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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: February 24, 2011                                    RCIS, AIST
                                                                 Y. Ioku
                                                            Yahoo! Japan
                                                              T. Hayashi
                                                                 Lepidum
                                                         August 23, 2010


                Mutual Authentication Protocol for HTTP
                     draft-oiwa-http-mutualauth-07

Abstract

   This document specifies a mutual authentication method for Hyper-Text
   Transport Protocol (HTTP).  This method provides true mutual
   authentication between an HTTP client and an HTTP server using
   password-based authentication.  Unlike the Basic and Digest
   authentication methods, the Mutual authentication method specified in
   this document assures the user that the server truly knows the user's
   encrypted password.  This prevents common phishing attacks: a
   phishing attacker controlling a fake website cannot convince a user
   that he authenticated to the genuine website.  Furthermore, even when
   a user authenticates to an illegitimate server, the server cannot
   gain any information about the user's password.  The Mutual
   authentication method is designed as an extension to the HTTP
   protocol, and is intended to replace existing authentication methods
   used in HTTP (the Basic method, Digest method, and authentication
   using HTML forms).

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 February 24, 2011.



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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Document Structure Overview  . . . . . . . . . . . . . . .  5
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Messages . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Typical Flows of the protocol  . . . . . . . . . . . . . .  7
     2.3.  Alternative flows  . . . . . . . . . . . . . . . . . . . .  9
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Tokens and Extensive-tokens  . . . . . . . . . . . . . . . 12
     3.2.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.3.  Strings  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  401-B0 . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.2.  401-B0-stale . . . . . . . . . . . . . . . . . . . . . . . 15
     4.3.  req-A1 . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.4.  401-B1 . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.5.  req-A3 . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.6.  200-B4 . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.7.  200-Optional-B0  . . . . . . . . . . . . . . . . . . . . . 20
   5.  Authentication Realms  . . . . . . . . . . . . . . . . . . . . 21
     5.1.  Resolving ambiguities  . . . . . . . . . . . . . . . . . . 22
   6.  Session Management . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Validation Methods . . . . . . . . . . . . . . . . . . . . . . 25
   8.  Decision procedure for the client  . . . . . . . . . . . . . . 26
   9.  Decision procedure for the server  . . . . . . . . . . . . . . 31
   10. Authentication-Control header  . . . . . . . . . . . . . . . . 33
     10.1. Location-when-unauthenticated field  . . . . . . . . . . . 34
     10.2. Location-when-logout field . . . . . . . . . . . . . . . . 34
     10.3. Logout-timeout . . . . . . . . . . . . . . . . . . . . . . 35
   11. Authentication Algorithms  . . . . . . . . . . . . . . . . . . 35



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     11.1. Support functions and notations  . . . . . . . . . . . . . 36
     11.2. Common functions for both settings . . . . . . . . . . . . 37
     11.3. Functions for discrete-logarithm settings  . . . . . . . . 37
     11.4. Functions for elliptic-curve settings  . . . . . . . . . . 38
   12. Methods to extend this protocol  . . . . . . . . . . . . . . . 40
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 40
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 40
     14.1. Security Properties  . . . . . . . . . . . . . . . . . . . 40
     14.2. Denial-of-service attacks to servers . . . . . . . . . . . 41
     14.3. Implementation Considerations  . . . . . . . . . . . . . . 41
     14.4. Usage Considerations . . . . . . . . . . . . . . . . . . . 42
   15. Notice on intellectual properties  . . . . . . . . . . . . . . 43
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 43
     16.2. Informative References . . . . . . . . . . . . . . . . . . 44
   Appendix A.  (Informative) Generic syntax of headers . . . . . . . 45
   Appendix B.  (Informative) Group parameters for
                discrete-logarithm based algorithms . . . . . . . . . 47
   Appendix C.  (Informative) Derived numerical values  . . . . . . . 49
   Appendix D.  (Informative) Draft Remarks from the Authors  . . . . 50
   Appendix E.  (Informative) Draft Change Log  . . . . . . . . . . . 50
     E.1.  Changes in revision 07 . . . . . . . . . . . . . . . . . . 50
     E.2.  Changes in revision 06 . . . . . . . . . . . . . . . . . . 51
     E.3.  Changes in revision 05 . . . . . . . . . . . . . . . . . . 51
     E.4.  Changes in revision 04 . . . . . . . . . . . . . . . . . . 51
     E.5.  Changes in revision 03 . . . . . . . . . . . . . . . . . . 51
     E.6.  Changes in revision 02 . . . . . . . . . . . . . . . . . . 52
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52























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1.  Introduction

   This document specifies a mutual authentication method for Hyper-Text
   Transport Protocl (HTTP).  The method, called as "Mutual
   Authentication Protocol" in this document, provides provides true
   mutual authentication between an HTTP client and an HTTP server,
   using just a simple password as a credential.

   Currently available methods for authentication in HTTP and Web system
   have several deficiencies.  Basic authentication method [RFC2617]
   sends a plaintext password to a server without any protections;
   Digest method uses a hash function which suffers from simple
   dictionary-based off-line attacks, and people begins to think it
   obsolete.

   The authentication method proposed in this document solves these
   problems, substitutes these existing methods and serves as a long-
   term solution of Web authentications security. it has the following
   main characteristics:

   o  It provides "true" mutual authentication: as well as assuring the
      server that the user knows the password, it also assures the user
      that the server truly knows the user's encrypted password at the
      same time.  It makes impossible for fake website owners to
      persuade users that he authenticated to the original websites.

   o  It uses only a password as a user's credential: unlike public-key-
      based security algorithms, the method does not rely on secret keys
      or other cryptographic data which have to be stored inside users'
      computers.  The method can be used almost as a drop-in replacement
      to the current authentication methods like Basic or Digest, while
      ensuring much stronger security.

   o  It is secure: when the server failed to authentication user, the
      protocol will not reveal any bit of user's password.

   By using the proposed method, users can discriminate between true and
   fake Web servers using their own passwords.  Even when a user inputs
   his/her password to a fake website owned by illegitimate phishers,
   the user will certainly notices that the 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 input sensitive data to the web forms after
   confirming that the mutual authentication has succeeded, without fear
   of phishing attacks.

   The document also proposes several extensions to the current HTTP
   authentication framework, to replace current widely-used form-based



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   Web authentication.  Nowadays, majority of the Web sites on the
   Internet use custom application-layer authentication implementations
   using Web forms.  Reasons of these may vary, but many people consider
   that the current HTTP Basic (and Digest, too) authentication method
   does not have functionality (including a good-feeling user
   interfaces) enough for supporting realistic Web-based applications.
   However, the method is very weak against phishing attacks, because
   the whole behavior of the authentication is controlled from the
   servers' side.  To overcome this problem, we need to "modernize" the
   HTTP authentication framework so that better client-controlled secure
   methods can be used well with Web applications.  The extensions
   proposed in this document include:

   o  multi-host single authentication within an Internet domain
      (Section 5),

   o  non-mandatory, optional authentication on HTTP (Section 4.7),

   o  log out from both server and client side (Section 10), and

   o  finer control for redirection depending on authentication status
      (Section 10).

1.1.  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].

   Terms "encouraged" and "advised" are used for suggestions which do
   not constitute "SHOULD"-level requirements.  People MAY freely choose
   not to include the suggested items regarding [RFC2119], but following
   the suggestion will be a best practice; it will improve security,
   interoperability or operation performance.

   This document distinguishes the terms "client" and "user" in the
   following way: A "client" is an entity understanding and talking HTTP
   and the specified authentication protocol, usually a computer
   software; on the contrary, a "user" is a (usually natural) person who
   want to access data resources using "a client".

   The term "natural numbers" means non-negative integers (including
   zero) throughout this document.

1.2.  Document Structure Overview

   The whole document is organized as follows:




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   o  Section 2 gives an overview presentation of the protocol design.

   o  The Sections from 3 to 9 define a general framework of the Mutual
      authentication protocol.  This framework is independent from
      specific cryptographic primitives.

   o  Section 10 defines an optional extension to the the generic HTTP
      authentication framework, which is useful mostly to control the
      Web browser behavior of the authentication.

   o  Section 11 defines a few specific cryptographic algorithms to be
      used with this authentication framework.

   o  The sections after that contain general normative and informative
      information about the protocol.

   o  Appendices contain some information which may help developers to
      implement the protocol.


2.  Protocol Overview

   The protocol, as a whole, is designed as a natural extension to the
   HTTP protocol [RFC2616] using a framework defined in [RFC2617].
   Internally, the server and the client will first perform a
   cryptographic key exchange, using the secret password as a "tweak" to
   the exchange.  The key-exchange will only succeed when the secrets
   used by the both peer are correctly related (i.e. generated from the
   same password).  Then the both peer will verify the authentication
   results by checking the sharing of the exchanged key.  This section
   describes the brief image of the protocol and the exchanged messages.

2.1.  Messages

   The authentication protocol uses seven kinds of messages to perform
   mutual authentication.  These messages have a specific names within
   this specification.

   o  Authentication request messages: used by the servers to request
      clients to start mutual authentication.

      *  401-B0 message: a general message for start authentication
         protocol.  It is also used as a message indicating an
         authentication failure.

      *  200-Optional-B0 message: a variant of 401-B0 message indicating
         that an authentication is not mandatory.




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      *  401-B0-stale message: a message indicating that it has to start
         a new authentication trial.

   o  Authenticated key exchange messages: used by both peers to perform
      authentication and shares a cryptographic secret.

      *  req-A1 message: a message sent from the client.

      *  401-B1 message: a message sent from the server as a response to
         req-A1 message.

   o  Authentication verification messages: used by both peers to verify
      authentication results.

      *  req-A3 message: a message used by the client, requesting that
         the server authenticates and authorizes the client.

      *  200-B4 message: a successful response used by the server, also
         asserting that the server is authentic to the client at the
         same time.

   In addition to above, either a request or a response without any HTTP
   headers related to this specification will be hereafter called a
   "normal request" or a "normal response", respectively.

2.2.  Typical Flows of the protocol

   In the typical case, a first client access to a resource protected by
   the Mutual authentication will follow the following protocol
   sequence.





















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          Client                                 Server
            |                                      |
            |  ---- (1) normal request --------->  |
        GET / HTTP/1.1                             |
            |                                      |
            |  <------------------ (2) 401-B0 ---  |
            |            401 Authentication Required
            |            WWW-Authenticate: Mutual realm="a realm"
            |                                      |
   [user,   |                                      |
    pass]-->|                                      |
            |  ---- (3) req-A1 ----------------->  |
        GET / HTTP/1.1                             |
        Authorization: Mutual user="john",         |--> [user DB]
                       wa="...", ...               |<-- [user info]
            |                                      |
            |  <------------------ (4) 401-B1 ---  |
            |            401 Authentication Required
            |            WWW-Authenticate: Mutual sid=..., wb="...", ...
            |                                      |
        [compute] (5) compute session secret   [compute]
            |                                      |
            |                                      |
            |  ---- (6) req-A3 ----------------->  |
        GET / HTTP/1.1                             |--> [verify (6)]
        Authorization: Mutual sid=...,             |<-- OK
                       oa="...", ...               |
            |                                      |
            |  <------------------ (7) 200-B4 ---  |
   [verify  |            200 OK                    |
     (7)]<--|            Authentication-Info: Mutual ob="..."
            |                                      |
            v                                      v

   Figure 1: Typical communication flow on the first access to resource

   o  As usual in general HTTP protocol design, a client will first
      request a resource without any authentication attempt (1).  If the
      requested resource is protected by the Mutual authentication, The
      server will respond with a message requesting authentication
      (401-B0) (2).

   o  The client processes the body of the message, and wait a user for
      inputing a user name and a password.  If the user name and a
      password is available, The client will send a message with
      authenticated key exchange (req-A1) to start authentication (3).





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   o  If the server has received a req-A1 message, The server looks up
      its user database for the user's authentication information.  Then
      the server creates a new session identifier (sid) which will be
      used to identify sets of following messages, and responds back a
      message with a server-side authenticated key exchange
      value(401-B1) (4).

   o  At this point (5), both peers calculate a shared "session secret"
      using the exchanged values in key exchange messages.  Only when
      both the server and the client have used secret credentials
      generated from the same password, the session secret values will
      match.  This session secret will be used for the actual access
      authentication after this point.

   o  The client will send a request with a client-side authentication
      challenge (req-A3) (6), generated from the client-own session
      secret.  The server will check the validity of the challenge using
      its own session secret.

   o  If the challenge from the client was correct, it means that the
      client certainly owns a credential based on the expected password
      (i.e. the client authentication succeeded.)  The server will
      respond with a successful message (200-B4) (7).  On the contrary
      to the usual one-way authentication (e.g.  HTTP Basic
      authentication or POP APOP authentication), This message also
      contains a server-side authentication challenge.

      When the client's challenge was incorrect (e.g. because the user-
      supplied password was incorrect), the server will respond with the
      401-B0 message (used in (2)) instead.

   o  The client MUST first check validity of the server-side
      authentication challenge contained in the message (7).  If the
      challenge was equal to the expected value, the server
      authentication succeeded.

      If it is not the value expected, or if the message does not
      contain authentication challenge value, it means that the mutual
      authentication has been broken for some unexpected reasons.  The
      client MUST NOT process any body and header values contained in
      this case.  (Note: This case should not happen between a
      correctly-implemented server and a client.)

2.3.  Alternative flows

   As shown above, the typical flow for first authenticated request
   requires three request-response pairs.  To reduce the protocol
   overhead, the protocol enables several short-cut flows which requires



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   fewer messages.

   o  (case A) If the client knows that the resource is likely to
      require the authentication, the client MAY omit first
      unauthenticated request (1) and send req-A1 message immediately.
      This will reduce one round-trip of messages.

   o  (case B) If both the client and the server previously shared a
      session secret associated with a valid session identifier (sid),
      the client MAY directly send a req-A3 message using existing sid
      and corresponding session secret.  This will further reduce one
      round-trip of the messages.

      In such cases, the server MAY have been thrown out the
      corresponding sessions from the session table.  In this case, the
      server will send a 401-B0-stale message as a response to req-A3
      message, indicating a new key exchange is required.  The client
      SHOULD retry from constructing a req-A1 message in this case.

   Figure 2 depicts the shortcut flows described above.  Under
   appropriate setting and implementations, most of the requests to
   resources are expected to meet both criteria, and thus only one
   round-trip of request/response will be required for most cases.




























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       (A) omit first request
          (2 round trips)

        Client        Server
        |                  |
        | --- req-A1 ----> |
        |                  |
        | <---- 401-B1 --- |
        |                  |
        | --- req-A3 ----> |
        |                  |
        | <---- 200-B4 --- |
        |                  |


       (B) reusing session secret

         (B-1) key available      (B-2) key expired
                 (1 round trip)           (3 round trips)

        Client        Server     Client              Server
        |                  |     |                        |
        | --- req-A3 ----> |     | --- req-A3 ----------> |
        |                  |     |                        |
        | <---- 200-B4 --- |     | <---- 401-B0-stale --- |
        |                  |     |                        |
                                 | --- req-A1 ----------> |
                                 |                        |
                                 | <---------- 401-B1 --- |
                                 |                        |
                                 | --- req-A3 ----------> |
                                 |                        |
                                 | <---------- 200-B4 --- |
                                 |                        |

            Figure 2: Several alternative flows on the protocol

   For more details, see Section 8 and Section 9.


3.  Message Syntax

   The Mutual authentication protocol uses five headers:
   WWW-Authenticate (in responses with status code 401),
   Optional-WWW-Authenticate (in responses with non-401 status codes),
   Authentication-Control (in responses), Authorization (in requests),
   and Authentication-Info (in responses other than 401 status).  These
   headers follow a common framework of the one described in [RFC2617]



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   [Editorial Note: to be httpbis-p7].  The detailed syntax definitions
   for these headers are contained in Section 4.

   These headers use some common syntax elements described in Figure 3.
   The syntax is denoted in the augmented BNF syntax defined in
   [RFC5234].

    auth-scheme      = "Mutual"             ; see HTTP for other values
    extension-field  = extension-token "=" value
    token            = 1*(%x30-39 / %x41-5A / %x61-7A / "-" / "_")
    extensive-token  = token / extension-token
    extension-token  = "-" token 1*("." token)
    value            = extensive-token / integer
                     / hex-fixed-number
                     / base64-fixed-number / string
    integer          = "0" / (%x31-39 *%x30-39)      ; no leading zeros
    hex-fixed-number = 1*(%x30-39 / %x41-46 / %x61-66)
    base64-fixed-number = string
    string           = %x22 *(%x20-21 / %x23-5B / %x5D-FF
                              / %x5C.22 / "\\") %x22
    spaces           = 1*(" " / %x09)

   Figure 3: the BNF syntax for the common elements used in the protocol

3.1.  Tokens and Extensive-tokens

   The tokens are case insensitive; Senders should SHOULD send these in
   lower-case, and receivers MUST accept both upper- and lower-cases.
   When tokens are used as (partial) inputs to any hash or other
   mathematical functions, it MUST always be used in lower-case.  All
   hexadecimal numbers are also case-insensitive, and SHOULD be sent in
   lower-case.

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

3.2.  Numbers

   The syntax definition of integers only allows representations which
   do not contain extra leading zeros.

   The numbers represented as a hex-fixed-number MUST have even number
   of characters (i.e. multiple of eight bits).  When these are
   generated from cryptographic values, those SHOULD have their "natural



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   length": if these are generated from a hash function, these lengths
   SHOULD correspond to the hash size; if these are representing
   elements of a mathematical set (or group), its lengths SHOULD be the
   shortest which can represent all elements in the set.  See Appendix C
   for information about the length of the fields used in this
   specification.  Session-identifiers and other non-cryptographically
   generated values 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.

   Numbers represented as 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 [RFC4648] without any spaces and newlines, and then enclosed
   by two double-quotations.

3.3.  Strings

   All strings outside ASCII or equivalent character sets MUST be
   encoded using UTF-8 encoding [RFC3629] of the ISO 10646-1 character
   set [ISO.10646-1.1993].  Both peers are RECOMMENDED to reject any
   invalid UTF-8 sequences which cause decoding ambiguities (e.g.
   containing <"> in the second or later byte of the UTF-8 encoded
   characters).

   To encode character strings to header values, 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.  These escaping backslashes and
   enclosing quotes SHALL be removed before any processing other than
   using them in header fields.

   If strings are representing a domain name or URI which contains non-
   ASCII characters, host parts SHOULD be encoded as it is used in the
   HTTP protocol layer (e.g. in a Host: header); in current standard it
   will be the one defined in [RFC5890].  It SHOULD use lower-case ASCII
   characters.

   For base64-fixed-numbers, which use the string syntax, see the
   previous section.


4.  Messages

   In this section we define seven kinds of messages used in the
   authentication protocol, Along with formats and requirements of the
   headers for each message.



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   To determine which message are expected to be sent, see Section 8 and
   Section 9.

   In the descriptions below, the allowed type of values for each header
   field is shown in parenthesis after the key names.  The type
   "algorithm-determined" means that the acceptable value type for the
   field is one of the types defined in Section 3, and is determined by
   the value of the "algorithm" field.  The fields marked as "mandatory"
   SHALL be contained in the message.  The fields marked as "non-
   mandatory" MAY either be contained or omitted in the message.

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, version=-draft07

    header-401-B0 = "WWW-Authenticate" ":" [spaces]
                    auth-scheme spaces fields-401-B0
    fields-401-B0 = field-401-B0 *([spaces] "," spaces field-401-B0)
    field-401-B0  = version / algorithm / validation
                  / auth-domain / realm / pwd-hash / stale
                  / extension-field
    version       = "version"     "=" extensive-token
    algorithm     = "algorithm"   "=" extensive-token
    validation    = "validation"  "=" extensive-token
    auth-domain   = "auth-domain" "=" string
    realm         = "realm" "=" string
    pwd-hash      = "pwd-hash" "=" extensive-token
    stale         = token

         Figure 4: the BNF syntax for the header in 401-B0 header

   The header SHALL contain all of the fields marked as "mandatory"
   below, and MAY contain those marked as "non-mandatory".

   version:       (mandatory extensive-token) should be the token
                  "-draft07" in this specification.  The behavior when
                  other values are specified is undefined.

   algorithm:     (mandatory extensive-token) specifies the
                  authentication algorithm to be used.  The value MUST
                  be one of the tokens described in Section 11, or the
                  tokens specified in other supplemental specification



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                  documentations.

   validation:    (mandatory 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.

   auth-domain:   (non-mandatory string) specifies authentication
                  domain, the set of hosts on which authentication
                  credentials are valid.  It MUST be one of the strings
                  described in Section 5.  If the value is omitted, it
                  is assumed to be the host part of the requested URI.

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

   pwd-hash:      (non-mandatory extensive-token) specifies the hash
                  algorithm (hereafter 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) = MD5(username | ":" | realm |
                     ":" | p), the same value as MD5(A1) for "MD5"
                     algorithm in [RFC2617].

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

                  If omitted, the value "none" is assumed.  The use of
                  "none" is recommended.

   stale:         (mandatory token) MUST be "0".

   The algorithm specified in this header 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, version=-draft07



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   The header MUST contain the same fields as in 401-B0, except that
   stale field holds the token 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, realm="xxxx",
   user="xxxx", wa=xxxx, version=-draft07

    header-req-A1 = "Authorization" ":" [spaces]
                    auth-scheme spaces fields-req-A1
    fields-req-A1 = field-req-A1 *([spaces] "," spaces field-req-A1)
    field-req-A1  = version / algorithm / validation
                  / auth-domain / realm / user / wa
                  / extension-field
    user          = "user" "=" string
    wa            = "wa"   "=" value

         Figure 5: the BNF syntax for the header in req-A1 message

   The header SHALL contain the fields with the following keys:

   version:       (mandatory, extensive-token) should be the token
                  "-draft07" in this specification.  The behavior when
                  other values are specified is undefined.

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

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

   wa:            (mandatory, algorithm-determined) is the client-side
                  key exchange value w_A, which is specified by the used
                  algorithm (see Section 11).

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 algorithm=xxxx, validation=xxxx,
   realm="xxxx", sid=xxxx, wb=xxxx, nc-max=x, nc-window=x, time=x,
   path="xxxx", version=-draft07




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    header-401-B1 = "WWW-Authenticate" ":" [spaces]
                    auth-scheme spaces fields-401-B1
    fields-401-B1 = field-401-B1 *([spaces] "," spaces field-401-B1)
    field-401-B1  = version / algorithm / validation
                  / auth-domain / realm / sid / wb
                  / nc-max / nc-window / time / path
                  / extension-field
    sid           = "sid"       "=" string
    wb            = "wb"        "=" value
    nc-max        = "nc-max"    "=" integer
    nc-window     = "nc-window" "=" integer
    time          = "time"      "=" integer
    path          = "path"      "=" string

         Figure 6: the BNF syntax for the header in 401-B1 message

   The header SHALL contain the fields with the following keys:

   version:       (mandatory, extensive-token) should be the token
                  "-draft07" in this specification.  The behavior when
                  other values are specified is undefined.

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

   sid:           (mandatory, hex-fixed-number) MUST be a session
                  identifier, 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:            (mandatory, algorithm-determined) is the server-side
                  key exchange value w_B, which is specified by the
                  algorithm (see Section 11).

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

   nc-window:     (mandatory, integer) the number of available nonce
                  slots which the server will accept.  The value of
                  nc-window is RECOMMENDED to be 32 or more.







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

   path:          (non-mandatory, string) specifies for which path in
                  the URI space the same authentication is expected to
                  apply.  The value is a space-separated list of URIs,
                  in the same format as it is specified in domain
                  parameter [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, clients 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 algorithm=xxxx, validation=xxxx, realm="xxxx",
   sid=xxxx, nc=x, oa=xxxx, version=-draft07

    header-req-A3 = "Authorization" ":" [spaces]
                    auth-scheme spaces fields-req-A3
    fields-req-A3 = field-req-A3 *([spaces] "," spaces field-req-A3)
    field-req-A3  = version / algorithm / validation
                  / auth-domain / realm / sid / nc / oa
                  / extension-field
    nc            = "nc" "=" integer
    oa            = "oa" "=" value

         Figure 7: the BNF syntax for the header in req-A3 message

   The fields contained in the header are as follows:

   version:       (mandatory, extensive-token) should be the token
                  "-draft07" in this specification.  The behavior when
                  other values are specified is undefined.

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







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   sid:           (mandatory, hex-fixed-number) MUST be one of the sid
                  values which has been received from the server for the
                  same authentication realm.

   nc:            (mandatory, integer) is a nonce value which is unique
                  among the requests sharing the same sid.  Values of
                  nonces SHOULD satisfy the properties outlined in
                  Section 6.

   oa:            (mandatory, algorithm-determined) is the client-side
                  authentication challenge value o_A, which is specified
                  by the algorithm (see Section 11).

4.6.  200-B4

   Every 200-B4 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, version=-draft07

    header-200-B4  = "Authentication-Info" ":" [spaces]
                     auth-scheme spaces fields-200-B4
    fields-200-B4  = field-200-B4 *([spaces] "," spaces field-200-B4)
    field-200-B4   = version / sid / ob / logout-timeout
    ob             = "ob"             "=" value
    logout-timeout = "logout-timeout" "=" integer

         Figure 8: the BNF syntax for the header in 200-B4 message

   The fields contained in the header are as follows:

   version:       (mandatory, extensive-token) should be the token
                  "-draft07" in this specification.  The behavior when
                  other values are specified is undefined.

   sid:           (mandatory, hex-fixed-number) MUST be the value
                  received from the client.

   ob:            (mandatory, algorithm-determined) is the server-side
                  authentication challenge value o_B, which is specified
                  by the algorithm (see Section 11).

   logout-timeout:  (non-mandatory, 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-inputted password to the



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                  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
                  realm, it overrides the previous timeout.

   The header MUST be sent before the content body: it MUST NOT be sent
   in a trailer of a chunked-encoded response.  If a "100 Continue"
   response is sent from the server, The Authentication-Info header
   SHOULD be included in that response, instead of the final response.

4.7.  200-Optional-B0

   The 200-Optional-B0 messages enables a non-mandatory authentication,
   which is not possible under current HTTP authentication mechanism.
   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 HTTP cookies [RFC2965] and
   custom form-based authentications.  The new method of authentication
   using this message will provide a replacement for those
   authentication systems.  The support for this message is RECOMMENDED,
   unless the protocol is used for some specific applications in which
   authentication is always mandatory.

   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 (with one exception described
   below).  Such responses are hereafter called 200-Optional-B0
   responses.

   HTTP/1.1 200 OK
   Optional-WWW-Authenticate: Mutual version=-draft07, algorithm=xxxx,
   validation=xxxx, realm="xxxx", stale=0

    header-200-Optional-B0 = "Optional-WWW-Authenticate" ":" [spaces]
                             auth-scheme spaces fields-401-B0

     Figure 9: the BNF syntax for the header in 200-Optional-B0 header

   The fields contained in the Optional-WWW-Authenticate header is the
   same as the 401-B0 message described in Section 4.1.  For
   authentication-related matters, a 200-Optional-B0 message will have
   the same meaning as a 401-B0 message with a corresponding
   WWW-Authenticate header.  (The behavior for other matters, such as
   caching, MAY be different between 200-Optional-B0 and 401-B0
   messages.)



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   The 200-Optional-B0 message is a only place where an
   Optional-WWW-Authenticate header is allowed.  If a server is to send
   a 401-B1 or a 401-B0-stale responses, it SHALL NOT replace it with
   200-Optional-B0 or similar responses.  Furthermore, if a server is
   going to send a 401-B0 message as a responses to req-A3 message with
   a correct realm, the server MUST send a 401-B0 message, not a
   200-Optional-B0 message.

   Servers requesting non-mandatory authentication SHOULD send the path
   field in 401-B1 messages with an appropriate value.  Clients
   supporting non-mandatory 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 a normal request without an
   Authorization header.


5.  Authentication Realms

   In this protocol, an "authentication realm" is defined as a set of
   resources (URIs) for which the same set of user names and passwords
   is valid for.  If the server requests authentication for the
   authentication realm which the client is already authenticated, the
   client will automatically perform authentication using the already-
   known secrets.  On the contrary, for the different authentication
   realms, clients SHOULD NOT automatically reuse the usernames and
   passwords for another realm.

   Just like Basic and Digest access authentication protocol, Mutual
   authentication protocol supports multiple, separate authentication
   realms to be set up inside each host.  Furthermore, the protocol
   supports that a single authentication realm spans over several hosts
   in the same Internet domain.

   Each authentication realm is defined and distinguished by the triple
   of an "authentication algorithm", an "authentication domain", and a
   "realm" parameter.  Server operators are NOT RECOMMENDED to use the
   same pair of an authentication domain and a realm for different
   authentication algorithms, however.

   Authentication algorithms are defined in Section 4 and Section 11.
   The realm parameter is a string as defined in Section 4.
   Authentication domains are described in the rest of this section.

   An authentication domain specifies the range of hosts which the
   authentication realm spans over.  In the protocol, it MUST be one of
   the following strings.





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   o  The string in format "<scheme>://<host>:<port>", where scheme,
      host and port are the URI parts of the requested URI.  Even if the
      request-URI does not have a port part, the string will include the
      one (i.e. 80 for http and 443 for https).  Use this when
      authentication is only valid for specific protocol (such as
      https).

   o  The "host" part of the requested URI.  This is the default value.
      Authentication realms in this kind of authentication domain will
      span over several protocols (i.e. http and https) and ports, but
      not over different hosts.

   o  String in format "*.<domain-postfix>", where "domain-postfix" is
      either the host part of the requested URI, or any domain in which
      the requested host is included (this means that the specification
      "*.example.com" is valid for all of hosts "www.example.com",
      "web.example.com", "www.sales.example.com" and "example.com").
      The domain-postfix sent from servers MUST be equal to or included
      in a valid Internet domain assigned to specific organization: if
      clients knows, by some means such as blacklists for HTTP cookies,
      that the specified domain is not to be assigned to any specific
      organization (e.g. "*.com" or "*.jp"), clients are RECOMMENDED to
      reject the authentication request.

   In the above specifications, every "scheme", "host" and "domain" MUST
   be in lower-case, and any internationalized domain names beyond the
   ASCII character set SHALL be represented in the way these are sent in
   the underlying HTTP protocol, represented in lower-case characters;
   i.e.&nbsp these SHALL be in the form of the LDH labels in IDNA
   [RFC5890].  All "port"s MUST be in the shortest, unsigned, decimal
   number notation.  Not obeying these requirements will cause failure
   of valid authentication attempts.

5.1.  Resolving ambiguities

   In the above definition of authentication domains, several domains
   will overwrap with each other.  Depending on the "path" parameters
   given in the "401-B1" message (see Section 4), There may be several
   candidates when the client is to send a request with authentication
   credentials included (at the Steps 3 and 4 of the decision procedure
   shown in Section 8).

   If such choices are required, the following procedure SHOULD be
   followed.

   o  If the client has previously sent a request to the same URI, and
      it remembers the authentication realm requested by 401-B0 messages
      at that time, use that realm.



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   o  In other cases, use one of authentication realms which represents
      most-specific authentication domains.  In the list of possible
      domain specifications shown above, one described earlier has
      priority over ones described after that.

      If there are several choices with different domain-postfix
      specifications, the one which has the longest domain-postfix has
      priority over ones with shorter domain-postfix.

   o  If there are realms with the same specifications of authentication
      domain, there is no defined priority: client MAY choose any one of
      possible choices.

   If possible, server operators are encouraged to avoid such
   ambiguities by setting "path" parameters properly.


6.  Session Management

   In the Mutual authentication protocol, a session represented by an
   sid is set up by the first 4 messages (first request, 401-B0, req-A1
   and 401-B1), and a "session secret" (z) associated to the session is
   established.  After having a session secret, this session, along with
   the secret, can be used for one or more requests for resources
   protected by the same realm in the same server.  Note that the
   session management is only an inside detail of the protocol and
   usually not visible to normal users.  If a session expires, the
   client and server SHOULD automatically reestablish another session
   without telling it to the users.

   The sessions are local to each port of a host inside an
   authentication domain; clients MUST establish separate sessions for
   each port of a host to be accessed.

   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 two or more requests using a single session
   specified by the sid.  However, for all such requests, each value of
   the nonce (in the nc field) MUST satisfy the following conditions:

   o  It is a natural number.





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   o  The same nonce has not yet sent previously in the same session.

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

   o  It is larger 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 the server in the session.

   The last condition allows servers to reject any nonce values which is
   "significantly" smaller than the "current" value (defined by the
   value of nc-window) of the nonce used in the session involved.  In
   other words, servers MAY treat such nonces as "already received".
   This restriction enables servers to implement duplicated nonce
   detection in a constant amount of memory (for each session).

   Servers 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-B0-stale message, as outlined in Section 9.  The
   server MAY also reject other invalid nonce values (such as ones above
   the nc-max limit) by sending a 401-B0-stale message.

   For example, consider the nc-window value of the current session to
   be 32, the nc-max to be 100, and that the client has already used the
   following nonce values beforehand: {1-20, 22, 24, 30-38, 45-60,
   63-72}.  Then the nonce values which can be used for next request is
   one of the following set: {41-44, 61-62, 73-100}.  The values {0, 21,
   23, 25-29, 39-40} MAY be rejected by the server because these are not
   above the current "window limit" (40 = 72 - 32).

   Typically, clients can ensure the above property by using a
   monotonically-increasing integer counter counting from zero upto the
   value of nc-max.

   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.






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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: The value v will be the ASCII
                  string in the following format:
                  "<scheme>://<host>:<port>", where scheme, host and
                  port are the URI components correspond to the
                  currently accessing resource.  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: The value v will be the
                  octet string of the hash value of the public key
                  certificate used in underlying TLS [RFC5246] (or SSL)
                  connection.  The hash value is defined as the value of
                  the whole signed certificate (specified as
                  "Certificate" in [RFC5280]), hashed by the hash
                  algorithm specified by the authentication algorithm
                  used.

   tls-key:       TLS shared-key validation: The value v will be the
                  octet string of the shared master secret negotiated in
                  underlying TLS (or SSL) connection.

   If the HTTP protocol is used on non-encrypted channel (TCP and SCTP,
   for example), 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).

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

   However, when the client is a Web browser with any scripting
   capabilities, underlying TLS channel used with HTTP/TLS MUST provide
   server identity verification.  This means (1) anonymous Diffie-
   Hellman key exchange ciphersuite MUST NOT be used, and (2) the
   verification of server certificate provided from the server MUST be
   employed.



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   For other systems, when underlying TLS channel used with HTTP/TLS
   does not provide server identity verification, the client SHOULD
   ensure that all responses are validated using the Mutual
   authentication protocol, regardless of the existence of the 401-B0
   responses.

   Note: The protocol defines two variants for validation on TLS
   connections.  The method "tls-key" method is more secure.  However,
   there are some situations where tls-cert is more preferable.

   o  When TLS accelerating proxies are used.  In this case, it is
      difficult for the authenticating server to acquire the TLS key
      information which is used between the client and the proxy.  It is
      not the case for client-side "tunneling" proxies using CONNECT
      method extension of HTTP.

   o  When a black-box implementation of the TLS protocol is used on
      either peer.

   Implementations supporting Mutual authentication over HTTPS protocol
   SHOULD support "tls-cert" validation.  Support for "tls-key"
   validation is OPTIONAL for both servers and clients.


8.  Decision procedure for the client

   To securely implement the protocol, the user client must be careful
   for accepting authenticated responses from the server.  This also
   holds upon reception of "normal responses" (responses which do not
   contain Mutual-related headers) from HTTP servers.

   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.)  Especially, clients
   SHOULD NOT accept "normal responses" unless explicitly allowed below.
   The labels on the steps are for informational purpose only.  Entries
   within each step are checked in top-to-bottom order, and the first
   clause satisfied SHOULD be taken.

   Step 1 (step_new_request):
       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 by the Mutual
       authentication scheme.  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 expect.  If there is one, go to Step 3.  Otherwise, go
       to Step 4.

   Step 3 (step_send_a3_1):
       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 200-Optional-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 200-B4 message, go to Step 14.

       *  If you receive a normal response, go to Step 11.

   Step 4 (step_send_a1_1):
       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 200-Optional-B0 message with a different
          authentication realm than expected, go to Step 6.

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

       *  If you receive a 401-B0 message with the same authentication
          realm, go to Step 13 (see Note 1).

       *  If you receive a normal response, go to Step 11.

   Step 5 (step_send_normal_1):
       Send a request without any Mutual authentication headers.

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

       *  If you receive a 200-Optional-B0 message, go to Step 6.

       *  If you receive a normal response, go to Step 11.





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   Step 6 (step_rcvd_b0):
       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 expect.  If there is one, go to Step 8.  Otherwise, go
       to Step 9.

   Step 8 (step_send_a3):
       Send a req-A3 request.

       *  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 200-B4 message, go to Step 14.

   Step 9 (step_send_a1):
       Send a req-A1 request.

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

       *  If you receive a 401-B0 message, go to Step 13 (See Note 1).

   Step 10 (step_rcvd_b1):
       Send a req-A3 request.

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

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

   Step 11 (step_rcvd_normal):
       This case means that the resource requested is out of the
       authenticated area.  The client will be in the "UNAUTHENTICATED"
       status.  If the response contains a request for authentications
       other than Mutual, it MAY be handled normally.

   Step 12 (step_rcvd_b0_unknown):
       This case means that the resource requested requires Mutual
       authentication, and the user is not authenticated yet.  The
       client will be in the "AUTH_REQUESTED" status, is RECOMMENDED to
       process the content sent from the server and ask user a username
       and a password.  If the user has inputted those, go to Step 9.






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   Step 13 (step_rcvd_b0_failed):
       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 (step_rcvd_b4):
       Check the validity of the received o_b value.  If it is equal to
       the expected value, it means that the mutual authentication has
       been succeeded.  The client will be in the "AUTH_SUCCEEDED"
       status.

       If the value is unexpected, it is a fatal communication error.

       If a user requests to log out explicitly (via user interfaces),
       the client MUST forget user's password, go to step 5 and reload
       the current resource without authentication credential.

   Note 1:  These transitions are valid for clients, but not recommended
       for servers to initiate.

   Any kind of response (including a normal response) other than those
   shown in the above procedure SHOULD be interpreted as fatal
   communication error, and in such cases user clients MUST NOT process
   any data (response body and other content-related headers) sent from
   the server.  However, as a handling for exceptional error cases,
   clients MAY accept a message without an Authentication-Info header,
   if it is a Server-Error (5xx) status.  The client will be in the
   "UNAUTHENTICATED" status in these cases.

   The client software SHOULD display the three client status to the
   end-user.  For an interactive client, however, if a request is a sub-
   request for a resource included to another page (e.g. embedded
   images, style sheets, frames etc.), its status MAY be omitted from
   being shown, and any "AUTH_REQUESTED" statuses MAY be treated in the
   same way as an "UNAUTHENTICATED" status.

   Figure 10 shows the full client-side state diagram.













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         ===========                                  -(11)------------
         NEW REQUEST                                 ( UNAUTHENTICATED )
         ===========                                  -----------------
              |                                               ^
              |                                               |normal
              v                                               |response
   +(1)-------------------+ NO                         +(5)----------+
   | The requested URI    |--------------------------->| send normal |
   | known to be auth'ed? |                            |   request   |
   +----------------------+                            +-------------+
          YES |   401-B0, 200-Optional-B0               401-B0|
              |   with a different realm       200-Optional-B0|
              |          -----------------------------------. |
              |         /                                   v v
              |        |       -(12)------------    NO  +(6)--------+
              |        |      ( AUTH_REQUESTED  )<------| user/pass |
              |        |       -----------------        |   known?  |
              |        |                                +-----------+
              |        |                                      |YES
              v        |                                      v
        +(2)--------+  |                                +(7)--------+
        | session   |  |                                | session   | NO
    NO /| available?|  |                                | available?|\
      / +-----------+  |                                +-----------+ |
     /        |YES     |                                      |YES    |
    |         |       /|                                      |       |
    |         v      / |   401-                401-           v       |
    |   +(3)--------+  |   B0  --(13)----------  B0     +(8)--------+ |
    |   |   send    |--+----->/ AUTH_REQUESTED \<-------|   send    | |
    |  /|  req-A3   |  |      \forget password /        |  req-A3   | |
     \/ +-----------+ /        ----------------        /+-----------+ |
     /\           \ \/                 ^ 401-B0       |      |401-B0- |
    |  -------.    \/\  401-B0-stale   |              |      |stale  /
    |         |    /\ -----------------+--------------+----. |      /
    |         v   /  \                 |              |    v v     v
    |   +(4)--------+ |   401-B1 +(10)-------+ 401-B1 | +(9)--------+
    |   |   send    |-|--------->|   send    |<-------+-|   send    |
    | --|  req-A1   | |          |  req-A3   |        | |  req-A1   |
    |/  +-----------+ |          +-----------+        | +-----------+
    |                 |200-B4          |        200-B4|       ^
    |normal           |                |200-B4       /        |
    |response         |                v            / ==================
    v                  \         -(14)---------    /  USER/PASS INPUTTED
    -(11)------------   ------->( AUTH-SUCCEED )<--   ==================
   ( UNAUTHENTICATED )           --------------
    -----------------

                   Figure 10: State diagram for clients



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9.  Decision procedure for the server

   Each server SHOULD have a table of session states.  This table need
   not be persistent in a long term; it MAY be cleared upon server
   restart, reboot and others.  Each entry of the table SHOULD contain
   at least the following information:

   o  The session identifier, the value of the sid field

   o  The algorithm used

   o  The authentication realm

   o  The state of the protocol: one of "wa received", "authenticated",
      "rejected", and "inactive"

   o  The user name received from the client

   o  The boolean flag whether the session is fake

   o  When the state is "wa received", the values of wa and sb

   o  When the state is "authenticated", the following information:

      *  The value of the session secret z

      *  The largest nc received from the client (largest-nc)

      *  For each possible nc values between (largest-nc - nc-
         window + 1) and max_nc, a flag whether a request with
         corresponding nc has been received.

   The table MAY contain other information.

   Servers SHOULD respond to the client requests according to the
   following procedure:

   o  When the server receives a normal request:

      *  If the requested resource is not protected by the Mutual
         Authentication, send a normal response.

      *  If the resource is protected by the Mutual Authentication, send
         a 401-B0 response.

      *  If the resource is protected by the optional Mutual
         Authentication, send a 200-Optional-B0 response.




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   o  When the server receives a req-A1 request:

      *  If the requested resource is not protected by the Mutual
         Authentication, send a normal response.

      *  If the authentication realm specified in the req-A1 request is
         not the expected one, send either a 401-B0 or a 200-Optional-B0
         response.

      *  If the server cannot validate the field wa, send a 401-B0
         response.

      *  If the received user name is either invalid, unknown or
         unacceptable, create a new session, mark it as a "fake"
         session, compute a random value as wb, and send a fake 401-B1
         response.  (Note: the server SHOULD NOT send 401-B0 response in
         this case, because it will leak the information to the client
         that the specified user will not be accepted.  Instead,
         postpone it to the response for the next req-A3 request.)

      *  Otherwise, create a new session, compute wb and send a 401-B1
         response.

      The created session has "wa received" state.

   o  When the server receives a req-A3 request:

      *  If the requested resource is not protected by the Mutual
         Authentication, send a normal response.

      *  If the authentication realm specified in the req-A3 request is
         not the expected one, send either a 401-B0 or a 200-Optional-B0
         response.

      If none of above is hold, the server will lookup the session
      corresponding to the received sid and the authentication realm.

      *  If the session corresponding to the received sid could not be
         found, or it is inactive, send a 401-B0-stale response.

      *  If the session is in "rejected" state, send either a 401-B0 or
         a 401-B0-stale message.

      *  If the session is a "fake" session, or if the received oa is
         incorrect, then send a 401-B0 response.  If the session is "wa
         received" state, it SHOULD be changed to a "rejected" state;
         otherwise, it MAY either be changed to a "rejected" status or
         keep the previous state.



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      *  If the session is in "active" state, and request has a nc value
         which was previously received from the client, send either a
         401-B0-stale message.  The session SHOULD be changed to
         "inactive" status.

      *  If the nc value in the request is larger than the nc-max field
         sent from the server, or if it is not larger then (largest-nc -
         nc-window) (when in "authenticated" status), the server MAY
         (not REQUIRED) send either a 401-B0-stale message.  The session
         SHOULD be changed to a "inactive" status if did so.

      *  Otherwise, send a 200-B4 response.  If the session was "wa
         received" state, the session SHOULD be changed to an
         "authenticated" state.  The maximum nc and the nc flags of the
         state SHOULD be updated properly.

   At any time, the server MAY change any state entries with both
   "rejected" and "authenticated" status to "inactive" status, and MAY
   discard any "inactive" states from the table.  The entries with "wa
   received" status SHOULD be kept unless there is an emergency
   situation such as server reboot and table capacity overflow.


10.  Authentication-Control header

    Authentication-Control-header
                      = "Authentication-Control" ":" [spaces]
                        auth-scheme spaces Auth-Ctrl-fields
    Auth-Ctrl-fields  = Auth-Ctrl-field
                        *([spaces] "," spaces Auth-Ctrl-field)
    Auth-Ctrl-field   = loc-when-unauthed / loc-when-logout
                      / logout-timeout
                      / extension-field
    loc-when-unauthed = "location-when-unauthenticated" "=" string
    loc-when-logout   = "location-when-logout" "=" string

      Figure 11: the BNF syntax for the Authentication-Control header

   The Authentication-Control header gives more precise control for the
   client behavior for Web applications using Mutual Access Control
   Protocol.  This headers may usually be generated in an application
   layer, as opposed to WWW-Authenticate headers which will be generated
   by Web servers.

   Support of this header is RECOMMENDED for interactive clients and not
   required for non-interactive clients.  Web applications SHOULD
   consider security impacts of behavior of clients which do not support
   this header.



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   The "auth-scheme" of this header and other authentication-related
   headers within the same message MUST be equal.  This document does
   not define any behavior associated with this header, when the
   "auth-scheme" of this header is not "Mutual".

10.1.  Location-when-unauthenticated field

   Authentication-Control: Mutual
   location-when-unauthenticated="http://www.example.com/login.html"

   The field "location-when-unauthenticated" specifies a location which
   any unauthenticated clients should be redirected to.  This header may
   be used, for example, when there is a central login page for the
   whole Web application.  The value of this field MUST be a string that
   contains an absolute URL location.  If a given URL is not absolute,
   clients MAY consider it as a relative URL from the current location.

   This field MAY be used with 401-B0 and 200-Optional-B0 messages;
   however, use of this with 200-Optional-B0 messages is not
   recommended.  If there is a 200-B4, 401-B0-stale or 401-B1 message
   with this field, clients MUST ignore this field.

   When a client receives a message with this field, if and only if the
   client's state after the processing the response is either Step 12 or
   Step 13 (i.e., a state in which the client will process response body
   and ask user's password), the client will treat the whole response as
   if it were a 303 "See Other" response with a Location header with the
   value of this field (i.e., client will be redirected to the specified
   location with a GET request).  Unlike a normal 303 response, if the
   client can proceed authentication without user's interaction (like
   steps 3, 4, 8, 9 and 10), this field is ignored.

   The specified location SHOULD be included in a set of locations
   specified in the "auth-domain" field of the corresponding 401-B0
   message.  If this is not satisfied, clients MAY ignore this field.

10.2.  Location-when-logout field

   Authentication-Control: Mutual
   location-when-logout="http://www.example.com/byebye.html"

   The field "location-when-logout" specifies a location where the
   client is to be redirected when users request logout explicitly.  The
   value of this field MUST be a string that contains an absolute URL
   location.  If a given URL is not absolute, clients MAY consider it as
   a relative URL from the current location.

   This field MAY be used with 200-B4 messages.  If there is a 401-B0,



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   401-B1, 401-B0-stale, 200-Optional-B0 or normal 200 message with this
   field, clients MUST ignore this field.

   When users of a client request to terminate an authentication
   session, and if the client currently displays a page supplied by a
   response with this field, the client will be redirected to the
   specified location by a new GET request (like received a 303
   response), instead of reloading the page without authentication
   credentials.  It is recommendable for Web applications to send this
   field with an appropriate value for any responses (except those with
   redirection (3XX) statuses) for non-GET requests.

10.3.  Logout-timeout

   Authentication-Control: Mutual logout-timeout=300

   The field "logout-timeout" has the same meaning as the field of the
   same name in "Authentication-Info" headers.  This field will be used
   with 200-B4 messages.  If both are specified, clients are recommended
   to use the one with the smaller value.


11.  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  iso-kam3-ec-p256-sha256: for the 256-bit prime-field elliptic-
      curve setting with SHA-256 hash function.

   o  iso-kam3-ec-p521-sha512: for the 521-bit prime-field elliptic-
      curve setting with SHA-512 hash function.

   o  iso-kam3-dl-2048-sha256: for the 2048-bit discrete-logarithm
      setting with SHA-256 hash function.

   o  iso-kam3-dl-4096-sha512: for the 4096-bit discrete-logarithm
      setting with SHA-512 hash function.

   For the elliptic-curve settings, the underlying groups are the
   elliptic curves over prime fields P-256 and P-521, respectively,
   specified in the appendix D.1.2 of FIPS PUB 186-3 [FIPS.186-3.2009]
   specification.  The hash functions H 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



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   is hex-fixed-number.

   For discrete-logarithm settings, the underlying groups are 2048-bit
   and 4096-bit MODP groups defined in [RFC3526], respectively.  See
   Appendix B for the exact specification of the group and associated
   parameters.  The hash functions H are SHA-256 for the 2048-bit group
   and SHA-512 for the 4096-bit group, respectively.  The representation
   of fields wa, wb, oa, and ob is base64-fixed-number.

   The clients SHOULD support at least "iso-kam3-dl-2048-sha256"
   algorithm, and are advised to support all of the above four
   algorithms whenever possible.  The server software implementations
   SHOULD support at least "iso-kam3-dl-2048-sha256" algorithm, unless
   it is known that users will not use it.

   Note: This algorithm is based on the Key Agreement Mechanism 3 (KAM3)
   defined in Section 6.3 of ISO/IEC 11770-4 [ISO.11770-4.2006] with a
   few modifications/improvements.  However, implementers should use
   this document as the normative reference, because the algorithm has
   been changed in several minor details as well as major improvements.

11.1.  Support functions and notations

   The algorithm definitions use several support functions and notations
   defined below:

   The integers in the specification is decimal, or hexadecimal when
   prefixed with "0x".

   The function octet(c) generates a single octet string whose code
   value is equal to c.  The operator |, when applied to octet strings,
   denotes the concatenation of two operands.

   The function VI encodes natural numbers into octet strings in the
   following manner: numbers are represented in big-endian radix-128
   string, where each digit is represented by a octet within 0x80-0xff
   except the last digit represented by a octet within 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 for subcomponents
   of object identifiers in the ASN.1 encoding [ITU.X690.1994], and
   available as a "w" conversion in the pack function of several
   scripting languages.

   The function VS encodes variable-length octet string into uniquely-
   decoded, self-delimited octet string, as in the following manner:

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



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   where length(s) is a number of octets (not characters) in s.

   [Editorial note: Unlike the colon-separated notion used in the Basic/
   Digest HTTP authentication scheme, the string generated by a
   concatenation of the VS-encoded strings will be unique, regardless of
   the characters included in the strings encoded.]

   The function OCTETS converts an integer to corresponding radix-256
   big-endian octet string having its natural length: See Section 3.2
   for the definition of the "natural length".

   Note: The definition of OCTETS() is different from the function
   GE2OS_x in the original ISO specification, which takes the shortest
   representation.

11.2.  Common functions for both settings

   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 (or 200-Optional-B0, hereafter
   implied) 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 password SHALL be encoded as a UTF-8 string before passed to ph.

   The values o_A and o_B are derived by the following equation.

   o_A = H(octet(4) | OCTETS(w_A) | OCTETS(w_B) | OCTETS(z) | VI(nc) |
   VS(v))

   o_B = H(octet(3) | OCTETS(w_A) | OCTETS(w_B) | OCTETS(z) | VI(nc) |
   VS(v))

   The equations for J, w_A, T, z, and w_B are specified differently for
   the discrete-logarithm setting and the elliptic-curve setting.  These
   equations are defined later in this section.

11.3.  Functions for discrete-logarithm settings

   In this section, the equation (x / y mod z) denotes a natural number
   w less than z which satisfies (w * y) mod z = x mod z.




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   For the discrete-logarithm, we refer some of the domain parameters by
   the following symbols:

   o  q: for "the prime" of the group.

   o  g: for "the generator" associated with the group.

   o  r: for the order of the subgroup generated by g.

   The function J is defined as

   J(pi) = g^(pi) mod q.

   The value of w_A is derived as

   w_A = g^(s_A) mod q,

   where s_A is a random integer within range [1, r-1] and r is the size
   of the subgroup generated by g.  In addition, s_A MUST be larger than
   log(q)/log(g) (so that g^(s_A) > q).

   The value of w_A SHALL satisfy 1 < w_A < q-1.  The server MUST check
   this condition upon reception.

   The value of w_B is derived from J(pi) and w_A as:

   w_B = (J(pi) * w_A^(H(octet(1) | OCTETS(w_A))))^s_B mod q,

   where s_B is a random number within range [1, r-1].  The value of w_B
   MUST satisfy 1 < w_B < q-1.  If this condition is not hold, the
   server MUST retry with another value of s_B. The client MUST check
   this condition upon reception.

   The value z in the client side is derived by the following equation:

   z = w_B^((s_A + H(octet(2) | OCTETS(w_A) | OCTETS(w_B))) / (s_A *
   H(octet(1) | w_A) + pi) mod r) mod q.

   The value z in the server side is derived by the following equation:

   z = (w_A * g^(H(octet(2) | OCTETS(w_A) | OCTETS(w_B))))^s_B mod q.

11.4.  Functions for elliptic-curve settings

   For the elliptic-curve setting, we refer some of the domain
   parameters by the following symbols:





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   o  q: for the prime used to define the group,

   o  G: for the defined point called the generator,

   o  r: for the order of the subgroup generated by G.

   The function P(p) converts a curve point p to an integer representing
   the point p, by computing x * 2 + (y mod 2), where (x, y) are the
   coordinates of the point p.  P'(z) is the inverse of function P, that
   is, it converts an integer z to a point p which satisfies P(p) = z.
   If such p is exist, it is uniquely defined.  Otherwise, z does not
   represent a valid curve point.  The operation [x] * p denotes an
   integer-multiplication of point p: it calculates p + p + ... (x
   times) ... + p.  See literatures on elliptic-curve cryptography for
   the exact algorithms for those. 0_E represents the infinity point.
   The equation (x / y mod z) denotes an natural number w less than z
   which satisfies (w * y) mod z = x mod z.

   the function J is defined as

   J(pi) = [pi] * G.

   The value of w_A is derived as

   w_A = P(W_A), where W_A = [s_A] * G.

   where s_A is a random number within range [1, r-1].  The value of w_A
   MUST represent a valid curve point, and W_A SHALL NOT be 0_E. The
   server MUST check this condition upon reception.

   The value of w_B is derived from J(pi) and W_A = P'(w_A) as:

   w_B = P(W_B), where W_B = [s_B] * (J(pi) + [H(octet(1) |
   OCTETS(w_A))] * W_A).

   where s_B is a random number within range [1, r-1].  The value of w_B
   MUST represent a valid curve point and satisfy [4] * P'(w_B) <> 0_E.
   If this condition does not hold, the server MUST retry with another
   value of s_B. The client MUST check this condition upon reception.

   The value z in the client side is derived by the following equation:

   z = P([(s_A + H(octet(2) | OCTETS(w_A) | OCTETS(w_B))) / (s_A *
   H(octet(1) | OCTETS(w_A)) + pi) mod r] * W_B), where W_B = P'(w_B).

   The value z in the server side is derived by the following equation:

   z = P([s_B] * (W_A + [H(octet(2) | OCTETS(w_A) | OCTETS(w_B))] * G)),



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   where W_A = P'(w_A).


12.  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.  Two-octet keys from
   "wc" to "wz" and from "oc" to "oz" are reserved for this purpose.  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.

   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.


13.  IANA Considerations

   The tokens used for authentication-algorithm, pwd-hash, and
   validation fields MUST be allocated by IANA.  To acquire registered
   tokens, a specification for the use of such tokens MUST be available
   as an RFC, as outlined in [RFC5226].

   Note: More formal declarations will be added in future drafts to meet
   RFC 5226 requirements.


14.  Security Considerations

14.1.  Security Properties

   o  The protocol is secure against passive eavesdropping and replay
      attacks.  However, the protocol relies on transport security
      including DNS integrity for data secrecy and integrity.  HTTP/TLS
      SHOULD be used where transport security is not assured and/or data
      secrecy is important.

   o  When used with HTTP/TLS, if TLS server certificates are reliably
      verified, the protocol gives true protection against active man-
      in-the-middle attacks.




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   o  Even if the server certificate is not used or is unreliable, the
      protocol gives protection against active man-in-the-middle attacks
      for each HTTP request/response pair.  However, in such cases,
      JavaScript or similar scripting facilities can be used to affect
      Mutually-authenticated contents from other contents not protected
      by this authentication mechanism.  This is the reason why this
      protocol requires that valid TLS server certificates MUST be
      presented (Section 7).

14.2.  Denial-of-service attacks to servers

   The protocol requires a server-side table of active sessions, which
   may become a critical point of the server resource consumptions.  For
   proper operation, the protocol requires that at least one key
   verification request is processed for each session identifier.  After
   that, servers MAY discard sessions internally at any time, without
   causing any operational problems to clients.  Clients will silently
   reestablishes a new session then.

   However, if a malicious client sends too many requests of key
   exchanges (req-A1 messages) only, resource starvation might occur.
   In such critical situations, servers MAY discard any kind of existing
   sessions regardless of these statuses.  One way to mitigate such
   attacks are that servers MAY have a number and a time limits for
   unverified pending key exchange requests (in the "wa received"
   status).

   This is a common weakness of authentication protocols with almost any
   kind of negotiations or states, including Digest authentication
   method and most Cookie-based authentication implementations.
   However, regarding the resource consumption, a situation of the
   mutual authentication method is a slightly better than the Digest,
   because HTTP requests without any kind of authentication requests
   will not generate any kind of sessions.  Session identifiers are only
   generated after a client starts a key negotiation.  It means that
   simple clients such as web crawlers will not accidentally consume
   server-side resources for session managements.

14.3.  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 fails, the client MUST NOT process any
      content sent with the message, including the body part.  Non-
      compliance to this requirement will allow 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



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      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 for this protocol.

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

   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:
      100-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 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.

14.4.  Usage Considerations

   o  The user-names inputted by user may be sent automatically to any
      servers sharing the same auth-domain.  This means that when host-
      type auth-domain is used for authentication in HTTPS site, and
      when an HTTP server on the same host requests Mutual
      authentication with the same realm, the client will send the user-
      name in a clear text.  If user-names have to be kept secret
      against eavesdropping, the server must use full-scheme-type auth-
      domain parameter.  On the contrary, passwords are not exposed to
      eavesdroppers even on HTTP requests.

   o  "Pwd_hash" field is only provided for backward compatibility for
      password databases, and using "none" function is the mostly secure
      choice and RECOMMENDED.  If values other than "none" is used, you
      must ensure that the hash values of the passwords were not exposed
      to the public.  Note that hashed password databases for plain-text
      authentications are usually not considered secret.

   o  If the server provides several ways of storing server-side
      password database, it is advised to store the values encrypted by
      one-way function J(pi), instead of the real passwords, those
      hashed by ph, or pi.




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15.  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.  For the detail of the patent application and
   its status, please 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.


16.  References

16.1.  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.186-3.2009]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-3, June 2009, <htt
              p://csrc.nist.gov/publications/fips/fips186-3/
              fips186-3.pdf>.

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

   [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.

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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.



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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

16.2.  Informative References

   [ISO.10646-1.1993]
              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.

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

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, October 2000.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.



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   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.


Appendix A.  (Informative) Generic syntax of headers

   Several headers (e.g.  WWW-Authenticate: headers in 401-B0,
   401-B0-stale, and 401-B1 messages) shares common header names.  To
   parse these headers, one MAY use the following general syntax
   definition of the message syntax:

    header           = header-name ":" [spaces] auth-scheme
                       spaces fields
    header-name      = "WWW-Authenticate" / "Optional-WWW-Authenticate"
                     / "Authorization" / "Authentication-info"
                     / "Authentication-Control"
    auth-scheme      = "Mutual"             ; see HTTP for other values
    fields           = field *([spaces] "," spaces field)
    field            = key "=" value        ; either a specific or
                                            ;        an extension field
    key              = extensive-token
    token            = 1*(%x30-39 / %x41-5A / %x61-7A / "-" / "_")
    extensive-token  = token / extension-token
    extension-token  = "-" token 1*("." token)
    value            = extensive-token / integer
                     / hex-fixed-number
                     / base64-fixed-number / string
    integer          = "0" / (%x31-39 *%x30-39)      ; no leading zeros
    hex-fixed-number = 1*(%x30-39 / %x41-46 / %x61-66)
    base64-fixed-number = string
    string           = %x22 *(%x20-21 / %x23-5B / %x5D-FF
                              / %x5C.22 / "\\") %x22
    spaces           = 1*(" " / %x09)

     Figure 12: the common BNF syntax for the headers in the protocol

   In this way of parsing, messages will be distinguished by the fields
   contained in a header corresponding to the authentication.  The
   procedure below determines the kind of a message which each HTTP
   request/response belongs to.

   o  If the message is a response with a "401" status:

      *  If it does not contain any WWW-Authenticate header, it is an
         error.

      *  If the WWW-Authenticate header specifies a scheme other than
         "Mutual", it is a normal response in this draft's scope.



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      *  Otherwise, the response contains a "WWW-Authenticate: Mutual"
         header.  If the header contains both sid and stale fields, it
         is an error.

      *  If the header contains a stale field with a value of 0, it is a
         401-B0 message.

      *  If the header contains a stale field with a value of 1, it is a
         401-B0-stale message.

      *  If the header contains an sid field, it is a 401-B1 message.

   o  If the message is a response other than a "401" status:

      *  If it contains both Authentication-Info and
         Optional-WWW-Authenticate headers, it is an error.

      *  If it contains a Authentication-Info header with a scheme
         "Mutual", it is a 200-B4 message.

      *  If it contains a Optional-WWW-Authenticate header with "Mutual"
         scheme, it is a 200-Optional-B0 message.

      *  If it contains a Optional-WWW-Authenticate header with a scheme
         other than "Mutual", it is either an error or a normal
         response, and the behavior is not defined in this
         specification.

      *  Otherwise, it is a normal response.

   o  If the message is a request:

      *  If it does not contain an Authorization header, or it contains
         an Authorization header with a scheme other than Mutual, it is
         a normal request.

      *  Otherwise, the request contains a "Authorization: Mutual"
         header.  If the header contains an sid field, it is a req-A3
         message.

      *  If the header do not contain an sid field, it is a req-A1
         message.

   Implementations MAY perform checks stricter than the procedure above,
   according to the definitions in Section 3.






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Appendix B.  (Informative) Group parameters for discrete-logarithm based
             algorithms

   The MODP group used for the iso-kam3-dl-2048-sha256 algorithm is
   defined by the following parameters.

   The prime is:

    q = 0xFFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
          29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
          EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
          E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
          EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
          C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
          83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
          670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
          E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
          DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
          15728E5A 8AACAA68 FFFFFFFF FFFFFFFF.

   The generator is:

    g = 2.

   The size of the subgroup generated by g is:

    r = (q - 1) / 2 =
        0x7FFFFFFF FFFFFFFF E487ED51 10B4611A 62633145 C06E0E68
          94812704 4533E63A 0105DF53 1D89CD91 28A5043C C71A026E
          F7CA8CD9 E69D218D 98158536 F92F8A1B A7F09AB6 B6A8E122
          F242DABB 312F3F63 7A262174 D31BF6B5 85FFAE5B 7A035BF6
          F71C35FD AD44CFD2 D74F9208 BE258FF3 24943328 F6722D9E
          E1003E5C 50B1DF82 CC6D241B 0E2AE9CD 348B1FD4 7E9267AF
          C1B2AE91 EE51D6CB 0E3179AB 1042A95D CF6A9483 B84B4B36
          B3861AA7 255E4C02 78BA3604 650C10BE 19482F23 171B671D
          F1CF3B96 0C074301 CD93C1D1 7603D147 DAE2AEF8 37A62964
          EF15E5FB 4AAC0B8C 1CCAA4BE 754AB572 8AE9130C 4C7D0288
          0AB9472D 45565534 7FFFFFFF FFFFFFFF.

   The MODP group used for the iso-kam3-dl-4096-sha512 algorithm is
   defined by the following parameters.

   The prime is:








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    q = 0xFFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
          29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
          EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
          E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
          EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
          C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
          83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
          670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
          E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
          DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
          15728E5A 8AAAC42D AD33170D 04507A33 A85521AB DF1CBA64
          ECFB8504 58DBEF0A 8AEA7157 5D060C7D B3970F85 A6E1E4C7
          ABF5AE8C DB0933D7 1E8C94E0 4A25619D CEE3D226 1AD2EE6B
          F12FFA06 D98A0864 D8760273 3EC86A64 521F2B18 177B200C
          BBE11757 7A615D6C 770988C0 BAD946E2 08E24FA0 74E5AB31
          43DB5BFC E0FD108E 4B82D120 A9210801 1A723C12 A787E6D7
          88719A10 BDBA5B26 99C32718 6AF4E23C 1A946834 B6150BDA
          2583E9CA 2AD44CE8 DBBBC2DB 04DE8EF9 2E8EFC14 1FBECAA6
          287C5947 4E6BC05D 99B2964F A090C3A2 233BA186 515BE7ED
          1F612970 CEE2D7AF B81BDD76 2170481C D0069127 D5B05AA9
          93B4EA98 8D8FDDC1 86FFB7DC 90A6C08F 4DF435C9 34063199
          FFFFFFFF FFFFFFFF.

   The generator is:

    g = 2.

   The size of the subgroup generated by g is:























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    r = (q - 1) / 2 =
        0x7FFFFFFF FFFFFFFF E487ED51 10B4611A 62633145 C06E0E68
          94812704 4533E63A 0105DF53 1D89CD91 28A5043C C71A026E
          F7CA8CD9 E69D218D 98158536 F92F8A1B A7F09AB6 B6A8E122
          F242DABB 312F3F63 7A262174 D31BF6B5 85FFAE5B 7A035BF6
          F71C35FD AD44CFD2 D74F9208 BE258FF3 24943328 F6722D9E
          E1003E5C 50B1DF82 CC6D241B 0E2AE9CD 348B1FD4 7E9267AF
          C1B2AE91 EE51D6CB 0E3179AB 1042A95D CF6A9483 B84B4B36
          B3861AA7 255E4C02 78BA3604 650C10BE 19482F23 171B671D
          F1CF3B96 0C074301 CD93C1D1 7603D147 DAE2AEF8 37A62964
          EF15E5FB 4AAC0B8C 1CCAA4BE 754AB572 8AE9130C 4C7D0288
          0AB9472D 45556216 D6998B86 82283D19 D42A90D5 EF8E5D32
          767DC282 2C6DF785 457538AB AE83063E D9CB87C2 D370F263
          D5FAD746 6D8499EB 8F464A70 2512B0CE E771E913 0D697735
          F897FD03 6CC50432 6C3B0139 9F643532 290F958C 0BBD9006
          5DF08BAB BD30AEB6 3B84C460 5D6CA371 047127D0 3A72D598
          A1EDADFE 707E8847 25C16890 54908400 8D391E09 53C3F36B
          C438CD08 5EDD2D93 4CE1938C 357A711E 0D4A341A 5B0A85ED
          12C1F4E5 156A2674 6DDDE16D 826F477C 97477E0A 0FDF6553
          143E2CA3 A735E02E CCD94B27 D04861D1 119DD0C3 28ADF3F6
          8FB094B8 67716BD7 DC0DEEBB 10B8240E 68034893 EAD82D54
          C9DA754C 46C7EEE0 C37FDBEE 48536047 A6FA1AE4 9A0318CC
          FFFFFFFF FFFFFFFF.


Appendix C.  (Informative) Derived numerical values

   This section gives several numerical values for implementing this
   protocol, derived from the above specifications.  The values shown in
   this section are for informative purpose only.

   +----------------+---------+---------+---------+---------+----------+
   |                | dl-2048 | dl-4096 | ec-p256 | ec-p521 |          |
   +----------------+---------+---------+---------+---------+----------+
   | Size of w_A    | 2048    | 4096    | 257     | 522     | (bits)   |
   | etc.           |         |         |         |         |          |
   | Size of H(...) | 256     | 512     | 256     | 512     | (bits)   |
   | length of      | 256     | 512     | 33      | 66      | (octets) |
   | OCTETS(w_A)    |         |         |         |         |          |
   | etc.           |         |         |         |         |          |
   | length of wa,  | 346 *   | 686 *   | 66      | 132     | (octets) |
   | wb field       |         |         |         |         |          |
   | values.        |         |         |         |         |          |
   | length of oa,  | 46 *    | 90 *    | 64      | 128     | (octets) |
   | ob field       |         |         |         |         |          |
   | values.        |         |         |         |         |          |





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   | minimum        | 2048    | 4096    | 1       | 1       |          |
   | allowed s_A    |         |         |         |         |          |
   +----------------+---------+---------+---------+---------+----------+

   (The numbers marked with * include enclosing quotation marks.)


Appendix D.  (Informative) Draft Remarks from the Authors

   The following items are currently under consideration for future
   revisions by the authors.

   o  Restructuring of the draft, possibly separating it to several
      parts, e.g. introduction, general HTTP extensions and Mutual
      authentication.

   o  Format of the "Authentication-Control" header and other header
      fields extending the general HTTP authentication scheme, and
      harmonization of those with other draft proposals.

   o  Whether to keep TLS-key validation or not.

   o  When keeping tls-key validation, whether to use "TLS channel
      binding" [RFC5929] for "tls-key" verification (Section 7).  Note
      that existing implementations of TLS should be considered to
      determine this.

   o  Adding test vectors for ensuring implementation correctness.

   o  Possibly adding a method for servers to detect availability of
      Mutual authentication on client-side.

   o  Applying the protocol for proxy authentication/authorization.


Appendix E.  (Informative) Draft Change Log

E.1.  Changes in revision 07

   o  Adapt to httpbis HTTP/1.1 drafts:

      *  Changed definition of extensive-token.

      *  LWSP continuation-line (%0D.0A.20) deprecated.

   o  To simplify the whole spec, the type of nonce-counter related
      fields are change from hex-integer to integer.




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   o  Algorithm tokens are renamed to include names of hash algorithms.

   o  Clarified the session management, added details of server-side
      protocol decisions.

   o  The whole draft was reorganized; introduction and overview has
      been rewritten.

E.2.  Changes in revision 06

   o  Integrated Optional Mutual Authentication to the main part.

   o  Clarified the decision procedure for message recognitions.

   o  Clarified that a new authentication request for any sub-requests
      in interactive clients may be silently discarded.

   o  Typos and confusing phrases are fixed.

   o  Several "future considerations" are added.

   The field "version" is NOT changed from the previous draft, as the
   semantics has not been changed.

E.3.  Changes in revision 05

   o  A new field "version" is added for supporting future incompatible
      changes with a single implementation.  In the (first) final
      specification its value will be changed to 1.

   o  A new header "Authentication-Control" added for precise control of
      application-level authentication behavior.

E.4.  Changes in revision 04

   o  Changed text of patent licenses: the phrase "once the protocol is
      accepted as an Internet standard" is removed so that the sentence
      also covers the draft versions of this protocol.

   o  The "tls-key" verification is now OPTIONAL.

   o  Several description fixes and clarifications.

E.5.  Changes in revision 03

   o  Wildcard domain specifications (e.g. "*.example.com") is allowed
      for auth-domain parameters (Section 4.1).




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   o  Specification of the "tls-cert" verification is updated
      (incompatible change).

   o  State transitions fixed.

   o  Requirements for servers about w_a values clarified.

   o  RFC references are updated.

E.6.  Changes in revision 02

   o  Auth-realm is extended to allow full-scheme type.

   o  A decision diagram for clients and decision procedures for servers
      are added.

   o  401-B1 and req-A3 messages are changed to have authentication
      realm information.

   o  Bugs on equations for o_A and o_B is fixed.

   o  Detailed equations for the whole algorithm is included.

   o  Elliptic-curve algorithms are updated.

   o  Several clarifications and other minor updates.


Authors' Addresses

   Yutaka Oiwa
   National Institute of Advanced Industrial Science and Technology
   Research Center for Information Security
   Room #1003, Akihabara Daibiru
   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







Oiwa, et al.            Expires February 24, 2011              [Page 52]


Internet-Draft   Mutual Authentication Protocol for HTTP     August 2010


   Hiromitsu Takagi
   National Institute of Advanced Industrial Science and Technology


   Yuichi Ioku
   Yahoo! Japan, Inc.
   Midtown Tower
   9-7-1 Akasaka
   Minato-ku, Tokyo
   JP


   Tatsuya Hayashi
   Lepidum Co. Ltd.
   #602, Village Sasazuka 3
   1-30-3 Sasazuka
   Shibuya-ku, Tokyo
   JP

































Oiwa, et al.            Expires February 24, 2011              [Page 53]


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