFC 2069.

Acknowledgments

   To provide a complete description for the Digest mechanism and its
   operation, this document borrows text heavily from [RFC2617].  The
   authors of this document would like to thank John Franks, Phillip M.
   Hallam-Baker, Jeffery L. Hostetler, Scott D. Lawrence, Paul J. Leach,
   Ari Luotonen, and Lawrence C. Stewart for their work on that
   specification.

   Special thanks to Julian Reschke for his many 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, Gary Mort, Alexey Melnikov, Benjamin Kaduk, Kathleen
   Moriarty, Francis Dupont, Hilarie Orman, and Ben Campbell 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.




Shekh-Yusef, et al.          Standards Track                   [Page 31]

RFC 7616            HTTP Digest Access Authentication     September 2015


   The authors would like to thank Barry Leiba for his help with the
   registry.

Authors' Addresses

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

   Phone: +1-613-967-5267
   Email: rifaat.ietf@gmail.com


   David Ahrens
   Independent
   California
   United States

   Email: ahrensdc@gmail.com


   Sophie Bremer
   Netzkonform
   Germany

   Email: sophie.bremer@netzkonform.de























Shekh-Yusef, et al.          Standards Track                   [Page 32]

