HTTPAuth                                             R. Shekh-Yusef, Ed.Internet-Draft                                                     AvayaObsoletes: 2617 (if approved)                                  D. AhrensIntended status: Standards Track                             IndependentExpires: October 25, 2015                                      S. Bremer                                                             Netzkonform                                                          April 23, 2015                   HTTP Digest Access Authentication                     draft-ietf-httpauth-digest-19Abstract   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 (http-auth@ietf.org), which is archived at [1].Status of This Memo   This Internet-Draft is submitted in full conformance with the   provisions of BCP 78 and BCP 79.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF).  Note that other groups may also distribute   working documents as Internet-Drafts.  The list of current Internet-   Drafts is at http://datatracker.ietf.org/drafts/current/.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on October 25, 2015.Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.Shekh-Yusef, et al.     Expires October 25, 2015                [Page 1]Internet-Draft      HTTP Digest Access Authentication         April 2015   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.Table of Contents   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4   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 Field  . . . . . . .   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  . . . . . . . . .  12       3.4.6.  Various Considerations  . . . . . . . . . . . . . . .  12     3.5.  The Authentication-Info and Proxy-Authentication-Info           Header Fields . . . . . . . . . . . . . . . . . . . . . .  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 . . .  18Shekh-Yusef, et al.     Expires October 25, 2015                [Page 2]Internet-Draft      HTTP Digest Access Authentication         April 2015   4.  Internationalization Considerations . . . . . . . . . . . . .  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  . . . . . . . . . . . . . . . . . . . . .  22     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. Parameter Randomness  . . . . . . . . . . . . . . . . . .  26     5.13. Summary . . . . . . . . . . . . . . . . . . . . . . . . .  26   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26     6.1.  Hash Algorithms for HTTP Digest Authentication  . . . . .  26     6.2.  Digest Scheme Registration  . . . . . . . . . . . . . . .  27   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  27   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  28     8.2.  Informative References  . . . . . . . . . . . . . . . . .  29   Appendix A.  Changes from RFC 2617  . . . . . . . . . . . . . . .  30   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  301.  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 [BASIC], "The Hypertext Transfer Protocol   (HTTP) Authentication-Info and Proxy-Authentication-Info Response   Header Fields" [AUTHINFO], and [RFC7235] obsolete [RFC2617].Shekh-Yusef, et al.     Expires October 25, 2015                [Page 3]Internet-Draft      HTTP Digest Access Authentication         April 20151.1.  Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].2.  Syntax Convention2.1.  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 Scheme3.1.  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   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 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 toShekh-Yusef, et al.     Expires October 25, 2015                [Page 4]Internet-Draft      HTTP Digest Access Authentication         April 2015   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      example might be "registered_users@gotham.news.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 an 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.Shekh-Yusef, et al.     Expires October 25, 2015                [Page 5]Internet-Draft      HTTP Digest Access Authentication         April 2015      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.   opaque      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.   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 staleShekh-Yusef, et al.     Expires October 25, 2015                [Page 6]Internet-Draft      HTTP Digest Access Authentication         April 2015      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 a      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      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 3rd 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"Shekh-Yusef, et al.     Expires October 25, 2015                [Page 7]Internet-Draft      HTTP Digest Access Authentication         April 2015      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.   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".   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:   response      A string of the hex digits computed as defined below, which proves      that the user knows a password.   username      The user's name in the specified realm.  The quoted string      contains the name in plain text 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 error.Shekh-Yusef, et al.     Expires October 25, 2015                [Page 8]Internet-Draft      HTTP Digest Access Authentication         April 2015   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].   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.   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.Shekh-Yusef, et al.     Expires October 25, 2015                [Page 9]Internet-Draft      HTTP Digest Access Authentication         April 2015   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   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 >Shekh-Yusef, et al.     Expires October 25, 2015               [Page 10]Internet-Draft      HTTP Digest Access Authentication         April 2015   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 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 needs only use the hash of the user   credentials in order to create the A1 value, this construction could   be used in conjunction with a third party authentication service so   that the web server would not need the actual password value.  The   specification of such a protocol is beyond the scope of this   specification.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 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.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 11]Internet-Draft      HTTP Digest Access Authentication         April 2015   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   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.   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 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".  TheShekh-Yusef, et al.     Expires October 25, 2015               [Page 12]Internet-Draft      HTTP Digest Access Authentication         April 2015   "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]).3.5.  The Authentication-Info and Proxy-Authentication-Info Header      Fields   The Authentication-Info header field and the Proxy-Authentication-   Info header field [AUTHINFO] 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 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 theShekh-Yusef, et al.     Expires October 25, 2015               [Page 13]Internet-Draft      HTTP Digest Access Authentication         April 2015         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.   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=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.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 14]Internet-Draft      HTTP Digest Access Authentication         April 20153.6.  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 aShekh-Yusef, et al.     Expires October 25, 2015               [Page 15]Internet-Draft      HTTP Digest Access Authentication         April 2015   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   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 4.2 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 theShekh-Yusef, et al.     Expires October 25, 2015               [Page 16]Internet-Draft      HTTP Digest Access Authentication         April 2015   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-Authentication-Info   with parameters the same as those for the Authentication-Info header   field.   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.  Examples3.9.1.  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:Shekh-Yusef, et al.     Expires October 25, 2015               [Page 17]Internet-Draft      HTTP Digest Access Authentication         April 2015   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"   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 the 'a' actually   is Unicode code point U+00E4 ("LATIN SMALL LETTER A WITH DIAERES"),   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:Shekh-Yusef, et al.     Expires October 25, 2015               [Page 18]Internet-Draft      HTTP Digest Access Authentication         April 2015   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   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 can not 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="ae66e67d6b427bd3f120414a82e4acff38e8ecd9101d6          c861229025f607a79dd",       opaque="HRPCssKJSGjCrkzDg8OhwpzCiGPChXYjwrI2QmXDnsOS",       userhash=falseShekh-Yusef, et al.     Expires October 25, 2015               [Page 19]Internet-Draft      HTTP Digest Access Authentication         April 20154.  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 [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].   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.   When usernames can not be sent hashed and include non-ASCII   characters, clients can include the "username*" parameter instead   (using the value encoding defined in [RFC5987]).5.  Security Considerations5.1.  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].Shekh-Yusef, et al.     Expires October 25, 2015               [Page 20]Internet-Draft      HTTP Digest Access Authentication         April 20155.2.  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   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,Shekh-Yusef, et al.     Expires October 25, 2015               [Page 21]Internet-Draft      HTTP Digest Access Authentication         April 2015   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   appropriate.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   him.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 22]Internet-Draft      HTTP Digest Access Authentication         April 2015   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   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.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 23]Internet-Draft      HTTP Digest Access Authentication         April 20155.7.  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   Authentication.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 24]Internet-Draft      HTTP Digest Access Authentication         April 20155.9.  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.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 25]Internet-Draft      HTTP Digest Access Authentication         April 20155.12.  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   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 Considerations6.1.  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.   ReferenceShekh-Yusef, et al.     Expires October 25, 2015               [Page 26]Internet-Draft      HTTP Digest Access Authentication         April 2015      A reference to the specification adding the algorithm to this      registry.   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.   To clarify the purpose of the existing "HTTP Digest Algorithm Values"   registry and to avoid confusion between the two registries, IANA is   asked to add 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   Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry and   adds a new reference to this specification.      Authentication Scheme Name: Digest      Pointer to specification text: this specification7.  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.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 27]Internet-Draft      HTTP Digest Access Authentication         April 2015   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, and Hilarie Orman 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.   The authors would like to thank Barry Leiba for his help with the   registry.8.  References8.1.  Normative References   [AUTHINFO]              Reschke, J., "The Hypertext Transfer Protocol (HTTP)              Authentication-Info and Proxy-Authentication-Info Response              Header Fields", draft-ietf-httpbis-auth-info-02 (work in              progress), February 2015.   [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.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 28]Internet-Draft      HTTP Digest Access Authentication         April 2015   [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              2011.   [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.Shekh-Yusef, et al.     Expires October 25, 2015               [Page 29]Internet-Draft      HTTP Digest Access Authentication         April 2015Appendix 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  Deprecates backward compatibility with RFC2069.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   USA   EMail: ahrensdc@gmail.com   Sophie Bremer   Netzkonform   Germany   EMail: sophie.bremer@netzkonform.deShekh-Yusef, et al.     Expires October 25, 2015               [Page 30]