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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 draft-oiwa-httpbis-mutualauth

Internet Engineering Task Force                                  Y. Oiwa
Internet-Draft                                               H. Watanabe
Intended status: Standards Track                               H. Takagi
Expires: May 15, 2008                                         RCIS, AIST
                                                               H. Suzuki
                                                            Yahoo! Japan
                                                       November 12, 2007


                Mutual Authentication Protocol for HTTP
                     draft-oiwa-http-mutualauth-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 15, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document defines the "Mutual authentication protocol for Hyper-
   Text Transport Protocol".  This protocol provides true mutual
   authentication between HTTP clients and servers using simple
   password-based authentication.  Unlike Basic and Digest HTTP access
   authentication protocol, the protocol ensures that server knows the



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   user's entity (encrypted password) upon successful authentication.
   This prevents common phishing attacks: phishing attackers cannot
   convince users that the user has been authenticated to the genuine
   website.  Furthermore, even when user has been authenticated against
   an illegitimate server, the server can not gain any bit of
   information about user's passwords.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Tokens and Ext-tokens  . . . . . . . . . . . . . . . . . .  6
     3.2.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Strings  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  401-B0 . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  401-B0-stale . . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  req-A1 . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.4.  401-B1 . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.5.  req-A3 . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.6.  200-B4 . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Decision procedure for the client  . . . . . . . . . . . . . . 11
   6.  Authentication Algorithms  . . . . . . . . . . . . . . . . . . 14
   7.  Validation Methods . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Session Management . . . . . . . . . . . . . . . . . . . . . . 16
   9.  Extension 1: Optional Mutual Authentication  . . . . . . . . . 17
   10. Methods to extend this protocol  . . . . . . . . . . . . . . . 18
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   13. Notice on intellectual properties  . . . . . . . . . . . . . . 19
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     14.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21













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1.  Introduction

   This document defines the "Mutual authentication protocol for Hyper-
   Text Transport Protocol".  This protocol provides true mutual
   authentication between HTTP clients and servers using simple
   password-based authentication.  Unlike Basic and Digest HTTP access
   authentication protocol, the protocol ensures that server knows the
   user's entity (encrypted password) upon successful authentication.
   This prevents common phishing attacks: phishing attackers cannot
   convince users that the user has been authenticated to the genuine
   website.  Furthermore, even when user has been authenticated against
   an illegitimate server, the server can not gain any bit of
   information about user's passwords.

   Recently, phishing attacks are getting more and more sophisticated.
   Phishers not only steal user's password directly, but imitate
   successful authentication to steal user's sensitive information,
   check the password validity by forwarding the password to the
   legitimate server, or employ a man-in-the-middle attack to hijack
   user's login session.  Existing countermeasures such as one-time
   passwords can not completely solve these problems.

   The protocol prevents such attacks by providing users a way to
   discriminate between true and fake web servers using their own
   passwords.  Even when a user inputs his/her password to a fake
   website, using this authentication method, any information about the
   password does not leak to the phisher, and the user certainly notices
   that the mutual authentication has failed.  Phishers cannot make such
   authentication attempt succeed, even if they forward received data
   from a user to the legitimate server or vice versa.  Users can safely
   input sensitive data to the web forms after confirming that the
   mutual authentication has succeeded.

   To achieve this goal, this protocol use a mechanism in ISO/IEC
   11770-4 [1], a kind of PAKE (Password-Authenticated Key Exchange)
   authentication algorithms as a basis.  The use of PAKE mechanism
   allows users to use familiar ID/password based accesses, without fear
   of leaking any password information to the communication peer.  The
   protocol, as a whole, is designed as a natural extension to the
   current HTTP authentication schema such as Basic and Digest access
   authentication (RFC 2617).  To use PAKE mechanism for such a purpose,
   we had to modify it to prevent credential forwarding (man-in-the-
   middle) attacks.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in <RFC2119>.


2.  Protocol Overview

   The following sequence is a typical sequence for the first access to
   the resource.

   o  If the server (S) has received a request for mutual-authentication
      protected resources from the Client (C) (which is not a req-A1 nor
      a req-A3 message), it sends a 401-B0 message to C.

      When C has received a 401-B0 message, C SHOULD check validity of
      the message.  If succeed, C processes the body of the message, and
      enables the password entry field.

   o  If the user has input the username and password as a response to
      the 401-B0 message, C creates a value s_A, calculates the value
      w_A, and construct and send a req-A1 message.

   o  If S has received an req-A1 message, S should record the received
      w_A value, validate w_A using T(w_A), and then look up the
      username from the user table. if the user is found, S prepares a
      new session id (sid), record it into a session table, and then
      construct s_B, calculate w_B, and then send an 401-B1 message.

      If there is no matching user found, the server SHOULD construct a
      fake w_B value, and let the protocol going on by sending an 401-B1
      message.

   o  When C has received an 401-B1 message as a response for a req-A1
      message, C should compute z and K_i, compute o_A, and send an
      req-A3 message.

      If C receives any messages other than 401-B1, C MUST NOT process
      the message body and treat it as a fatal communication error
      condition.  This case includes the reception of HTTP OK (200-
      status) message.

   o  If S has received an req-A3 message, S should look up the received
      sid from the session table.  If no matching sid message is
      received, or if S has not received the corresponding req-A1
      message beforehand, S SHOULD send an 401-B0-stale message.

      Otherwise, S should computes o_A' and check its value.  If the
      validation has failed, the server SHOULD send an 401-B0 message.

      If the validation has succeed, the server SHOULD calculate o_B,



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      and send a 200-B4 message.

   o  When C has received a 401-B0-stale message as a response to req-A3
      message, and it is the first time that it has received a 401-B0-
      stale message, C SHOULD redo constructing req-A1 message.

      Instead, when C has received an 401-B0 message, it means the
      authentication has been failed, possibly due to that the wrong
      password has been given.  C MAY ignore the body of the 401-B0
      message in this case.

      When C has received an 200-B4 message, C MUST first compute the
      value of o_B' and validates the value o_B sent from the server.
      If it has not verified successfully, C MUST ignore the body of the
      message, and treat it as a fatal communication error condition.
      If it has succeed, C will process the body of the message.

      If C receives any messages other than 401-B0-stale or 401-B1, C
      SHOULD NOT process the message body and other headers and treat it
      as a fatal communication error condition.  This case includes the
      reception of usual HTTP OK (200-status) messages.

   For the second or later request to the server, if the client knows
   that the resource is likely to require the authentication, the client
   MAY omit first unauthenticated request and send req-A1 message
   immediately.  Furthermore, if client owns a valid session ID (sid),
   the client can send a req-A3 message using existing sid.  In either
   case, the first (and only the first) response from the server MAY be
   a normal, unauthenticated message, and client MAY accept such
   messages.  For more detail, see Section 5.


3.  Message Syntax

   The Mutual authentication protocol uses four headers: WWW-
   Authenticate (in responses with status code 401), Optional-WWW-
   Authenticate (in responses with positive status codes), Authorization
   (in requests), and Authentication-info (in positive responses).
   These three headers share the common syntax described in Figure 1.
   The syntax is denoted in the augmented BNF syntax defined in
   <RFC2616>, except that "a"..."b" means any ASCII characters between
   "a" and "b" inclusive.  The syntax is a subset of the one described
   in <RFC2617>.








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              header = header-name ":" spaces "Mutual" spaces fields
         header-name = "WWW-Authenticate" | "Optional-WWW-Authenticate"
                       | "Authorization" | "Authentication-info"
              spaces = 1*(SP | HT | CR LF 1*(SP | HT))+
              fields = field 0*("," spaces fields)
               field = key "=" value
                 key = extensive-token
     extensive-token = token | extension-token
     extension-token = token "@" token
               token = 1*("0"..."9" | "A"..."Z" | "a"..."z"
                        | "-" | "_" | ".")
               value = extensive-token | integer | hex-integer
                     | hex-fixed-number
                     | base64-fixed-number | string
             integer = "0" | ("1"..."9") 0*("0"..."9")
         hex-integer = "0" |
                       ("1"..."9" | "A"..."F" | "a"..."f")
                       0*("0"..."9" | "A"..."F" | "a"..."f")
    hex-fixed-number = 1*("0"..."9" | "A"..."F" | "a"..."f")
    base64-fixed-number = string
              string = <"> *(<TEXT except <"> and "\"> | "\\" |
                             "\" <"> | "\,")* <">

         Figure 1: BNF syntax for the headers used in the protocol

3.1.  Tokens and Ext-tokens

   The tokens MUST be interpreted case-insensitive, and SHOULD be sent
   in the same case as shown in the specification.  All hex-fixed-number
   or hex-integer numbers are case-insensitive, and SHOULD be sent in
   lower-case.

   Extensive-tokens are used where the set of acceptable tokens are
   extensible.  Any non-standard extensions of this protocol MUST use
   the extension-tokens of format "<token>@<domain-name>", where domain-
   name is the valid registered (sub-)domain name on the Internet owned
   by the party who defines extensions.

3.2.  Numbers

   The syntax definitions of integer and hex-integer only allow
   representations which do not contain extra leading 0s.

   The numbers represented as a hex-fixed-number SHALL have even
   characters (i.e. multiple of eight bits).  When these are generated
   from the cryptographic values, those SHOULD have the natural length:
   if these are generated from a hash function, its lengths SHOULD
   correspond to the hash size; if these are representing elements of a



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   mathematical group, its lengths SHOULD be the shortest which can
   represent all elements in the group.  Other values such as session-id
   are represented in any (even) length determined by the side who
   generates it first, and the same length SHALL be used throughout the
   whole communications by both peers.

   The numbers represented as a base64-fixed-number MUST be generated as
   follows: first, the number is converted to a big-endian octet-string
   representation.  The length of the representation is determined in
   the same way as above.  Then, the string is BASE64-converted, and
   then surrounded by two double-quotations.

3.3.  Strings

   All strings outside ASCII or equivalent character sets SHOULD be
   encoded using UTF-8 encoding of the ISO 10646-1 character set.  Both
   peers SHOULD reject any invalid UTF-8 sequences which causes decoding
   ambiguities (e.g. containing <"> in the second or later byte of the
   UTF-8 encoded characters).  To encode character strings, these will
   first be encoded according to UTF-8, then all occurrences of
   characters <"> and "\" will be escaped by prepending "\", and two
   <">s will be put around the string.  If the contents of the strings
   are comma-separated values, the commas in the values are also quoted
   by "\".

   If strings are representing a domain name or URI which contains non-
   ASCII characters, the host parts SHOULD be encoded using puny-code
   instead of UTF-8, and SHOULD use lower-case ASCII characters.

   For Base64-fixed-numbers, which use the string syntax, see the
   previous section.


4.  Messages

4.1.  401-B0

   Every 401-B0 message must be an valid HTTP 401 (Authentication
   Required) message containing one (and only one: hereafter not
   explicitly noticed) "WWW-Authenticate" header of the following
   format.

   WWW-Authenticate: Mutual algorithm=xxxx, validation=xxxx, realm=xxxx,
   stale=0

   The header MUST contain the fields with the following keys:





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   algorithm:     (extensive-token) specifies the authentication
                  algorithm to be used.  The value MUST be one of the
                  tokens described in Section 6, or the tokens specified
                  in other supplemental specification documentations.

   validation:    (extensive-token) specifies the method of host
                  validation.  The value MUST be one of the tokens
                  described in Section 7, or the tokens specified in
                  other supplemental specification documentations.

   realm:         (string) is a UTF-8 encoded name of the authentication
                  domain inside the server.

   pwdhash:       (optional, extensive-token) specifies the hash
                  algorithm (referred to by ph) used for additionally
                  hashing the password.  The valid tokens are

                  *  none: ph(p) = p

                  *  md5: ph(p) = MD5(p)

                  *  digest-md5: ph(p) defined as A1 in <RFC2617>.

                  *  sha1: ph(p) = SHA1(p)

                  If omitted, the value "none" is assumed.

   auth-domain:   (optional, string) MUST currently be equal to the
                  host-part of the requested URI, and assumed to have
                  that value if omitted.  The triple of auth-domain,
                  algorithm, and realm determines the "authentication
                  realm" which defines the area where the same user-name
                  and passwords are applicable.

   stale:         (token) MUST be "0".

   Any additional fields MUST NOT be contained in the header, except
   those explicitly specified in supplement specifications of the
   "authentication algorithm".

   The algorithm will determine the values for w_A, w_B, o_A and o_B.

4.2.  401-B0-stale

   A 401-B0-stale message is a variant of 401-B0 message, which means
   that the client has sent a request message which is not for any
   active session.




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   WWW-Authenticate: Mutual algorithm=xxxx, validation=xxxx, realm=xxxx,
   stale=1

   The header MUST contain the same fields as in 401-B0, except the
   stale field holds the integer 1.

4.3.  req-A1

   Every req-A1 message MUST be a valid HTTP request message containing
   a "Authorization" header of the following format.

   Authorization: Mutual algorithm=xxxx, validation=xxxx, user=xxxx,
   realm=xxxx, wa=xxxx

   The header MUST contain the fields with the following keys:

   algorithm, validation, auth-domain, realm:  MUST be the same value as
                  it is received from S.

   user:          (string) is the UTF-8 encoded name of the user.

   wa:            (algorithm-determined) is the value of w_A specified
                  by the used algorithm.

4.4.  401-B1

   Every 401-B1 message MUST be an valid HTTP 401 (Authentication
   Required) message containing a "WWW-Authenticate" header of the
   following format.

   WWW-Authenticate: Mutual sid=xxxx, wb=xxxx, nc-max=x, nc-window=x,
   time=x, path=xxxx

   The header MUST contain the fields with the following keys:

   sid:           (hex-fixed-number) MUST be a session id, which is a
                  random integer.  The sid SHOULD have uniqueness of at
                  least 80 bits or the square of the maximal estimated
                  transactions concurrently available in the session
                  table, whichever is larger.  Sids are local to each
                  authentication realm concerned: the same sids for
                  different authentication realms SHALL be treated as
                  independent ones.

   wb:            (algorithm-determined) is the value of w_B specified
                  by the algorithm.





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   nc-max:        (hex-integer) is the maximal value of nonce counts
                  which S accepts.

   nc-window:     (hex-integer) the number of available nonce slots
                  which S prepares.  The value of nc-window is
                  RECOMMENDED to be thirty-two ("20" in hex-integer) or
                  more.

   time:          (integer) represents the suggested time (in seconds)
                  which C can reuse the session key represented by sid.
                  It SHOULD at least be 60.  The value of this field is
                  not directly linked to the duration that S keeps track
                  of the session represented by sid.

   path:          (string) specifies for which path in the URI space the
                  same authentication is expected to apply.  The value
                  is the same format as it is specified in <RFC2617> for
                  the Digest authentications, and client is RECOMMENDED
                  to recognize it.  The all paths contained in the field
                  SHALL be inside the specified auth-domain: if not,
                  client SHOULD ignore such elements.

4.5.  req-A3

   Every req-A3 message MUST be a valid HTTP request message containing
   a "Authorization" header of the following format.

   Authorization: Mutual sid=xxxx, nc=x, oa=xxxx

   The fields contained in the header is as follows:

   sid:           (hex-fixed-number) MUST be one of the sid values which
                  has been received from S.

   nc:            (hex-integer) is a nonce value which is unique among
                  the requests sharing the same sid.  The valie of nc
                  MUST satisfy the following properties:

                  *  It is not larger than the nc-max value which has
                     been sent from S in the session represented by the
                     sid.

                  *  C have not sent the same value in the same session.

                  *  It is not smaller than (largest-nc - nc-window),
                     where largest-nc is the maximal value of nc which
                     has previously been sent in the session, and nc-
                     window is the value of the nc-window field which



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                     has been sent from S in the session.

   oa:            (algorithm-determined) is the value of o_A specified
                  by the algorithm.

4.6.  200-B4

   Every 200-B1 message MUST be an valid HTTP message which is not 401
   (Authentication Required) type, containing an "Authentication-Info"
   header of the following format.

   Authentication-Info: Mutual sid=xxxx, ob=xxxx

   The fields contained in the header is as follows:

   sid:           (hex-fixed-number) MUST be the value received from C.

   ob:            (algorithm-determined) is the value of o_B specified
                  by the algorithm.

   logout-timeout:  (optional, integer) is a number of seconds after
                  which the client should re-validate the user's
                  password for the authentication realm.  As a special
                  case, the value 0 means that the client SHOULD
                  automatically forget the user-inputed password to the
                  current authentication realm and revert to the
                  unauthenticated state (i.e.~server-initiated logout).
                  This does not, however, mean that the long-term
                  memories for the passwords (such as password reminders
                  and auto fill-in) should be removed.  If new value of
                  timeout is received for the same authentication realm,
                  it overrides the previous timeout.


5.  Decision procedure for the client

   To securely implementing the protocol, the user client must be
   careful to accepting authenticated responses from the server.

   Clients SHOULD implement the decision procedure equivalent to the one
   shown below.  (Unless implementers understand what is required for
   security, they should not alter this.)

   Step 1   If the client software need to get a new Web resource, check
            whether the resource is expected to be inside some
            authentication realm for which the user has already
            authenticated. if yes, go to Step 2.  Otherwise, go to Step
            5.



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   Step 2   Check whether there is an available sid for the
            authentication realm you expects.  If there is one, go to
            Step 3.  Other wise, go to Step 4.

   Step 3   Send a req-A3 request.

            *  If you receive a 401-B0 message with a different
               authentication realm than expected, go to Step 6.

            *  If you receive a 401-B0-stale message, go to Step 9.

            *  If you receive a 401-B0 message, go to Step 13.

            *  If you receive a valid 200-B4 message, go to Step 14.

            *  If you receive a normal response (without Mutual-specific
               headers), go to Step 11.

   Step 4   Send a req-A1 request.

            *  If you receive a 401-B0 message with a different
               authentication realm than expected, go to Step 6.

            *  If you receive a 401-B0-stale message, go to Step 9.

            *  If you receive a 401-B1 message, go to Step 10.

            *  If you receive a normal response (without Mutual-specific
               headers), go to Step 10.

   Step 5   Send a request without any authentication headers.

            *  If you receive a 401-B0 message, go to Step 6.

            *  If you receive a normal response (without Mutual-specific
               headers), go to Step 11.

   Step 6   Check whether you know the user's password for the requested
            authentication realm.  If yes, go to Step 7.  Otherwise, go
            to Step 12.

   Step 7   Check whether there is an available sid for the
            authentication realm you expects.  If there is one, go to
            Step 8.  Other wise, go to Step 9.







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   Step 8   Send a req-A3 request.

            *  If you receive a 401-B0 message with a different
               authentication realm than expected, go to Step 6.

            *  If you receive a 401-B0-stale message, go to Step 9.

            *  If you receive a 401-B0 message, go to Step 13.

            *  If you receive a valid 200-B4 message, go to Step 14.

   Step 9   Send a req-A1 request.

            *  If you receive a 401-B1 message, go to Step 10.

   Step 10  Send a req-A3 request.

            *  If you receive a 401-B0 message, go to Step 13.

            *  If you receive a valid 200-B4 message, go to Step 14.

   Step 11  This case means that the resource requested is out of the
            authenticated contents area.  The client will be in
            "UNAUTHENTICATED" status.

   Step 12  This case means that the resource requested requires mutual
            authentication, and the user is not authenticated yet.  The
            client will be in "AUTH_REQUESTED" status, is RECOMMENDED to
            process the content sent from the server and, SHOULD ask
            user a username and password.  If the user has input those,
            go to Step 9.

   Step 13  This case means that in some reason the authentication
            failed: possibly the password or username is invalid for the
            authenticated resource.  Forget the password for the
            authentication realm and go to Step 12.

   Step 14  This case means that the mutual authentication has been
            succeeded.  The client will be in "AUTH_SUCCEEDED" status.

   All other kind of responses than shown in above procedure SHOULD be
   interpreted as fatal communication error, and in such cases user
   clients MUST NOT process any data sent from the server.

   The client software SHOULD show the three client status to the end-
   user.





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6.  Authentication Algorithms

   This specification specified only one family of the authentication
   algorithm.  The family consist of four authentication algorithms,
   which only differ in underlying mathematical groups and security
   parameters.  The algorithms do not add any additional fields.  The
   tokens for algorithms are

   o  "iso11770-4-ec-p256" for the 256-bit prime-field elliptic-curve
      setting.

   o  "iso11770-4-ec-p521" for the 521-bit prime-field elliptic-curve
      setting.

   o  "iso11770-4-dl-2048" for the 2048-bit discrete-logarithm setting.

   o  "iso11770-4-dl-4096" for the 4096-bit discrete-logarithm setting.

   The clients SHOULD support at least "iso11770-4-dl-2048" algorithm,
   and RECOMMENDED to support all of the above four algorithms if
   possible.  The server software implementations SHOULD support at
   least "iso11770-4-dl-2048" algorithm, unless it is known that users
   will not use it.

   This algorithm uses Key-exchange-method-3 defined in ISO-11770-4 as a
   basis.

   For the elliptic-curve settings, the underlying fields and the curves
   used for elliptic-curve cryptography are the prime field and the
   Curve P-256 and P-511, respectively, specified in the appendix of
   FIPS PUB 186-2 specification.  The hash functions H are SHA-256 for
   P-256 curve and SHA-512 for P-521 curve, respectively.  The
   representation of fields wa, wb, oa, and ob is hex-fixed-number.

   For discrete-logarithm settings, the underlying groups are 2048-bit
   and 4096-bit MODP group defined in <RFC3526> respectively.  The hash
   functions H are SHA-256 for 2048-bit field and SHA-512 for 4096-bit
   field, respectively.  The representation of fields wa, wb, oa, and ob
   is base64-fixed-number.

   The password-based string pi used by this authentication is derived
   in the following manner:

   pi = H(VS(algorithm) | VS(auth-domain) | VS(realm) | VS(username) |
   VS(ph(password)).

   The values of algorithm, realm and auth-domain are taken from the
   values contained in the 401-B0 message.  When pi is used in the



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   context of an octet string, it MUST have the natural length derived
   from the bit-length of the output of function H (e.g. 32 octets for
   SHA-256).  The function ph is defined by the value of the pwdhash
   given in a 401-B0 message.

   The function VS encodes variable-length octet string into decodable
   octet string, as in the following manner:

   VS(s) = VI(length(s)) | s,

   where length(s) is a number of octets (not characters) in s and the
   function VI converts integer into octet string in the manner which is
   used for the length field in DER format.  Shortly, integers are
   represented in big-endian radix-128 string, where each digit is
   represented by a octet 0x80-0xff except for the last digit
   represented by 0x00-0x7f.  The first octet MUST NOT be 0x80.  For
   example, VI(i) = octet(i) for i < 128, and VI(i) = octet(0x80 | (i >>
   7)) | octet(i & 127) for 128 <= i < 16384.

   The equation for J, w_A, T, z, K_i, and w_B is the one specified in
   the documentation of ISO-11770-4, given that pi is derived in the
   above equation.

   The value o_A (o_A') and o_B (o_B') is derived by the following
   equation, instead of ones specified in ISO 11770-4.

   o_A = H(hex(04) | GEtoOS_x(w_A) | GEtoOS_x(W_B) | GEtoOS_x(z) |
   GEtoOS_x(g_1) | VI(nc) | VS(v))

   o_B = H(hex(03) | GEtoOS_x(w_A) | GEtoOS_x(W_B) | GEtoOS_x(z) |
   GEtoOS_x(g_1) | VI(nc) | VS(v))


7.  Validation Methods

   The "validation methods" specifies a method to "relates" the mutual
   authentication processed by this protocol with other authentications
   already performed in the underlying layers.  It decides the value of
   v, which is an input to the authentication protocol.

   The meaning of the validation method field and corresponding values
   of v is as follows:

   host:          hostname validation: v will be the ASCII string in the
                  following format: "scheme://host:port".  The scheme
                  and host are lower-case, and the port is in a shortest
                  decimal representation.  Even if the request-URI do
                  not have a port part, v will include the one.



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   tls-cert:      TLS certificate validation: v will be the octet string
                  of the fingerprint of the public key certificate used
                  in underlying TLS/SSL connection.

   tls-key:       TLS shared-key validation: v will be the octet string
                  of the pre-shared secret negotiated in underlying TLS/
                  SSL connection.

   If HTTP protocol is used on unencrypted channel, the validation type
   MUST be "host".  If HTTPS protocol with server certificates is used,
   the validation type MUST be either "tls-cert" or "tls-key".  If HTTPS
   protocol is used with anonymous Diffie-Hellman key exchange, the
   validation type MUST be "tls-key" (but see the note below).

   The client MUST validate this field upon reception of 401-B0
   messages.

   However, when the protocol is used on web browsers with any scripting
   capabilities, anonymous Diffie-Hellman family of TLS/SSL cipher-suite
   MUST NOT be used even if "tls-key" validated Mutual authentication
   has been employed, and the certificate shown in TLS/SSL negotiation
   MUST be verified using PKI.  For other systems, if the "tls-key"
   validation is used on TLS/SSL protocol without certificate
   verification using PKI, those systems MUST ensure that all
   transactions with authenticated peer server MUST use and be validated
   by the Mutual authentication protocol, regardless of the existence of
   the 401-B0 responses.


8.  Session Management

   By the first 4 messages (first request, 401-B0, req-A1 and 401-B1), a
   session represented by a sid is generated.  This session can be used
   for 1 or more requests for resources protected by the same realm in
   the same server.

   The server SHOULD accept at least one req-A3 request for each
   sessions (given that the request reaches to the server in a time
   window specified by the timeout field specified in the 401-B1
   message).  After that, the server can discard any session at any time
   and send 401-B0-stale messages for any req-A3 requests.

   The client can send more than 1 requests using a single session
   specified by the sid.  However, for all such requests, the values of
   the nonce-counter (nc field) MUST be different from each other.  The
   server MUST check for duplication of the received nonces, and if any
   duplication is detected, the server MUST discard the session and
   respond by a 401-B1-stale message.



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   In addition, for each sessions, if the client has already sent a
   request with nonce value x, it SHOULD NOT sent requests with nonce
   value not larger than (x - nc-window) afterwards.  The server MAY
   reject any requests with nonces violating this rule with 401-B1-stale
   responses.

   The value of nonces and nonce-related values MUST always be treated
   as a natural number within infinite range.  Implementations using
   fixed-width integers or fixed-precision floating numbers MUST handle
   integer overflow correctly and carefully.  Such implementations are
   RECOMMENDED to accept any larger values from the peer which cannot be
   represented in the fixed-width integer representations, as long as
   other limits such as internal header-length restrictions are
   involved.  The protocol is designed carefully so that both clients
   and servers can be implemented the protocol only with fixed-width
   integers, by rounding any overflowed value into the maximum possible
   value.


9.  Extension 1: Optional Mutual Authentication

   In several Web applications, users can access the same contents both
   as a guest user and as a authenticated users.  In usual Web
   applications, it is implemented using Cookies and custom form-based
   authentications.  The extension described in this section provides a
   replacement for those authentication systems.

   Every HTTP successful responses (response code 200, 206 and others)
   to GET requests which do not contain Authentication-Info: header can
   contain the Optional-WWW-Authenticate header.  Such messages are
   hereafter called 200-Optional-B0 messages.

      HTTP/1.1 200 OK
      Optional-WWW-Authenticate: Mutual algorithm=xxxx, validation=xxxx,
      realm=xxxx, stale=0

   The fields contained in the Optional-WWW-Authenticate header is the
   same as the 401-B0 message described in Section 4.1.  The client
   software supporting the mutual authentication protocol receiving a
   200-Optional-B0 message will process the contents of the message and
   enables an authentication input field.

   When the user input the username and password, the client resends the
   request with req-A1 header.  The server MUST respond with a 401-B1
   message. 200-Optional-B0 response SHOULD NOT be sent as a response to
   req-A1 and req-A3 messages, unless the authentication realm sent from
   the client (or indicated by sid) is different from the one which the
   server requests.



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   The client software supporting optional mutual authentication MUST
   recognize the "Path" field contained in 200-B4 message, and MUST send
   either req-A1 or req-A3 request for the URI space inside the
   specified "path"s, instead of unauthenticated requests.


10.  Methods to extend this protocol

   If a non-standard extensions to the this protocol is implemented, it
   SHOULD use the extension-tokens defined in Section 3 to avoid
   conflicts with this protocol and other extensions.

   Authentication algorithms other than those defined in this document
   MAY use other representations for keys "wa", "wb", "oa" and "ob",
   replace those keys, and/or add fields to the messages containing
   those fields by supplemental specifications.  Those specifications
   are RECOMMENDED to use extension-tokens to avoid any key-name
   conflict with the future extension of this protocol.


11.  IANA Considerations

   The tokens used for authentication-algorithm, pwd-hash, validation
   MUST be registered to and assigned by IANA.  All registered tokens
   must be defined by at least Informational RFC or similar
   standardization processes.  Extension-tokens MAY be freely used for
   any non-standard or experimental uses for those fields provided that
   the domain part in the token is appropriately used.


12.  Security Considerations

   o  To securely implement the protocol, the Authentication-Info
      headers in the 200-B4 messages MUST always be validated by the
      client.  If the validation is failed, the client MUST NOT process
      any content sent with the message, including the body part.  Non-
      compliance to this will enable the phishing attacks.

   o  The authentication status on the client-side SHOULD be visible to
      the users of the client.  In addition, the method for asking
      user's name and passwords SHOULD be carefully designed so that (1)
      the user can easily distinguish request of this authentication
      methods from existing insecure authentication methods such as
      Basic methods, and (2) the Web contents cannot imitate the user-
      interfaces of this protocol.

   o  For TLS communications, when a web form is submitted from PAKE-
      authenticated pages with the validation methods of "tls-cert" to



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      an URI which is protected by the same realm (so indicated by the
      path parameter), if server certificate has been changed since the
      pages has been received, the peer is RECOMMENDED to be revalidated
      using a req-A1 message with an "Expect: continue" header.  The
      same applies when the page is received with the validation methods
      of "tls-key", and when the TLS session has been expired.

   o  Server-side storage of the user passwords is RECOMMENDED to have
      the values encrypted by one-way function J(pi), instead of the
      real passwords, those hashed by ph, or pi.


13.  Notice on intellectual properties

   The National Institute of Advanced Industrial Science and Technology
   (AIST) and Yahoo!  Japan, Inc. has jointly submitted a patent
   application about the protocol proposed in this documentation to the
   Patent Office of Japan.  The patent is intended to be open to any
   implementors of this protocol and its variants under non-exclusive
   royalty-free manner without any contracts with the patent owners once
   the protocol is accepted as a Internet standard.  For the detail of
   the patent application, contact the author of this document.

   The elliptic-curve based authentication algorithms might involve
   several existing patents of third-parties.  The authors of the
   document take no position regarding the validity or scope of such
   patents, and other patents as well.


14.  References

14.1.  Normative References

   [ISO-11770-4]
              International Organization for Standardization, "ISO/IEC
              11770-4:2006: Information technology - Security techniques
              - Key management - Part 4: Mechanisms based on weak
              secrets", 2006.

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

   [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
              Diffie-Hellman groups for Internet Key Exchange (IKE)",
              RFC 3526, May 2003.






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14.2.  Informative References

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [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.

URIs

   [1]  <ISO-11770-4>


Authors' Addresses

   Yutaka Oiwa
   National Institute of Advanced Industrial Science and Technology
   Research Center for Information Security
   Akihabara Daibiru #1102
   1-18-13 Sotokanda
   Chiyoda-ku, Tokyo
   JP

   Phone: +81 3-5298-4722
   Email: y.oiwa@aist.go.jp


   Hajime Watanabe
   National Institute of Advanced Industrial Science and Technology


   Hiromitsu Takagi
   National Institute of Advanced Industrial Science and Technology


   Hirofumi Suzuki
   Yahoo! Japan, Inc.
   Roppongi Hills Mori Tower
   6-10-1 Roppongi
   Minato-ku, Tokyo
   JP

   Phone: +81 3-6440-6290





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Full Copyright Statement

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   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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