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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 22, 2008                                         RCIS, AIST
                                                               H. Suzuki
                                                            Yahoo! Japan
                                                       November 19, 2007


                Mutual Authentication Protocol for HTTP
                     draft-oiwa-http-mutualauth-01

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 22, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document specifies 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 a user has been authenticated
   against an illegitimate server, the server cannot gain any bit of
   information about user's passwords.  The protocol is designed as an
   extension to the HTTP protocol, and the protocol design intends to
   replace existing authentication mechanism such as Basic/Digest access
   authentications and form-based authentications.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Tokens and Extensive-tokens  . . . . . . . . . . . . . . .  6
     3.2.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Strings  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  401-B0 . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  401-B0-stale . . . . . . . . . . . . . . . . . . . . . . .  9
     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  . . . . . . . . . . . . . . 12
   6.  Authentication Algorithms  . . . . . . . . . . . . . . . . . . 14
   7.  Validation Methods . . . . . . . . . . . . . . . . . . . . . . 16
   8.  Session Management . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Extension 1: Optional Mutual Authentication  . . . . . . . . . 17
   10. Methods to extend this protocol  . . . . . . . . . . . . . . . 18
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     12.1. General Assumptions  . . . . . . . . . . . . . . . . . . . 19
     12.2. Implementation Considerations  . . . . . . . . . . . . . . 19
   13. Notice on intellectual properties  . . . . . . . . . . . . . . 20
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     14.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23








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1.  Introduction

   This document specifies 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 [RFC2617], 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 a user has been
   authenticated against an illegitimate server, the server cannot 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 cannot 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 uses a mechanism in ISO/IEC
   11770-4 [ISO.11770-4.2006], 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 HTTP protocol [RFC2616].

   The design also considers to replace current form-based Web
   authentication, which is very vulnerable against phishing attacks.
   To this purpose, several extensions to current  HTTP authentication
   mechanism [RFC2617] are introduced.






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1.1.  Requirements Language

   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.  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.




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      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 succeeded, the server SHOULD calculate o_B,
      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 validate 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
      MUST 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 MAY 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], plus 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: the BNF syntax for the headers used in the protocol

3.1.  Tokens and Extensive-tokens

   The tokens MUST be interpreted case-insensitive, and SHOULD be sent
   in the same case as shown in the specification.  When these are used
   as (partial) inputs to any hash or other mathematical functions, it
   MUST be used in lower-case.  All hex-fixed-number or hex-integer
   numbers are also 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, these lengths SHOULD



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   correspond to the hash size; if these are representing elements of a
   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 SHALL 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 encoded by
   the Base 64 encoding [RFC3548], and then enclosed by two double-
   quotations.

3.3.  Strings

   All strings outside ASCII or equivalent character sets SHOULD be
   encoded using UTF-8 encoding [RFC3629] of the ISO 10646-1 character
   set [ISO.10646-1.1993].  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
   without leading BOM, 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
   defined in [RFC3492] 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

   In this section, formats and requirements of the headers for each
   message are presented.  The allowed type for values for each header
   field is shown in parenthesis after the key names.

   Note: The term "optional" here means that omitting the field is
   allowed and has specific meanings in communications (i.e. it is not
   generally "OPTIONAL" defined in [RFC2119]).






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4.1.  401-B0

   Every 401-B0 message SHALL be a 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 SHALL contain the fields with the following keys:

   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.

   pwd-hash:      (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.






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   stale:         (token) MUST be "0".

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

   The algorithm will determine the types and 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.

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

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

4.3.  req-A1

   Every req-A1 message SHALL 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 SHALL 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 SHALL be a 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




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   The header SHALL 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 SHOULD be treated as
                  independent ones.

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

   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 will accept.  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 is RECOMMENDED to be at least 60.  The value of
                  this field is not directly linked to the duration that
                  S keeps track of the session represented by sid.

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

4.5.  req-A3

   Every req-A3 message SHALL 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:






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   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 value of nc
                  SHOULD 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
                     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 SHALL be a 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 current 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-ins) should be removed.  If a new value
                  of timeout is received for the same authentication



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                  realm, it overrides the previous timeout.


5.  Decision procedure for the client

   To securely implement 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
   the security, they should not alter this.)

   Step 1   If the client software needs 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.

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




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   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.  Otherwise, go to Step 9.

   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 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 ask user a
            username and password.  If the user has input those, go to
            Step 9.





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   Step 13  This case means that in some reason the authentication
            failed: possibly the password or the 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 (contents and other content-related
   headers) sent from the server.

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


6.  Authentication Algorithms

   This document specifies only one family of the authentication
   algorithm.  The family consists 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 are advised to support all of the above four algorithms whenever
   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 Agreement Mechanism 3 (KAM3) defined in
   Section 6.3 of ISO/IEC-11770-4 [ISO.11770-4.2006] 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 [FIPS.186-2.2000] specification.  The hash functions H



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   are SHA-256 for P-256 curve and SHA-512 for P-521 curve,
   respectively, defined in FIPS PUB 180-2 [FIPS.180-2.2002].  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 groups defined in [RFC3526] respectively.  The hash
   functions H are SHA-256 for the 2048-bit field and SHA-512 for the
   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
   context of an octet string, it SHALL have the natural length derived
   from the size of the output of function H (e.g. 32 octets for SHA-
   256).  The function ph is defined by the value of the pwd-hash field
   given in a 401-B0 message.

   The function VI encodes natural numbers into octet strings in the
   following manner: integers are represented in big-endian radix-128
   string, where each digit is represented by a octet 0x80-0xff except
   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.  This
   encoding is the same as the one used in the length field in the ASN.1
   encoding [ITU.X690.1994].

   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.

   The equations for J, w_A, T, z, K_i, and w_B are those specified for
   KAM3 in [ISO.11770-4.2006], given that pi is derived in the above
   equation.

   The values o_A (o_A') and o_B (o_B') are derived by the following
   equation, instead of ones specified in [ISO.11770-4.2006].

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



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   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 method" specifies a method to "relate" the mutual
   authentication processed by this protocol with other authentications
   already performed in the underlying layers and to prevent man-in-the-
   middle attacks.  It decides the value of v which is an input to
   authentication protocols.

   The valid tokens for the validation field and corresponding values of
   v are 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 does
                  not have a port part, v will include the one.

   tls-cert:      TLS certificate validation: v will be the octet string
                  of the fingerprint of the public key certificate used
                  in underlying TLS/SSL [RFC4346] connection.

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

   If the HTTP protocol is used on unencrypted channel, the validation
   type MUST be "host".  If HTTP/TLS [RFC2818] (https) protocol is used
   with server certificates, the validation type MUST be either "tls-
   cert" or "tls-key".  If HTTP/TLS 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, the 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 servers MUST use and be
   validated by the Mutual authentication protocol, regardless of the



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   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
   session, given that the request reaches the server in a time window
   specified by the timeout field in the 401-B1 message, and that there
   are no emergent reasons (such as flooding attacks) to forget the
   sessions.  After that, the server MAY discard any session at any time
   and MAY send 401-B0-stale messages for any req-A3 requests.

   The client MAY send more than one 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.

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

   Values of nonces and nonce-related values MUST always be treated as
   natural numbers 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 which cannot be represented
   in the fixed-width integer representations, as long as other limits
   such as internal header-length restrictions are not involved.  The
   protocol is designed carefully so that both clients and servers can
   implement the protocol only with fixed-width integers, by rounding
   any overflowed values to 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.  The support for this



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   extension is RECOMMENDED.

   Servers MAY send HTTP successful responses (response code 200, 206
   and others) containing the Optional-WWW-Authenticate header, when it
   is allowed to send 401-B0 responses and the requests do not contain
   Authentication-Info: headers.  Such responses are hereafter called
   200-Optional-B0 responses.

      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.  In terms of the state management in Section 5, 200-
   Optional-B0 responses are treated as if it is 401-B0 response: these
   messages 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 expects.

   Servers requesting optional mutual authentication SHOULD send the
   path field in 401-B1 messages with an appropriate value.  Client
   software supporting optional mutual authentication MUST recognize the
   field, and MUST send either req-A1 or req-A3 request for the URI
   space inside the specified paths, instead of unauthenticated
   requests.


10.  Methods to extend this protocol

   If a non-standard extension to the this protocol is implemented, it
   MUST 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.  If those specifications
   use keys other than shown above, it is RECOMMENDED to use extension-
   tokens to avoid any key-name conflict with the future extension of
   this protocol.




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11.  IANA Considerations

   The tokens used for authentication-algorithm, pwd-hash, and
   validation fields MUST be allocated by IANA.  To acquire registered
   token, IESG Approval outlined in [RFC2434] is required.  Extension-
   tokens MAY be freely used for any non-standard, private and/or
   experimental uses for those fields provided that the domain part in
   the token is appropriately used.


12.  Security Considerations

12.1.  General Assumptions

   o  The protocol, by itself, does not assure any secrecy and relies on
      transport security including DNS security.  HTTP/TLS SHOULD be
      used where transport security is not assured and data secrecy is
      important.  The protocol is secure against passive eavesdropping
      and replay attacks, though.

   o  When used with HTTP/TLS, the protocol gives true protection
      against active man-in-the-middle attacks for each HTTP request/
      response pair, even when the server certificate is not used or is
      unreliable.  However, in such cases, JavaScript or similar
      scripting facilities can be used to affect Mutually-authenticated
      contents from those not protected by this authentication
      mechanism.  This is why this memo requires that sane TLS server
      certificates MUST be presented.

12.2.  Implementation 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 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 other existing authentication methods such as Basic
      and Digest methods, and (2) the Web contents cannot imitate the
      user-interfaces of this protocol.

      An informational memo regarding user-interface considerations and
      recommendations for implementing this protocol will be separately
      published.



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   o  For HTTP/TLS communications, when a web form is submitted from
      Mutually-authenticated pages with the validation methods of "tls-
      cert" to a URI which is protected by the same realm (so indicated
      by the path field), 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 storages of user passwords are advised 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 once the protocol is accepted as an 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

   [FIPS.180-2.2002]
              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002, <http://
              csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.

   [FIPS.186-2.2000]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-2, January 2000, <
              http://csrc.nist.gov/publications/fips/fips186-2/
              fips186-2-change1.pdf>.

   [ISO.10646-1.1993]



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              International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Part 1: Architecture and Basic
              Multilingual Plane", ISO Standard 10646-1, May 1993.

   [ISO.11770-4.2006]
              International Organization for Standardization,
              "Information technology - Security techniques - Key
              management - Part 4: Mechanisms based on weak secrets",
              ISO Standard 11770-4, May 2006.

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

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

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

   [RFC3548]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

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

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

14.2.  Informative References

   [ITU.X690.1994]
              International Telecommunications Union, "Information
              Technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T Recommendation
              X.690, 1994.

   [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



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              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.


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: mutual-auth-contact@m.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
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Acknowledgment

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





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