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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: November 19, 2012                                   RISEC, AIST
                                                               B. Kihara
                                                              T. Hayashi
                                                                 Lepidum
                                                                 Y. Ioku
                                                            Yahoo! Japan
                                                            May 18, 2012


                Mutual Authentication Protocol for HTTP
                     draft-oiwa-http-mutualauth-11

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."




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   This Internet-Draft will expire on November 19, 2012.

Copyright Notice

   Copyright (c) 2012 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.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Relations to other technologies  . . . . . . . . . . . . .  6
       1.1.1.  Technologies updated or superceded by this proposal  .  6
         1.1.1.1.  HTTP Basic and Digest authentication . . . . . . .  6
         1.1.1.2.  HTML Form authentication . . . . . . . . . . . . .  6
       1.1.2.  Technologies not updated by this proposal  . . . . . .  7
         1.1.2.1.  Federated identity/authorization management  . . .  7
         1.1.2.2.  HTTPS and HTTPS client-certificate
                   authentication . . . . . . . . . . . . . . . . . .  8
         1.1.2.3.  Relationship with local identity-management
                   frameworks . . . . . . . . . . . . . . . . . . . .  8
         1.1.2.4.  HTTP and HTTP authentication architecture  . . . .  8
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  9
     1.3.  Document Structure and Related Documents . . . . . . . . .  9
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 10
     2.1.  Messages Overview  . . . . . . . . . . . . . . . . . . . . 10
     2.2.  Typical Flows of the Protocol  . . . . . . . . . . . . . . 11
     2.3.  Alternative Flows  . . . . . . . . . . . . . . . . . . . . 14
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.1.  Values . . . . . . . . . . . . . . . . . . . . . . . . . . 16
       3.1.1.  Tokens . . . . . . . . . . . . . . . . . . . . . . . . 16
       3.1.2.  Strings  . . . . . . . . . . . . . . . . . . . . . . . 17
       3.1.3.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . 17
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.1.  401-INIT and 401-STALE . . . . . . . . . . . . . . . . . . 19
     4.2.  req-KEX-C1 . . . . . . . . . . . . . . . . . . . . . . . . 21
     4.3.  401-KEX-S1 . . . . . . . . . . . . . . . . . . . . . . . . 22
     4.4.  req-VFY-C  . . . . . . . . . . . . . . . . . . . . . . . . 23
     4.5.  200-VFY-S  . . . . . . . . . . . . . . . . . . . . . . . . 24
   5.  Authentication Realms  . . . . . . . . . . . . . . . . . . . . 25
     5.1.  Resolving Ambiguities  . . . . . . . . . . . . . . . . . . 26
   6.  Session Management . . . . . . . . . . . . . . . . . . . . . . 27
   7.  Validation Methods . . . . . . . . . . . . . . . . . . . . . . 29
   8.  Authentication Extensions  . . . . . . . . . . . . . . . . . . 30
   9.  Decision Procedure for Clients . . . . . . . . . . . . . . . . 31
   10. Decision Procedure for Servers . . . . . . . . . . . . . . . . 36
   11. Authentication Algorithms  . . . . . . . . . . . . . . . . . . 38
     11.1. Support Functions and Notations  . . . . . . . . . . . . . 39
     11.2. Default Functions for Algorithms . . . . . . . . . . . . . 40
   12. Application Channel Binding  . . . . . . . . . . . . . . . . . 41
   13. Application for Proxy Authentication . . . . . . . . . . . . . 41
   14. Methods to Extend This Protocol  . . . . . . . . . . . . . . . 42
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 43
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 43
     16.1. Security Properties  . . . . . . . . . . . . . . . . . . . 43
     16.2. Denial-of-service Attacks to Servers . . . . . . . . . . . 44



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     16.3. Implementation Considerations  . . . . . . . . . . . . . . 44
     16.4. Usage Considerations . . . . . . . . . . . . . . . . . . . 45
   17. Notice on Intellectual Properties  . . . . . . . . . . . . . . 45
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 46
     18.2. Informative References . . . . . . . . . . . . . . . . . . 47
   Appendix A.  (Informative) Draft Remarks from Authors  . . . . . . 48
   Appendix B.  (Informative) Draft Change Log  . . . . . . . . . . . 48
     B.1.  Changes in Revision 11 . . . . . . . . . . . . . . . . . . 48
     B.2.  Changes in Revision 10 . . . . . . . . . . . . . . . . . . 49
     B.3.  Changes in Revision 09 . . . . . . . . . . . . . . . . . . 49
     B.4.  Changes in Revision 08 . . . . . . . . . . . . . . . . . . 50
     B.5.  Changes in Revision 07 . . . . . . . . . . . . . . . . . . 50
     B.6.  Changes in Revision 06 . . . . . . . . . . . . . . . . . . 50
     B.7.  Changes in Revision 05 . . . . . . . . . . . . . . . . . . 51
     B.8.  Changes in Revision 04 . . . . . . . . . . . . . . . . . . 51
     B.9.  Changes in Revision 03 . . . . . . . . . . . . . . . . . . 51
     B.10. Changes in Revision 02 . . . . . . . . . . . . . . . . . . 51
     B.11. Changes in Revision 01 . . . . . . . . . . . . . . . . . . 52
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52































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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.  This makes it impossible for fake website owners
      to persuade users that they have 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, along with [I-D.oiwa-http-auth-extension], also



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   proposes several extensions to the current HTTP authentication
   framework, to replace current widely-used form-based Web
   authentication.  The extensions provided include:

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

   o  non-mandatory, optional authentication on HTTP (Section 8),

   o  log out from both server and client side (Section 8), and

   o  finer control for redirection depending on authentication status
      (Section 8).

1.1.  Relations to other technologies

1.1.1.  Technologies updated or superceded by this proposal

1.1.1.1.  HTTP Basic and Digest authentication

   The main purpose of this proposal is obviously updating the two
   existing HTTP authentication methods, Basic and Digest [RFC2617].

   HTTP Basic authentication, as its name suggests, provides very simple
   authentication mechanism using plain-text password directly upon the
   HTTP transport.  HTTP Digest authentication focuses on mitigating the
   fundamental weakness of Basic authentication by using MD5-based
   hashing to the authentication, but that has almost failed to deploy
   due to improper implementations, interoperability problems, and
   missing feature implementations before MD5 has deprecated by its
   cryptographic weakness.  Digest also has a fundamental problem that
   the server-side must posses a password-equivalent to perform
   authentication, which increases risks of server-side data leakage.

1.1.1.2.  HTML Form authentication

   Another aim of this protocol is (at least) partially replacing the
   HTML form authentication.  Because of inflexibility of the HTTP Basic
   authentication, recent Web applications tend to use application-level
   implementations for user authentication using HTML Forms and Web
   browser rendering engines.  However, that method has many potential
   security weaknesses as same as the HTTP Basic authentication as it
   uses plaintext.  Considering server-impersonations and existence of
   human-forging rogue servers (i.e. phishing), script-based
   implementations of hash-based authentication does not help, because
   its behavior is completely controlled by the web-page content itself,
   which is possibly provided by such a rogue server.  This also closes
   any possibilities for extending HTML forms to implement cryptography,



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   as its user-interface could not be prevented from being imitated
   using plain-text forms.  Using HTTP-level authentication is better in
   this field, because it is under the control of the client software
   (Web browsers), which can enforce security checks regardless of
   server-provided contents.

   Of course, we could not ignore the strong reasons of favoring Form
   authentication over Basic authentication: its flexibility.  HTTP
   authentication framework lacks many features for recent Web
   applications, mainly for interactions between HTTP-level
   authentications and application-level management of "authentication
   sessions".  As long as current HTTP-layer (and lower-layer)
   authentication are used, the new method would share the same problem.
   To solve this problem, this protocol has a companion mechanism for
   application-level control of authentication behaviors as a separate
   draft [I-D.oiwa-http-auth-extension].  By using this additional
   mechanism, Web applications can implement most of these required
   features as easy as just calling an already-provided API for them.

1.1.2.  Technologies not updated by this proposal

1.1.2.1.  Federated identity/authorization management

   There are several technologies (protocols, frameworks, or systems)
   for managing authentications/authorizations involving multiple-
   parties: some of those examples are OAuth, OpenID Connect, SAML etc.
   These technologies can be further divided to two categories:
   federated authentication and authorization delegation, although some
   of these technologies cover both.

   Federated authentication provides so-called "three-legged
   authentication": provided the result of user authentication to a
   single entity (identity provider) and the user's consent, the
   mechanism can provide other entities assertion of the user's identity
   without performing a separate identity management by every entity.
   Authorization delegation gives a mechanism for transferring a part of
   the user's privilege on an entity (resource owners) to another entity
   without requiring users give away the full credential for the
   authentication.

   Essentially, both of those technologies are transforming a result of
   conventional, one-by-one (two-legged) authentication into a multi-
   party privilege management.  The purpose of this protocol is to
   secure the very part of the two-legged authentication, and so it can
   be naturally combined with existing federated management frameworks
   for increasing security of the entire system.

   Additionally, this protocol can provide a secure peer-to-peer shared



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   key generated during authentication to the higher-layer applications
   Section 12.  These keys can be possibly used by such federating
   mechanisms in future for simplifying/securing the framework.

1.1.2.2.  HTTPS and HTTPS client-certificate authentication

   This protocol will not replace the wide-spread and widely-accepted
   technology of SSL/TLS and HTTPS.  This protocol will still relying on
   the HTTPS for the integrity and secrecy of the HTTP payload.  This
   protocol ensures users the integrity and secrecy of the
   authentication credentials, and authenticity of the talking peer
   server.

   Client certificate (and other public-key-based) authentications have
   a fair-amount of applications (mainly for high-assurance
   applications), and there are possible needs for redesigning/updating
   the whole framework.  However, currently public-key-based user-
   authentication and connection-based user identification is out-of-
   scope of this proposal.

1.1.2.3.  Relationship with local identity-management frameworks

   There are several existing frameworks for managing user identity of
   tightly-managed, closed group of users, such as Kerberos etc.  Some
   of these have defined a bridging protocol for HTTP authentication.
   This protocol does not currently aim to replace such existing
   frameworks.

   More precisely, requirements for those framework and usual Web user
   authentication differ fundamentally.  In such framework, user
   authentication in performed first, and the result of the
   authentication tends to be shared in all applications, sometimes even
   shared regardless of the underlying protocols.  In those systems, it
   is almost never likely to use a multiple identity to be used inside a
   single server and inside a single client machine at the same time.
   In such applications, connection-based or even a machine-based
   authentication can be used without a trouble.  This is not a case for
   the general Web authentication applications.

1.1.2.4.  HTTP and HTTP authentication architecture

   Although HTTP and generic HTTP authentication architecture lacks some
   required features (see above), the whole structure of per-request,
   per-resource authentication is well-suited for general Web
   applications compared with connection-based or machine-based
   authentication/authorization framework (those which tie user identity
   to either connections or machines).  The whole protocol in this
   specification is designed on top of the framework of



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   [I-D.ietf-httpbis-p7-auth].  Small extensions to the framework in
   this specification and [I-D.oiwa-http-auth-extension], which are
   designed for filling the missing features, are carefully designed so
   that it can be implemented easily only by the client-side without
   changing the whole framework.

1.2.  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.

   This document treats target (codomain) of hash functions to be
   natural numbers.  The notation OCTETS(H(s)) gives a usual octet-
   string output of hash function H applied to string s.

1.3.  Document Structure and Related Documents

   The entire document is organized as follows:

   o  Section 2 presents an overview of the protocol design.

   o  Sections 3 to 10 define a general framework of the Mutual
      authentication protocol.  This framework is independent of
      specific cryptographic primitives.

   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.





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   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.

   In addition, there are two companion documents which are referred
   from/related to this specification:

   o  [I-D.oiwa-http-mutualauth-algo]: defines a cryptographic
      primitives which can be used with this protocol framework. [draft
      note: it is separated so that it may be replaced with another
      crypto in future.  We need at least one example for testing/
      implementing this protocol, so here it is.]

   o  [I-D.oiwa-http-auth-extension]: defines a small but useful
      extensions to the current HTTP authentication framework so that it
      can support application-level semantics of existing Web systems.


2.  Protocol Overview

   The protocol, as a whole, is designed as a natural extension to the
   HTTP protocol [I-D.ietf-httpbis-p1-messaging] using a framework
   defined in [I-D.ietf-httpbis-p7-auth].  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 Overview

   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.

      *  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.



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      *  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]
                       kc1="...", ...              |<-- [user info]
            |                                      |
            |  <-------------- (4) 401-KEX-S1 ---  |
            |           401 Authentication Required
            |           WWW-Authenticate: Mutual sid=..., ks1="...", ...
            |                                      |
        [compute] (5) compute session secret   [compute]
            |                                      |
            |                                      |
            |  ---- (6) req-VFY-C -------------->  |
        GET / HTTP/1.1                             |--> [verify (6)]
        Authorization: Mutual sid=...,             |<-- OK
                       vkc="...", ...              |
            |                                      |
            |  <--------------- (7) 200-VFY-S ---  |
   [verify  |           200 OK                     |
     (7)]<--|           Authentication-Info: Mutual vks="..."
            |                                      |
            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 9 and 10.


3.  Message Syntax

   Throughout this specification, The syntax is denoted in the extended
   augmented BNF syntax defined in [I-D.ietf-httpbis-p1-messaging] and
   [RFC5234].  The following elements are quoted from [RFC5234],
   [I-D.ietf-httpbis-p1-messaging] and [I-D.ietf-httpbis-p7-auth]:
   DIGIT, ALPHA, SP, auth-scheme, quoted-string, auth-param, header-
   field, token, challenge, and credential.



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   The Mutual authentication protocol uses three headers:
   WWW-Authenticate (in responses with status code 401), Authorization
   (in requests), and Authentication-Info (in responses other than 401
   status).  These headers follow a common framework described in
   [I-D.ietf-httpbis-p7-auth].  The detailed meanings for these headers
   are contained in Section 4.

   The framework in [I-D.ietf-httpbis-p7-auth] defines the syntax for
   the headers WWW-Authenticate and Authorization as the syntax elements
   "challenge" and "credentials", respectively.  The "auth-scheme"
   contained in those headers MUST be "Mutual" throughout this protocol
   specification.  The syntax for "challenge" and "credentials" to be
   used with the "Mutual" auth-scheme SHALL be name-value pairs (#auth-
   param), not the "b64token" defined in [I-D.ietf-httpbis-p7-auth].

   The Authentication-Info: header used in this protocol SHALL contain
   the value in same syntax as those the "WWW-Authenticate" header, i.e.
   the "challenge" syntax element.

   In HTTP, the WWW-Authenticate header may contain more than one
   challenges.  Client implementations SHOULD be aware of and be capable
   of handle those cases correctly.

3.1.  Values

   The parameter values contained in challenge/credentials MUST be
   parsed strictly conforming to the HTTP semantics (especially un-
   quoting of the string parameter values).  In this protocol, those
   values are further categorized into the following value types: tokens
   (bare-token and extensive-token), string, integer, hex-fixed-number,
   and base64-fixed-number.

   For clarity, implementations are encouraged to use the canonical
   representations specified in the following subsections for sending
   values.  Recipients SHOULD accept both quoted and unquoted
   representations interchangeably as specified in HTTP.

3.1.1.  Tokens

   For sustaining both security and extensibility at the same time, this
   protocol defines a stricter sub-syntax for the "token" to be used.
   The extensive-token values SHOULD follow the following syntax (after
   HTTP value parsing):

    bare-token       = 1*(DIGIT / ALPHA / "-" / "_")
    extension-token  = "-" bare-token 1*("." bare-token)
    extensive-token  = bare-token / extension-token




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                   Figure 3: BNF syntax for token values

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

   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 SHOULD use the extension-tokens
   with format "-<bare-token>.<domain-name>", where <domain-name> is a
   validly registered (sub-)domain name on the Internet owned by the
   party who defines the extensions.

   Bare-tokens and extensive-tokens are also used for parameter names
   (of course in the unquoted form).  Requirements for using the
   extension-token for the parameter names are the same as the above.

   The canonical format for bare-tokens and tokens are unquoted tokens.

3.1.2.  Strings

   All character 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], without any leading BOM characters.  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).

   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.

   The canonical format for strings are quoted-string.

3.1.3.  Numbers

   The following syntax definitions gives a syntax for number-type
   values:

    integer          = "0" / (%x31-39 *DIGIT)      ; no leading zeros
    hex-fixed-number = 1*(2(DIGIT / %x41-46 / %x61-66))
    base64-fixed-number = 1*( ALPHA / DIGIT /
                           "-" / "." / "_" / "~" / "+" / "/" ) *"="




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                   Figure 4: BNF syntax for number types

   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).  Those values
   are case-insensitive, and SHOULD be sent in lower-case.  When these
   values are generated from any cryptographic values, they SHOULD have
   their "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 radix-256
   binary representation as an octet string.  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.  Implementations decoding base64-
   fixed-number SHOULD reject any input data with invalid characters,
   excess/insufficient paddings, or non-canonical pad bits (See Sections
   3.1 to 3.5 of [RFC4648]).

   The canonical format for integer and hex-fixed-number are unquoted
   tokens, and that for base64-fixed-number is quoted-string.


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 9
   and 10.

   In the descriptions below, the type of allowable values for each
   header parameter is shown in parenthesis after each parameter name.
   The "algorithm-determined" type means that the acceptable value for



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   the parameter is one of the types defined in Section 3, and is
   determined by the value of the "algorithm" parameter.  The parameters
   marked "mandatory" SHALL be contained in the message.  The parameters
   marked "non-mandatory" MAY either be contained or omitted in the
   message.  Each parameter SHALL appear in each headers exactly once at
   most.

   All credentials and challenges MAY contain any parameters not
   explicitly specified in the following sections.  Recipients who do
   not understand such parameters MUST silently ignore those.  However,
   all credentials and challenges MUST meet the following criteria:

   o  For responses, the parameters "reason", any "ks*" (where * stands
      for any decimal integers), and "vks" are mutually exclusive: any
      challenge MUST NOT contain two or more parameters among them.
      They MUST NOT contain any "kc*" and "vkc" parameters.

   o  For requests, the parameters "kc*" (where * stands for any decimal
      integers), and "vks" are mutually exclusive and any challenge
      MUST NOT contain two or more parameters among them.  They MUST NOT
      contain any "ks*" and "vks" parameters.

4.1.  401-INIT and 401-STALE

   Every 401-INIT or 401-STALE message SHALL be a valid HTTP 401-status
   (Authentication Required) message containing one (and only one:
   hereafter not explicitly noticed) "WWW-Authenticate" header
   containing a "reason" parameter in the challenge.  The challenge
   SHALL contain all of the parameters marked "mandatory" below, and MAY
   contain those marked "non-mandatory".

   version:       (mandatory extensive-token) should be the token
                  "-draft11" 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
                  [I-D.oiwa-http-mutualauth-algo] 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.






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   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 "single-port" type domain in
                  Section 5.

   realm:         (mandatory string) is a UTF-8 encoded string
                  representing the name of the authentication realm
                  inside the authentication domain.  As specified in
                  [I-D.ietf-httpbis-p7-auth], this value MUST always be
                  sent in the quoted-string form.

   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.

   reason:        (mandatory extensive-token) SHALL be an extensive-
                  token which describes the possible reason of the
                  failed authentication/authorization.  Both servers and
                  clients SHALL understand and support the following
                  three tokens:

                  *  initial: authentication was not tried because there
                     was no Authorization header in the corresponding
                     request.

                  *  stale-session: the provided sid; in the request was
                     either unknown to or expired in the server.

                  *  auth-failed: authentication trial was failed by
                     some reasons, possibly with a bad authentication
                     credentials.




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                  Implementations MAY support the following tokens or
                  any extensive-tokens defined outside this
                  specification.  If clients has received any unknown
                  tokens, these SHOULD treat these as if it were "auth-
                  failed" or "initial".

                  *  reauth-needed: server-side application requires a
                     new authentication trial, regardless of the current
                     status.

                  *  invalid-parameters: authentication was not even
                     tried in the server-side because some parameters
                     are not acceptable.

                  *  internal-error: authentication was not even tried
                     in the server-side because there is some troubles
                     on the server-side.

                  *  user-unknown: a special case of auth-failed,
                     suggesting that the provided user-name is invalid.
                     The use of this parameter is NOT RECOMMENDED for
                     security implications, except for special-purpose
                     applications which makes this value sense.

                  *  invalid-credential: ditto, suggesting that the
                     provided user-name was valid but authentication was
                     failed.  The use of this parameter is
                     NOT RECOMMENDED as the same as the above.

   The algorithm specified in this header will determine the types
   (among those defined in Section 3) and the values for K_c1, K_s1,
   VK_c and VK_s.

   Among these messages, those with the reason parameter of value
   "stale-session" will be called "401-STALE" messages hereafter,
   because these have a special meaning in the protocol flow.  Messages
   with any other reason parameters will be called "401-INIT" messages.

4.2.  req-KEX-C1

   Every req-KEX-C1 message SHALL be a valid HTTP request message
   containing an "Authorization" header with a credential containing a
   "kc1" parameter.

   The credential SHALL contain the parameters with the following names:






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   version:       (mandatory, extensive-token) should be the token
                  "-draft11" 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.

   kc1:           (mandatory, algorithm-determined) is the client-side
                  key exchange value K_c1, which is specified by the
                  algorithm that is used.

   rekey-sid:     (non-mandatory, hex-fixed-number): reserved for future
                  extensions (see rekey-method in "200-VFY-S" message).

4.3.  401-KEX-S1

   Every 401-KEX-S1 message SHALL be a valid HTTP 401-status
   (Authentication Required) response message containing a
   "WWW-Authenticate" header with a challenge containing a "ks1"
   parameter.

   The challenge SHALL contain the parameters with the following names:

   version:       (mandatory, extensive-token) should be the token
                  "-draft11" 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.
                  See Section 6 for more details.

   ks1:           (mandatory, algorithm-determined) is the server-side
                  key exchange value K_s1, which is specified by the
                  algorithm.






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   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 parameter 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 parameter 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 parameter MUST be
                  inside the specified auth-domain: if not, clients
                  SHOULD ignore such elements.

4.4.  req-VFY-C

   Every req-VFY-C message SHALL be a valid HTTP request message
   containing an "Authorization" header with a credential containing a
   "vkc" parameter.

   The parameters contained in the header are as follows:

   version:       (mandatory, extensive-token) should be the token
                  "-draft11" 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.




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

4.5.  200-VFY-S

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

   The parameters contained in the header are as follows:

   version:       (mandatory, extensive-token) should be the token
                  "-draft11" 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.

   vks:           (mandatory, algorithm-determined) is the server-side
                  authentication verification value VK_s, 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.  If logout-timeout
                  parameters are specified both in an
                  Authentication-Info header and an
                  Authentication-Control header
                  ([I-D.oiwa-http-auth-extension]), the client SHOULD
                  respect the smaller one of those and ignore the other.

   rekey-method:  (non-mandatory, extensive-token): defining a
                  credential used for reestablishing a new session with
                  a new sid.  It must be either omitted or the token
                  "passwords" at the current specification.  The bare-
                  tokens "refresh-key" and "refresh-key-global" are
                  reserved for future extensions.




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   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.


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 protection spaces
   to be set up inside each host.  Furthermore, the protocol 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  Single-server type: The string in format
      "<scheme>://<host>:<port>", where <scheme>, <host>, and <port> are
      the corresponding URI parts of the request 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).  The port part MUST NOT
      contain leading zeros.  Use this when authentication is only valid
      for specific protocol (such as https).

   o  Single-host type: 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



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      and https) and ports, but not over different hosts.

   o  Wildcard-domain type: 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
      [RFC6265], 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. these SHALL be in the form of the LDH labels in IDNA
   [RFC5890].  All "port"s MUST be in the shortest, unsigned, decimal
   number notation.  Not obeying these requirements will cause failure
   of valid authentication attempts.

5.1.  Resolving Ambiguities

   In the above 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 9).

   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
      if 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 earlier 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.



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   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.

   Sessions and session identifiers are local to each server (defined by
   scheme, host and port) inside an authentication domain; the clients
   MUST establish separate sessions for each port of a host to be
   accessed.  Furthermore, sessions and identifiers are also local to
   each authentication realm, even if these are provided from the same
   servers.  The same session identifiers provided either from different
   servers or for different realms SHOULD be treated as independent
   ones.

   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 parameter 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 parameter) MUST satisfy the following
   conditions:

   o  It is a natural number.

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





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   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 parameter 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 10.  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).

   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.







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7.  Validation Methods

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

   The valid tokens for the validation parameter 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.

   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 parameter upon reception of the 401-INIT
   messages.

   However, when the client is a Web browser with any scripting



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   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.


8.  Authentication Extensions

   The HTTP authentication extensions described in
   [I-D.oiwa-http-auth-extension] is a definitive part of this protocol.
   Interactive clients (e.g.  Web browsers) supporting this protocol are
   RECOMMENDED to support non-mandatory authentication and the
   Authentication-Control header defined there, except the "auth-style"
   parameter.  This specification also proposes (however, not mandates)
   default "auth-style" to be "non-modal".  Web applications SHOULD
   however consider the security impacts of the behaviors of clients
   that do not support these headers.

   Authentication-initializing messages with the
   Optional-WWW-Authenticate header are used where 401-INIT response is
   valid.  Such a message is called a 200-Optional-INIT message in this
   document.  (It will not replace other 401-type messages such as
   401-STALE and 401-KEX-S1.)




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9.  Decision Procedure for Clients

   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 VK_s 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.

   Note 1:  These transitions MAY be accepted by clients, but
       NOT RECOMMENDED for servers to initiate.

   Any kind of response (including a normal response) other than those



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   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 5 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 5: State diagram for clients



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10.  Decision Procedure for Servers

   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 parameter.

   o  The algorithm used.

   o  The authentication realm.

   o  The state of the protocol: one of "key exchanging",
      "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 "key exchanging", the values of K_c1 and S_s1.

   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 parameter kc1, 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 K_s1, 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 K_s1 and send a
         401-KEX-S1 response.

      The created session has the "key exchanging" 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 in the "inactive" state, 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 in the "authenticated" 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



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         "inactive" status.

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

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

      *  Otherwise, send a 200-VFY-S response.  If the session was in
         the "key exchanging" 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 "key exchanging" status SHOULD be kept unless there is an
   emergency situation such as a server reboot or a table capacity
   overflow.


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 K_c1, K_s1 (exchanged on wire) and S_c1, S_s1
      (kept secret in each peer).

   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.

   Some examples:

      VI(0) = "\000" (in C string notation)

      VI(100) = "d"

      VI(10000) = "\316\020"





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      VI(1000000) = "\275\204@"

      VS("") = "\000"

      VS("Tea") = "\003Tea"

      VS("Caf<e acute>" [in UTF-8]) = "\005Caf\303\251"

      VS([10000 "a"s]) = "\316\020aaaaa..." (10002 octets)

   [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 3.1.3 for the definition of "natural length".

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 parameter 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 VK_c and VK_s are derived by the following equation.

   VK_c = H(octet(4) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) | VI(nc)
   | VS(v))

   VK_s = H(octet(3) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) | VI(nc)
   | VS(v))




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


12.  Application Channel Binding

   Applications and upper-layer communication protocols may need
   authentication binding to the HTTP-layer authenticated user.  Such
   applications MAY use the following values as a standard shared
   secret.

   These values are parameterized with an optional octet string (t)
   which may be arbitrarily chosen by each applications or protocols.
   If there is no appropriate value to be specified, use a null string
   for t.

   For applications requiring binding to either an authenticated user or
   a shared-key session (to ensure that the requesting client is
   certainly authenticated), the following value b_1 MAY be used.

   b_1 = OCTETS(H(OCTETS(H(octet(6) | OCTETS(K_c1) | OCTETS(K_s1) |
   OCTETS(z) | VI(0) | VS(v))) | VS(t))).

   For applications requiring binding to a specific request (to ensure
   that the payload data is generated for the exact HTTP request), the
   following value b_2 MAY be used.

   b_2 = OCTETS(H(OCTETS(H(octet(7) | OCTETS(K_c1) | OCTETS(K_s1) |
   OCTETS(z) | VI(nc) | VS(v))) | VS(t))).

   Note: Channel bindings to lower-layer transports (TCP and TLS) are
   defined in Section 7.


13.  Application for Proxy Authentication

   The authentication scheme defined by the previous sections can be
   applied m.m. for proxy authentications.  In such cases, the following
   alterations MUST be applied:

   o  The 407 status is to be sent and recognized for places where the
      401 status is used,

   o  Proxy-Authenticate: header is to be used for places where WWW-
      Authenticate: is used,




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   o  Proxy-Authorization: header is to be used for places where
      Authorization: is used,

   o  Proxy-Authentication-Info: header is to be used for places where
      Authentication-Info: is used,

   o  The auth-domain parameter is fixed to the host-name of the proxy,
      which means to cover all requests processed through the specific
      proxy,

   o  The limitation for the paths contained in the path parameter of
      401-KEX-S1 messages is disregarded,

   o  The omission of the path parameter of 401-KEX-S1 messages means
      that the authentication realm will potentially cover all requests
      processed by the proxy,

   o  The scheme, host name and the port of the proxy is used for
      validation tokens, and

   o  Authentication extension in [I-D.oiwa-http-auth-extension] is not
      applicable.

   The requirements for client software to display the authentication
   status to the end-user is also not applicable for proxy
   authentication.  If the client software supports both end-to-end and
   proxy authentication using this protocol, it SHOULD be careful that
   the authentication status of the proxy communication will never be
   confused by users with authentication statuses of the end-to-end
   resource authentications.


14.  Methods to Extend This Protocol

   If a private 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. (standardized or being-
   standardizing extensions MAY use either bare-tokens or extension-
   tokens.)

   Specifications defining authentication algorithms MAY use other
   representations for the parameters "kc1", "ks1", "vkc", and "vks",
   replace those parameter names, and/or add parameters to the messages
   containing those parameters in supplemental specifications, provided
   that syntactic and semantic requirements in Section 3,
   [I-D.ietf-httpbis-p1-messaging] and [I-D.ietf-httpbis-p7-auth] are
   satisfied.  Any parameters starting with "kc", "ks", "vkc" or "vks"
   and followed by decimal natural numbers (e.g. kc2, ks0, vkc1, vks3



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   etc.) are reserved for this purpose.  If those specifications use
   names other than those mentioned above, it is RECOMMENDED to use
   extension-tokens to avoid any parameter 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 parameters provided that the
   domain part in the token is appropriately used.


15.  IANA Considerations

   When bare-tokens are used for the authentication-algorithm, pwd-hash,
   and validation parameters 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.


16.  Security Considerations

16.1.  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).








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16.2.  Denial-of-service Attacks to Servers

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

   However, if a malicious client sends too many requests of key
   exchanges (req-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.

16.3.  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



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      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 parameter), if the server certificate has been changed since
      the pages were received, the peer is RECOMMENDED to be revalidated
      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.

16.4.  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" parameter 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.


17.  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



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   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.


18.  References

18.1.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R., Lafon, Y., and J. Reschke, "HTTP/1.1, part
              1: URIs, Connections, and Message Parsing",
              draft-ietf-httpbis-p1-messaging-19 (work in progress),
              March 2012.

   [I-D.ietf-httpbis-p7-auth]
              Fielding, R., Lafon, Y., and J. Reschke, "HTTP/1.1, part
              7: Authentication", draft-ietf-httpbis-p7-auth-19 (work in
              progress), March 2012.

   [I-D.oiwa-http-auth-extension]
              Oiwa, Y., Watanabe, H., Takagi, H., Kihara, B., Hayashi,
              T., and Y. Ioku, "HTTP Authentication Extensions for
              Interactive Clients", draft-oiwa-http-auth-extension-01
              (work in progress), May 2012.

   [I-D.oiwa-http-mutualauth-algo]
              Oiwa, Y., Watanabe, H., Takagi, H., Kihara, B., Hayashi,
              T., and Y. Ioku, "Mutual Authentication Protocol for HTTP:
              KAM3-based Cryptographic Algorithms",
              draft-oiwa-http-mutualauth-algo-02 (work in progress),
              May 2012.

   [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.



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   [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.

18.2.  Informative References

   [I-D.ietf-precis-framework]
              Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation and Comparison of Internationalized Strings in
              Application Protocols", draft-ietf-precis-framework-03
              (work in progress), May 2012.

   [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.

   [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.

   [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



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              for TLS", RFC 5929, July 2010.

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


Appendix A.  (Informative) Draft Remarks from Authors

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

   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  Adopt [I-D.ietf-precis-framework] for replacing SASLprep
      reference.  Especially, use NFC canonicalization instead of NFKC.

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


Appendix B.  (Informative) Draft Change Log

B.1.  Changes in Revision 11

   o  Message syntax definition reverted to pre-07 style as httpbis-p1
      and p7 now defines a precise rule for parameter value parsing.

   o  Replaced "stale" parameter with more infomative/extensive "reason"
      parameter in 401-INIT and 401-STALE.

   o  Reserved "rekey-sid" and "rekey-method" parameters for future
      extensions.

   o  Added descriptions for replacing/non-replacing existing
      technologies.






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B.2.  Changes in Revision 10

   o  The authentication extension parts (non-mandatory authentication
      and authentication controls) are separated to yet another draft.

   o  The default auth-domain parameter is changed to the full scheme-
      host-port syntax, which is consistent with usual HTTP
      authentication framework behavior.

   o  Provision for application channel binding is added.

   o  Provision for proxy access authentication is added.

   o  Bug fix: syntax specification of sid parameter was wrong: it was
      inconsistent with the type specified in the main text (the bug
      introduced in -07 draft).

   o  Terminologies for headers are changed to be in harmony with
      httpbis drafts (e.g. field to parameter).

   o  Syntax definitions are changed to use HTTP-extended ABNF syntax,
      and only the header values are shown for header syntax, in harmony
      with httpbis drafts.

   o  Names of parameters and corresponding mathematical values are now
      renamed to more informative ones.  The following list shows
      correspondence between the new and the old names.

   +------------+----------+-------------------------------------------+
   | new name   | old name | description                               |
   +------------+----------+-------------------------------------------+
   | S_c1, S_s1 | s_a, s_b | client/server-side secret randoms         |
   | K_c1, K_s1 | w_a, w_b | client/server-side exchanged key          |
   |            |          | components                                |
   | kc1, ks1   | wa, wb   | parameter names for those                 |
   | VK_c, VK_s | o_a, o_b | client/server-side key verifiers          |
   | vkc, vks   | oa, ob   | parameter names for those                 |
   | z          | z        | session secrets                           |
   +------------+----------+-------------------------------------------+

B.3.  Changes in Revision 09

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

   o  Names of the messages are changed to more informative ones than
      before.  The following is the correspondence table of those names:




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   +-------------------+-----------------+-----------------------------+
   | new name          | old name        | description                 |
   +-------------------+-----------------+-----------------------------+
   | 401-INIT          | 401-B0          | initial response            |
   | 401-STALE         | 401-B0-stale    | session key expired         |
   | req-KEX-C1        | req-A1          | client->server key exchange |
   | 401-KEX-S1        | 401-B1          | server->client key exchange |
   | req-VFY-C         | req-A3          | client->server auth.        |
   |                   |                 | verification                |
   | 200-VFY-S         | 200-B4          | server->client auth.        |
   |                   |                 | verification                |
   | 200-Optional-INIT | 200-Optional-B0 | initial with non-mandatory  |
   |                   |                 | authentication              |
   +-------------------+-----------------+-----------------------------+

B.4.  Changes in Revision 08

   o  The English text has been revised.

B.5.  Changes in Revision 07

   o  Adapt to httpbis HTTP/1.1 drafts:

      *  Changed definition of extensive-token.

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

   o  To simplify the whole spec, the type of nonce-counter related
      parameters 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.

B.6.  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.





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   o  Typos and confusing phrases are fixed.

   o  Several "future considerations" are added.

B.7.  Changes in Revision 05

   o  A new parameter 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.

B.8.  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.

B.9.  Changes in Revision 03

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

   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.

B.10.  Changes in Revision 02

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

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

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





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   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.

B.11.  Changes in Revision 01

   o  Several texts are rewritten for clarification.

   o  Added several security consideration clauses.


Authors' Addresses

   Yutaka Oiwa
   National Institute of Advanced Industrial Science and Technology
   Research Institute for Secure Systems
   Tsukuba Central 2
   1-1-1 Umezono
   Tsukuba-shi, Ibaraki
   JP

   Email: mutual-auth-contact-ml@aist.go.jp


   Hajime Watanabe
   National Institute of Advanced Industrial Science and Technology


   Hiromitsu Takagi
   National Institute of Advanced Industrial Science and Technology


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


   Tatsuya Hayashi
   Lepidum Co. Ltd.





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













































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