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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: January 6, 2012                                      RCIS, AIST
                                                                 Y. Ioku
                                                            Yahoo! Japan
                                                              T. Hayashi
                                                                 Lepidum
                                                            July 5, 2011


                Mutual Authentication Protocol for HTTP
                     draft-oiwa-http-mutualauth-09

Abstract

   This document specifies a mutual authentication method for the Hyper-
   text Transport Protocol (HTTP).  This method provides a 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 the 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 January 6, 2012.



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Copyright Notice

   Copyright (c) 2011 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  . . . . . . . . . . . . . . .  6
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Messages . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Typical Flows of the protocol  . . . . . . . . . . . . . .  7
     2.3.  Alternative flows  . . . . . . . . . . . . . . . . . . . . 10
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Tokens and Extensive-tokens  . . . . . . . . . . . . . . . 12
     3.2.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.3.  Strings  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  401-INIT . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.2.  401-STALE  . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  req-KEX-C1 . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.4.  401-KEX-S1 . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.5.  req-VFY-C  . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.6.  200-VFY-S  . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.7.  200-Optional-INIT  . . . . . . . . . . . . . . . . . . . . 20
   5.  Authentication Realms  . . . . . . . . . . . . . . . . . . . . 21
     5.1.  Resolving ambiguities  . . . . . . . . . . . . . . . . . . 23
   6.  Session Management . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Validation Methods . . . . . . . . . . . . . . . . . . . . . . 25
   8.  Decision procedure for client  . . . . . . . . . . . . . . . . 27
   9.  Decision procedure for the server  . . . . . . . . . . . . . . 32
   10. Authentication-Control header  . . . . . . . . . . . . . . . . 34
     10.1. Location-when-unauthenticated field  . . . . . . . . . . . 35
     10.2. Location-when-logout field . . . . . . . . . . . . . . . . 35
     10.3. Logout-timeout . . . . . . . . . . . . . . . . . . . . . . 36
   11. Authentication Algorithms  . . . . . . . . . . . . . . . . . . 36



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     11.1. Support functions and notations  . . . . . . . . . . . . . 37
     11.2. Default functions for algorithms . . . . . . . . . . . . . 38
   12. Methods to extend this protocol  . . . . . . . . . . . . . . . 38
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 39
     14.1. Security Properties  . . . . . . . . . . . . . . . . . . . 39
     14.2. Denial-of-service attacks to servers . . . . . . . . . . . 39
     14.3. Implementation Considerations  . . . . . . . . . . . . . . 40
     14.4. Usage Considerations . . . . . . . . . . . . . . . . . . . 41
   15. Notice on intellectual properties  . . . . . . . . . . . . . . 41
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 42
     16.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Appendix A.  (Informative) Generic syntax of headers . . . . . . . 43
   Appendix B.  (Informative) Draft Remarks from Authors  . . . . . . 45
   Appendix C.  (Informative) Draft Change Log  . . . . . . . . . . . 46
     C.1.  Changes in revision 09 . . . . . . . . . . . . . . . . . . 46
     C.2.  Changes in revision 08 . . . . . . . . . . . . . . . . . . 46
     C.3.  Changes in revision 07 . . . . . . . . . . . . . . . . . . 46
     C.4.  Changes in revision 06 . . . . . . . . . . . . . . . . . . 47
     C.5.  Changes in revision 05 . . . . . . . . . . . . . . . . . . 47
     C.6.  Changes in revision 04 . . . . . . . . . . . . . . . . . . 47
     C.7.  Changes in revision 03 . . . . . . . . . . . . . . . . . . 47
     C.8.  Changes in revision 02 . . . . . . . . . . . . . . . . . . 48
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 48


























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1.  Introduction

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

   The currently available methods for authentication in HTTP and Web
   systems have several deficiencies.  The Basic authentication method
   [RFC2617] sends a plaintext password to a server without any
   protection; the Digest method uses a hash function that suffers from
   simple dictionary-based off-line attacks, and people have begun to
   think it is obsolete.

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

   o  It provides "true" mutual authentication: in addition to 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 it impossible for fake website owners to
      persuade users that thee authenticated with the original websites.

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

   o  It is secure: when the server fails to authenticate with a user,
      the protocol will not reveal any bit of the user's password.

   Users can discriminate between true and fake Web servers using their
   own passwords by using the proposed method.  Even when a user inputs
   his/her password to a fake website owned by illegitimate phishers,
   the user will certainly notice that the authentication has failed.
   Phishers will not be successful in their authentication attempts,
   even if they forward the received data from a user to a 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.  A majority of the recent Web-sites on the
   Internet use custom application-layer authentication implementations
   using Web forms.  The reasons for these may vary, but many people
   believe that the current HTTP Basic (and Digest, too) authentication
   method does not have enough functionality (including a good-feeling
   user interfaces) to support most of realistic Web-based applications.
   However, the method is very weak against phishing attacks, because
   the whole behavior of the authentication is controlled from the
   server side.  To overcome this problem, we need to "modernize" the
   HTTP authentication framework so that better client-controlled secure
   methods can be used 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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   The terms "encouraged" and "advised" are used for suggestions that do
   not constitute "SHOULD"-level requirements.  People MAY freely choose
   not to include the suggested items regarding [RFC2119], but complying
   with those suggestions would be a best practice; it will improve the
   security, interoperability, and/or operational 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 computer software;
   a "user" is a (usually natural) person who wants to access data
   resources using "a client".

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







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1.2.  Document Structure Overview

   The entire document is organized as follows:

   o  Section 2 presents an overview of the protocol design.

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

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

   o  Section 11 describes properties needed for cryptographic
      algorithms used with this protocol framework, and defines a few
      functions which will be shared among such cryptographic
      algorithms.

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

   o  The appendices contain some information that 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 peers are correctly related (i.e. generated from the
   same password).  Then, both peers will verify the authentication
   results by confirming the sharing of the exchanged key.  This section
   describes a 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 specific names within
   this specification.

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





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      *  401-INIT message: a general message to start the authentication
         protocol.  It is also used as a message indicating an
         authentication failure.

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

      *  401-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 the sharing of a cryptographic secret.

      *  req-KEX-C1 message: a message sent from the client.

      *  401-KEX-S1 message: a message sent from the server as a
         response to a req-KEX-C1 message.

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

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

      *  200-VFY-S message: a successful response used by the server,
         and also asserting that the server is authentic to the client
         simultaneously.

   In addition to the 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 typical cases, the 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-INIT ---  |
            |            401 Authentication Required
            |            WWW-Authenticate: Mutual realm="a realm"
            |                                      |
   [user,   |                                      |
    pass]-->|                                      |
            |  ---- (3) req-KEX-C1 ------------->  |
        GET / HTTP/1.1                             |
        Authorization: Mutual user="john",         |--> [user DB]
                       wa="...", ...               |<-- [user info]
            |                                      |
            |  <-------------- (4) 401-KEX-S1 ---  |
            |            401 Authentication Required
            |            WWW-Authenticate: Mutual sid=..., wb="...", ...
            |                                      |
        [compute] (5) compute session secret   [compute]
            |                                      |
            |                                      |
            |  ---- (6) req-VFY-C -------------->  |
        GET / HTTP/1.1                             |--> [verify (6)]
        Authorization: Mutual sid=...,             |<-- OK
                       oa="...", ...               |
            |                                      |
            |  <--------------- (7) 200-VFY-S ---  |
   [verify  |            200 OK                    |
     (7)]<--|            Authentication-Info: Mutual ob="..."
            |                                      |
            v                                      v

     Figure 1: Typical communication flow for first access to resource

   o  As usual in general HTTP protocol designs, a client will at 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-INIT) (2).

   o  The client processes the body of the message, and waits for the
      user to input the user name and a password.  If the user name and
      the password are available, the client will send a message with
      the authenticated key exchange (req-KEX-C1) to start the
      authentication (3).




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   o  If the server has received a req-KEX-C1 message, the server looks
      up the user's authentication information within its user database.
      Then the server creates a new session identifier (sid) that will
      be used to identify sets of the messages that follow it, and
      responds back with a message containing a server-side
      authenticated key exchange value (401-KEX-S1) (4).

   o  At this point (5), both peers calculate a shared "session secret"
      using the exchanged values in the key exchange messages.  Only
      when both the server and the client have used secret credentials
      generated from the same password will the session secret values
      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
      verification value (req-VFY-C) (6), generated from the client-
      owned session secret.  The server will check the validity of the
      verification value using its own session secret.

   o  If the authentication verification value from the client was
      correct, it means that the client definitely owns the credential
      based on the expected password (i.e. the client authentication
      succeeded.)  The server will respond with a successful message
      (200-VFY-S) (7).  Contrary to the usual one-way authentication
      (e.g.  HTTP Basic authentication or POP APOP authentication), this
      message also contains a server-side authentication verification
      value.

      When the client's verification value is incorrect (e.g. because
      the user-supplied password was incorrect), the server will respond
      with the 401-INIT message (the same one as used in (2)) instead.

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

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








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2.3.  Alternative flows

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

   o  (case A) If the client knows that the resource is likely to
      require the authentication, the client MAY omit the first
      unauthenticated request (1) and immediately send a key exchange
      (req-KEX-C1 message).  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-VFY-C message using the
      existing session identifier and corresponding session secret.
      This will further reduce one round-trip of messages.

      In such cases, the server MAY have thrown out the corresponding
      sessions from the session table.  In this case, the server will
      respond with a 401-STALE message, indicating a new key exchange is
      required.  The client SHOULD retry constructing a req-KEX-C1
      message in this case.

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






















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       (A) omit first request
          (2 round trips)

        Client            Server
        |                      |
        | --- req-KEX-C1 ----> |
        |                      |
        | <---- 401-KEX-S1 --- |
        |                      |
        | ---- req-VFY-C ----> |
        |                      |
        | <----- 200-VFY-S --- |
        |                      |


       (B) reusing session secret

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

        Client            Server   Client              Server
        |                      |   |                        |
        | ---- req-VFY-C ----> |   | --- req-VFY-C -------> |
        |                      |   |                        |
        | <----- 200-VFY-S --- |   | <------- 401-STALE --- |
        |                      |   |                        |
                                   | --- req-KEX-C1 ------> |
                                   |                        |
                                   | <------ 401-KEX-S1 --- |
                                   |                        |
                                   | --- req-VFY-C -------> |
                                   |                        |
                                   | <------- 200-VFY-S --- |
                                   |                        |

              Figure 2: Several alternative flows on protocol

   For more details, see Sections 8 and 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 described in [RFC2617] [Editorial



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   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: BNF syntax for common elements used in protocol

3.1.  Tokens and Extensive-tokens

   The tokens are case insensitive; Senders SHOULD send these in lower-
   case, and receivers MUST accept both upper- and lower-cases.  When
   tokens are used as the (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
   with format "-<token>.<domain-name>", where <domain-name> is a
   validly registered (sub-)domain name on the Internet owned by the
   party who defines the extensions.

3.2.  Numbers

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

   The numbers represented as a hex-fixed-number MUST include an even
   number of characters (i.e. multiples of eight bits).  When these are
   generated from any cryptographic values, they SHOULD have their



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   "natural 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 for representing all the elements in the set.  For example,
   any results of SHA-256 hash function will be represented by 64
   characters, and any elements in 2048-bit prime field (modulo a 2048-
   bit integer) will be represented by 512 characters, regardless of how
   much 0's will be appear in front of such representations.  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 all
   communications by both peers.

   The 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 mentioned above.  Then, the string is encoded using
   the Base 64 encoding [RFC4648] without any spaces and newlines, and
   then enclosed by two double-quotations.

3.3.  Strings

   All the strings outside ASCII character sets MUST be encoded using
   the UTF-8 encoding [RFC3629] for the ISO 10646-1 character set
   [ISO.10646-1.1993].  Both peers are RECOMMENDED to reject any invalid
   UTF-8 sequences that might 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, they will first be
   encoded according to UTF-8 without a leading BOM, then all
   occurrences of the 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 when using them in a header field.

   If strings are representing a domain name or URI that contains non-
   ASCII characters, the host parts SHOULD be encoded as it is used in
   the HTTP protocol layer (e.g. in a Host: header); under current
   standards 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.







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4.  Messages

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

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

   In the descriptions below, the type of allowable values for each
   header field is shown in parenthesis after the key names.  The
   "algorithm-determined" type means that the acceptable value 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 "mandatory"
   SHALL be contained in the message.  The fields marked "non-mandatory"
   MAY either be contained or omitted in the message.  Each field SHALL
   appear in each headers exactly once at most.

4.1.  401-INIT

   Every 401-INIT 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-INIT = "WWW-Authenticate" ":" [spaces]
                  auth-scheme spaces fields-401-INIT
  fields-401-INIT = field-401-INIT *([spaces] "," spaces field-401-INIT)
  field-401-INIT  = 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: BNF syntax for header in 401-INIT header

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





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

   algorithm:     (mandatory extensive-token) specifies the
                  authentication algorithm to be used.  The value MUST
                  be one of the tokens specified in &algo_draft; or
                  other supplemental specification documentation.

   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 documentation.

   auth-domain:   (non-mandatory string) specifies the authentication
                  domain, the set of hosts for which the 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.





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4.2.  401-STALE

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

   WWW-Authenticate: Mutual algorithm=xxxx, validation=xxxx,
   realm="xxxx", stale=1, version=-draft07

   The header MUST contain the same fields as in 401-INIT, except that
   the stale field contains token 1.

4.3.  req-KEX-C1

   Every req-KEX-C1 message SHALL be a valid HTTP request message
   containing an "Authorization" header of the following format.

   Authorization: Mutual algorithm=xxxx, validation=xxxx, realm="xxxx",
   user="xxxx", wa=xxxx, version=-draft07

    header-req-KEX-C1 = "Authorization" ":" [spaces]
                        auth-scheme spaces fields-req-KEX-C1
    fields-req-KEX-C1 = field-req-KEX-C1
                        *([spaces] "," spaces field-req-KEX-C1)
    field-req-KEX-C1  = 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-KEX-C1 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 is
                  undefined when other values are specified.

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

   user:          (mandatory, string) is the UTF-8 encoded name of the
                  user.  If this name comes from a user input, client
                  software SHOULD prepare the string using SASLprep
                  [RFC4013] before encoding it to UTF-8.






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

4.4.  401-KEX-S1

   Every 401-KEX-S1 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

    header-401-KEX-S1 = "WWW-Authenticate" ":" [spaces]
                        auth-scheme spaces fields-401-KEX-S1
    fields-401-KEX-S1 = field-401-KEX-S1
                        *([spaces] "," spaces field-401-KEX-S1)
    field-401-KEX-S1  = 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-KEX-S1 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 is
                  undefined when other values are specified.

   algorithm, validation, auth-domain, realm:  MUST be the same value as
                  it is when 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.
                  Session identifiers are local to each concerned
                  authentication realm: the same sids for different
                  authentication realms SHOULD be treated as independent



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                  ones.

   wb:            (mandatory, algorithm-determined) is the server-side
                  key exchange value w_B, which is specified by the
                  algorithm.

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

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

   time:          (mandatory, integer) represents the suggested time (in
                  seconds) that the client can reuse the session
                  represented by the 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 the sid.

   path:          (non-mandatory, string) specifies which path in the
                  URI space the same authentication is expected to be
                  applied.  The value is a space-separated list of URIs,
                  in the same format as it was 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-VFY-C

   Every req-VFY-C message SHALL be a valid HTTP request message
   containing an "Authorization" header of the following format.

   Authorization: Mutual algorithm=xxxx, validation=xxxx, realm="xxxx",
   sid=xxxx, nc=x, oa=xxxx, version=-draft07

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




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

   The fields contained in the header are as follows:

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

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

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

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

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

4.6.  200-VFY-S

   Every 200-VFY-S message SHALL be a valid HTTP message that is not of
   the 401 (Authentication Required) type, containing an
   "Authentication-Info" header of the following format.

   Authentication-Info: Mutual sid=xxxx, ob=xxxx, version=-draft07

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

           Figure 8: BNF syntax for header in 200-VFY-S message

   The fields contained in the header are as follows:








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

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

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

   logout-timeout:  (non-mandatory, integer) is the number of seconds
                  after which the client should re-validate the user's
                  password for the current authentication realm.  The
                  value 0 means that the client SHOULD automatically
                  forget the user-inputted password for the current
                  authentication realm and revert to the unauthenticated
                  state (i.e. server-initiated logout).  This does not,
                  however, mean that the long-term memories for the
                  passwords (such as the password reminders and auto
                  fill-ins) should be removed.  If a new timeout value
                  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 the 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-INIT

   The 200-Optional-INIT messages enable a non-mandatory authentication,
   which is not possible under the current HTTP authentication
   mechanism.  In several Web applications, users can access the same
   contents as both a guest user and an authenticated user.  In most Web
   applications, it is implemented using HTTP cookies [RFC6265] and
   custom form-based authentications.  The new authentication method
   using this message will provide a replacement for these
   authentication systems.  Support for this message is RECOMMENDED,
   unless the protocol is used for some specific applications in which
   the 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-INIT responses (with one exception described
   below).  Such responses are hereafter called 200-Optional-INIT
   responses.




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

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

        Figure 9: BNF syntax for header in 200-Optional-INIT header

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

   The 200-Optional-INIT message is the only place where an
   Optional-WWW-Authenticate header is allowed.  If a server is supposed
   to send a 401-KEX-S1 or a 401-STALE response, it SHALL NOT replace it
   with 200-Optional-INIT or similar responses.  Furthermore, if a
   server is going to send a 401-INIT message as a response to a
   req-VFY-C message with a correct realm, the server MUST send a
   401-INIT message, not a 200-Optional-INIT message.

   Servers requesting non-mandatory authentication SHOULD send the path
   field in the 401-KEX-S1 messages with an appropriate value.  Clients
   supporting non-mandatory authentication MUST recognize the field, and
   MUST send either a req-KEX-C1 or a req-VFY-C 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 an
   authentication realm that the client is already authenticated for,
   the client will automatically perform the authentication using the
   already-known secrets.  However, for the different authentication
   realms, the clients SHOULD NOT automatically reuse the usernames and
   passwords for another realm.

   Just like in Basic and Digest access authentication protocols, Mutual
   authentication protocol supports multiple, separate authentication
   realms to be set up inside each host.  Furthermore, the protocol



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   supports that a single authentication realm spans over several hosts
   within 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.  However, server operators are NOT RECOMMENDED to
   use the same pair of an authentication domain and a realm for
   different authentication algorithms.

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

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

   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 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 within this kind of authentication domain
      will span over several protocols (i.e. http and https) and ports,
      but not over different hosts.

   o  The 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 the servers MUST be
      equal to or included in a valid Internet domain assigned to a
      specific organization: if clients know, by some means such as a
      blacklist for HTTP cookies, that the specified domain is not to be
      assigned to any specific organization (e.g. "*.com" or "*.jp"),
      the 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 they 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



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   failure of valid authentication attempts.

5.1.  Resolving ambiguities

   In the above definitions of authentication domains, several domains
   will overlap each other.  Depending on the "path" parameters given in
   the "401-KEX-S1" message (see Section 4), there may be several
   candidates when the client is going to send a request including an
   authentication credential (Steps 3 and 4 of the decision procedure
   presented 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-INIT
      messages at that time, use that realm.

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

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

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

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


6.  Session Management

   In the Mutual authentication protocol, a session represented by an
   sid is set up using first four messages (first request, 401-INIT,
   req-KEX-C1 and 401-KEX-S1), and a "session secret" (z) associated
   with the session is established.  After sharing 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 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 informing the users.




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

   The server SHOULD accept at least one req-VFY-C request for each
   session, given that the request reaches the server in a time window
   specified by the timeout field in the 401-KEX-S1 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-STALE messages for any req-VFY-C 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.

   o  The same nonce was not sent within the same session.

   o  It is not larger than the nc-max value that was 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 was previously sent in the session,
      and nc-window is the value of the nc-window field which was
      received from the server in the session.

   The last condition allows servers to reject any nonce values that are
   "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 with a 401-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-STALE message.

   For example, assume the nc-window value of the current session is 32,
   nc-max is 100, and that the client has already used the following
   nonce values: {1-20, 22, 24, 30-38, 45-60, 63-72}.  Then the nonce
   values that 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 they are not above the current "window
   limit" (40 = 72 - 32).




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

   The values of the nonces and any nonce-related values MUST always be
   treated as natural numbers within an infinite range.  Implementations
   using fixed-width integers or fixed-precision floating numbers MUST
   correctly and carefully handle integer overflows.  Such
   implementations are RECOMMENDED to accept any larger values that
   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 the
   clients and servers can implement the protocol using only fixed-width
   integers, by rounding any overflowed values to the maximum possible
   value.


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 v that is an input to the
   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 corresponding to the
                  currently accessing resource.  The scheme and host are
                  in 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 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 the underlying TLS [RFC5246] (or
                  SSL) connection.  The hash value is defined as the
                  value of the entire signed certificate (specified as
                  "Certificate" in [RFC5280]), hashed by the hash
                  algorithm specified by the authentication algorithm
                  used.






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

   If the HTTP protocol is used on a non-encrypted channel (TCP and
   SCTP, for example), the validation type MUST be "host".  If HTTP/TLS
   [RFC2818] (https) protocol is used with the server certificates, the
   validation type MUST be either "tls-cert" or "tls-key".  If HTTP/TLS
   protocol is used with an anonymous Diffie-Hellman key exchange, the
   validation type MUST be "tls-key" (see the note below).

   If the validation type "tls-cert" is used, the server certificate
   provided on TLS connection MUST be verified to make sure that the
   server actually owns the corresponding secret key.

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

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

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

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

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

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

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




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8.  Decision procedure for client

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

   Clients SHOULD implement a decision procedure equivalent to the one
   shown below.  (Unless implementers understand what is required for
   the security, they should not alter this.)  In particular, clients
   SHOULD NOT accept "normal responses" unless explicitly allowed below.
   The labels on the steps are for informational purposes only.  Action
   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 access a new Web resource, check
       whether the resource is expected to be inside some authentication
       realm for which the user has already been authenticated by the
       Mutual authentication scheme.  If yes, go to Step 2.  Otherwise,
       go to Step 5.

   Step 2:
       Check whether there is an available sid for the authentication
       realm you expect.  If there is one, go to Step 3.  Otherwise, go
       to Step 4.

   Step 3 (step_send_vfy_1):
       Send a req-VFY-C request.

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

       *  If you receive a 200-Optional-INIT message with a different
          authentication realm than expected, go to Step 6.

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

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

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

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








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   Step 4 (step_send_kex1_1):
       Send a req-KEX-C1 request.

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

       *  If you receive a 200-Optional-INIT message with a different
          authentication realm than expected, go to Step 6.

       *  If you receive a 401-KEX-S1 message, go to Step 10.

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

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

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

   Step 6 (step_rcvd_init):
       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_vfy):
       Send a req-VFY-C request.

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

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

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








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   Step 9 (step_send_kex1):
       Send a req-KEX-C1 request.

       *  If you receive a 401-KEX-S1 message, go to Step 10.

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

   Step 10 (step_rcvd_kex1):
       Send a req-VFY-C request.

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

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

   Step 11 (step_rcvd_normal):
       The requested resource 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_init_unknown):
       The requested resource requires a Mutual authentication, and the
       user is not yet authenticated.  The client will be in the
       "AUTH_REQUESTED" status, and is RECOMMENDED to process the
       content sent from the server, and to ask user for a user name and
       a password.  When those are supplied from the user, proceed to
       Step 9.

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

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

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







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   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 a fatal
   communication error, and in such cases the clients MUST NOT process
   any data (response body and other content-related headers) sent from
   the server.  However, to handle 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 in 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 a diagram of the client-side state.































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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-INIT, 200-Optional-INIT         401-INIT|
              |   with a different realm     200-Optional-INIT|
              |          -----------------------------------. |
              |         /                                   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)--------+  |  INIT --(13)----------  INIT   +(8)--------+ |
    |   |   send    |--+----->/ AUTH_REQUESTED \<-------|   send    | |
    |  /| req-VFY-C |  |      \forget password /        | req-VFY-C | |
     \/ +-----------+ /        ----------------        /+-----------+ |
     /\           \ \/                 ^ 401-INIT     |     |401-     |
    |  ------      \/\  401-STALE      |              |     | STALE  /
    |        \     /\ -----------------+--------------+---. |       /
    |         |   /  \                 |              |   | |      /
    |         v  /    | 401-           |       401-   |   v v     v
    |   +(4)--------+ | KEX-S1   +(10)-------+ KEX-S1 | +(9)--------+
    |   |   send    |-|--------->|   send    |<-------+-|   send    |
    | --| req-KEX-C1| |          | req-VFY-C |        | | req-KEX-C1|
    |/  +-----------+ |          +-----------+        | +-----------+
    |                 |200-VFY-S      |      200-VFY-S|       ^
    |normal           |               |200-VFY-S     /        |
    |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 over a long term; it MAY be cleared upon server
   restart, reboot, or others.  Each entry in 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", or "inactive".

   o  The user name received from the client

   o  The boolean flag noting whether or not 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 or not a request with
         the 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-INIT response.

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




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   o  When the server receives a req-KEX-C1 request:

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

      *  If the authentication realm specified in the req-KEX-C1 request
         is not the expected one, send either a 401-INIT or a
         200-Optional-INIT response.

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

      *  If the received user name is either invalid, unknown or
         unacceptable, create a new session, mark it a "fake" session,
         compute a random value as wb, and send a fake 401-KEX-S1
         response.  (Note: the server SHOULD NOT send a 401-INIT
         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-VFY-C
         request.)

      *  Otherwise, create a new session, compute wb and send a
         401-KEX-S1 response.

      The created session has the "wa received" state.

   o  When the server receives a req-VFY-C request:

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

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

      If none of above holds true, 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-STALE response.

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

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



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         status or kept in the previous state.

      *  If the session is in the "active" state, and the request has an
         nc value that was previously received from the client, send a
         401-STALE message.  The session SHOULD be changed to the
         "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 to) send a 401-STALE message.  The session SHOULD
         be changed to the "inactive" status if so.

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

   At any time, the server MAY change any state entries with both the
   "rejected" and "authenticated" statuses to the "inactive" status, and
   MAY discard any "inactive" states from the table.  The entries with
   the "wa received" status SHOULD be kept unless there is an emergency
   situation such as a server reboot or a 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 provides a more precise control of
   the client behavior for Web applications using the Mutual
   authentication protocol.  This header will usually be generated in
   the application layer, as opposed to WWW-Authenticate headers which
   will be generated by the Web servers.

   Support of this header is RECOMMENDED for interactive clients and not
   required for non-interactive clients.  Web applications SHOULD



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   consider the security impacts of the behaviors of clients that do not
   support this header.

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

   This field MAY be used with a 401-INIT and 200-Optional-INIT message;
   however, use of this field with 200-Optional-INIT messages is not
   recommended.  If there is a 200-VFY-S, 401-STALE or 401-KEX-S1
   message with this field, the clients MUST ignore this field.

   When a client receives a message with this field, if and only if the
   client's state after processing the response is either Step 12 or 13
   (i.e., a state in which the client will process the response body and
   ask for the user's password), the client will treat the entire
   response as if it were a 303 "See Other" response with a Location
   header that contains 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 process authentication without
   the 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-INIT
   message.  If this is not satisfied, the 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



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

   This field MAY be used with 200-VFY-S messages.  If there is a
   401-INIT, 401-KEX-S1, 401-STALE, 200-Optional-INIT or normal 200
   message with this field, the clients MUST ignore this field.

   When the user 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 (as if it received a 303
   response), instead of reloading the page without the authentication
   credential.  Web applications are encouraged 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 the "Authentication-Info" header.  This field will be
   used with 200-VFY-S messages.  If both are specified, clients are
   RECOMMENDED to use the one with the smaller value.


11.  Authentication Algorithms

   Cryptographic authentication algorithms which are used with this
   protocol will be defined separately.  The algorithm definition MUST
   at least provide a definitions for the following functions:

   o  The server-side authentication credential J, derived from user-
      side authentication credential pi.

   o  Key exchange values w_A and w_B.

   o  Shared secret z, to be computed in both server-side and client
      side.

   o  A hash function H to be used with the protocol.

   All algorithm used with this protocol SHOULD provide secure mutual
   authentication between client and servers, and generate a
   cryptographically strong shared secret value z, equivalently strong
   to or stronger than the hash function H. If any passwords (or pass-



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   phrases or any equivalents, i.e. weak secrets) are involved, these
   SHOULD NOT be guessable from any data transmitted in the protocol,
   even if an attacker (either an eavesdropper or an active server)
   knows the possible thoroughly-searchable candidate list of the
   passwords.  Furthermore, if possible, the function for deriving
   server-side authentication credential J is RECOMMENDED to be one-way
   so that pi should not be easily computed from J(pi).

11.1.  Support functions and notations

   In this section we define several support functions and notations to
   be shared by several algorithm definitions:

   The integers in the specification are in decimal, or in 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 the
   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 a variable-length octet string into a
   uniquely-decoded, self-delimited octet string, as in the following
   manner:

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

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

   [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 to be encoded.]

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




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11.2.  Default functions for algorithms

   The functions defined in this section are common default functions
   among authentication algorithms.

   The client-side 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-INIT (or 200-Optional-INIT, 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
   determined by the value of the pwd-hash field given in a 401-INIT
   message.  If the password comes from a user input, it SHOULD first be
   prepared using SASLprep [RFC4013].  Then, 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))

   Specifications for cryptographic algorithms used with this framework
   MAY override the functions pi, o_A, and o_B defined above.  In such
   cases implementations MUST use the ones defined with such algorithm
   specifications.


12.  Methods to extend this protocol

   If a non-standard extension to 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 the fields "wa", "wb", "oa", and
   "ob", replace those keys, and/or add fields to the messages
   containing those fields in 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 those mentioned
   above, it is RECOMMENDED to use extension-tokens to avoid any key-



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   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 the 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 the future drafts to
   meet the 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 provides true protection against active
      man-in-the-middle attacks.

   o  Even if the server certificate is not used or is unreliable, the
      protocol provides 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 the 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



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   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-KEX-C1 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-VFY-S messages MUST always be validated by the
      client.  If the validation fails, the client MUST NOT process any
      content sent with the message, including other headers and 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 for
      the user's name and passwords SHOULD be carefully designed so that
      (1) the user can easily distinguish the request from this
      authentication method from any other 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 "tls-cert" validation method
      to a URI that is protected by the same realm (so indicated by the
      path field), if the server certificate has been changed since the
      pages were received, the peer is RECOMMENDED to be revalidated



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      using a req-KEX-C1 message with an "Expect: 100-continue" header.
      The same applies when the page is received with the "tls-key"
      validation method, and when the TLS session has expired.

   o  Server-side storages of user passwords are advised to contain 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 a 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 on an HTTPS site, and
      when an HTTP server on the same host requests Mutual
      authentication within 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.  Contrarily, passwords are not exposed to
      eavesdroppers even on HTTP requests.

   o  The "Pwd_hash" field is only provided for backward compatibility
      of password databases.  The use of "none" function is the most
      secure choice and is RECOMMENDED.  If values other than "none" are
      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 for storing server-side
      password secrets into the password database, it is advised to
      store the values encrypted by using the one-way function J(pi),
      instead of the real passwords, those hashed by ph, or pi.


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 on 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 details of the patent application and
   its status, please contact the author of this document.

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




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16.  References

16.1.  Normative References

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

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

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [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.

   [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.

   [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.

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




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   [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.

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

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.


Appendix A.  (Informative) Generic syntax of headers

   Several headers (e.g.  WWW-Authenticate: headers in 401-INIT,
   401-STALE, and 401-KEX-S1 messages) shares common header names.  To
   parse these headers, one MAY use the following general syntax
   definition of the message syntax:

























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    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: Common BNF syntax for 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 each HTTP request/response.

   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 within this draft's scope.

      *  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-INIT message.

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





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      *  If the header contains an sid field, it is a 401-KEX-S1
         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 the "Mutual"
         scheme, it is a 200-VFY-S message.

      *  If it contains a Optional-WWW-Authenticate header with the
         "Mutual" scheme, it is a 200-Optional-INIT 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-VFY-C
         message.

      *  If the header do not contain an sid field, it is a req-KEX-C1
         message.

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


Appendix B.  (Informative) Draft Remarks from Authors

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

   o  Restructuring of the draft, further separating the HTTP extensions
      part from this document.

   o  Format of the "Authentication-Control" header and other header
      fields extending the general HTTP authentication scheme, and



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      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 TLS implementations 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.

   o  Possible support for optional key renewal and cross-site federated
      authentication.


Appendix C.  (Informative) Draft Change Log

C.1.  Changes in revision 09

   Note: the token for the header field "version" is NOT changed from
   draft-07, because the protocol semantics has not been changed in this
   revision.

   o  The (default) cryptographic algorithms are separated to another
      draft.

   o  Names of the messages are changed to more informative one (like
      KEX-C for client key-exchange) than before.

C.2.  Changes in revision 08

   o  The English text has been revised.

C.3.  Changes in revision 07

   o  Adapt to httpbis HTTP/1.1 drafts:

      *  Changed definition of extensive-token.

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






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

   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.

C.4.  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.

C.5.  Changes in revision 05

   o  A new field called "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" is added for precise control
      of application-level authentication behavior.

C.6.  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.

C.7.  Changes in revision 03

   o  Wildcard domain specifications (e.g. "*.example.com") are 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 concerning w_a values are clarified.

   o  RFC references are updated.

C.8.  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-KEX-S1 and req-VFY-C messages are changed to contain
      authentication realm information.

   o  Bugs on equations for o_A and o_B are fixed.

   o  Detailed equations for the entire algorithm are 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







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   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

































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