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Versions: (draft-oiwa-httpbis-mutualauth) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 8120

HTTPAUTH Working Group                                           Y. Oiwa
Internet-Draft                                               H. Watanabe
Intended status: Experimental                                  H. Takagi
Expires: January 7, 2016                                      ITRI, AIST
                                                                K. Maeda
                                                              T. Hayashi
                                                                 Lepidum
                                                                 Y. Ioku
                                                              Individual
                                                            July 6, 2015


                Mutual Authentication Protocol for HTTP
                     draft-ietf-httpauth-mutual-05

Abstract

   This document specifies a mutual authentication method for the Hyper-
   text Transfer 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.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 7, 2016.

Copyright Notice

   Copyright (c) 2015 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



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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Document Structure and Related Documents . . . . . . . . .  5
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Messages Overview  . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Typical Flows of the Protocol  . . . . . . . . . . . . . .  6
     2.3.  Alternative Flows  . . . . . . . . . . . . . . . . . . . .  9
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Non-ASCII extended header parameters . . . . . . . . . . . 11
     3.2.  Values . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.2.1.  Tokens . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.2.2.  Strings  . . . . . . . . . . . . . . . . . . . . . . . 13
       3.2.3.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  401-INIT and 401-STALE . . . . . . . . . . . . . . . . . . 15
     4.2.  req-KEX-C1 . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.3.  401-KEX-S1 . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.4.  req-VFY-C  . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.5.  200-VFY-S  . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Authentication Realms  . . . . . . . . . . . . . . . . . . . . 20
     5.1.  Resolving Ambiguities  . . . . . . . . . . . . . . . . . . 22
   6.  Session Management . . . . . . . . . . . . . . . . . . . . . . 22
   7.  Host Validation Methods  . . . . . . . . . . . . . . . . . . . 24
     7.1.  Applicability notes  . . . . . . . . . . . . . . . . . . . 26
     7.2.  Notes on tls-unique  . . . . . . . . . . . . . . . . . . . 26
   8.  Authentication Extensions  . . . . . . . . . . . . . . . . . . 27
   9.  String Preparation . . . . . . . . . . . . . . . . . . . . . . 27
   10. Decision Procedure for Clients . . . . . . . . . . . . . . . . 28
     10.1. General Principles and Requirements  . . . . . . . . . . . 28
     10.2. State machine for the client-side (informative)  . . . . . 30
   11. Decision Procedure for Servers . . . . . . . . . . . . . . . . 34
   12. Authentication Algorithms  . . . . . . . . . . . . . . . . . . 36
     12.1. Support Functions and Notations  . . . . . . . . . . . . . 37
     12.2. Default Functions for Algorithms . . . . . . . . . . . . . 38
   13. Application Channel Binding  . . . . . . . . . . . . . . . . . 39
   14. Application for Proxy Authentication . . . . . . . . . . . . . 40



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   15. Methods to Extend This Protocol  . . . . . . . . . . . . . . . 40
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
     16.1. Registry for Authentication Algorithms . . . . . . . . . . 41
     16.2. Registry for Password Hashes . . . . . . . . . . . . . . . 42
     16.3. Registry for Validation Methods  . . . . . . . . . . . . . 42
   17. Security Considerations  . . . . . . . . . . . . . . . . . . . 43
     17.1. Security Properties  . . . . . . . . . . . . . . . . . . . 43
     17.2. Denial-of-service Attacks to Servers . . . . . . . . . . . 44
       17.2.1. On-line Active Password Attacks  . . . . . . . . . . . 44
     17.3. Communicating the status of mutual authentication with
           users  . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     17.4. Implementation Considerations  . . . . . . . . . . . . . . 45
     17.5. Usage Considerations . . . . . . . . . . . . . . . . . . . 46
   18. Notice on Intellectual Properties  . . . . . . . . . . . . . . 46
   19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     19.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     19.2. Informative References . . . . . . . . . . . . . . . . . . 48
   Appendix A.  (Informative) Draft Remarks from Authors  . . . . . . 49
   Appendix B.  (Informative) Draft Change Log  . . . . . . . . . . . 49
     B.1.  Changes in Httpauth WG Revision 05 . . . . . . . . . . . . 50
     B.2.  Changes in Httpauth WG Revision 04 . . . . . . . . . . . . 50
     B.3.  Changes in Httpauth WG Revision 03 . . . . . . . . . . . . 50
     B.4.  Changes in Httpauth WG Revision 02 . . . . . . . . . . . . 50
     B.5.  Changes in Httpauth WG Revision 01 . . . . . . . . . . . . 50
     B.6.  Changes in Httpauth Revision 00  . . . . . . . . . . . . . 51
     B.7.  Changes in HttpBis Revision 00 . . . . . . . . . . . . . . 51
     B.8.  Changes in Revision 12 . . . . . . . . . . . . . . . . . . 51
     B.9.  Changes in Revision 11 . . . . . . . . . . . . . . . . . . 51
     B.10. Changes in Revision 10 . . . . . . . . . . . . . . . . . . 51
     B.11. Changes in Revision 09 . . . . . . . . . . . . . . . . . . 52
     B.12. Changes in Revision 08 . . . . . . . . . . . . . . . . . . 53
     B.13. Changes in Revision 07 . . . . . . . . . . . . . . . . . . 53
     B.14. Changes in Revision 06 . . . . . . . . . . . . . . . . . . 53
     B.15. Changes in Revision 05 . . . . . . . . . . . . . . . . . . 54
     B.16. Changes in Revision 04 . . . . . . . . . . . . . . . . . . 54
     B.17. Changes in Revision 03 . . . . . . . . . . . . . . . . . . 54
     B.18. Changes in Revision 02 . . . . . . . . . . . . . . . . . . 54
     B.19. Changes in Revision 01 . . . . . . . . . . . . . . . . . . 55
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55












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1.  Introduction

   This document specifies a mutual authentication method for Hyper-Text
   Transfer 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 authentication method proposed in this document is a general
   framework for using password-based authenticated key exchange (PAKE)
   and similar stronger cryptographic primitives on the HTTP.  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 tiny bit of information about the
      user's password.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   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 octet



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   strings.  The notation INT(H(s)) gives a numerical (natural-number)
   output of hash function H applied to string s.

1.2.  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 11 define a general framework of the Mutual
      authentication protocol.  This framework is independent of
      specific cryptographic primitives.

   o  Section 12 describes properties needed for cryptographic
      algorithms used with this protocol framework, and defines a few
      functions which will be shared among such cryptographic
      algorithms.

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

   o  The appendices contain some information that may help developers
      to implement the protocol.

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

   o  [I-D.ietf-httpauth-mutual-algo]: defines a cryptographic
      primitives which can be used with this protocol framework.

   o  [I-D.ietf-httpauth-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 [RFC7230] using a framework defined in [RFC7235].
   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.





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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.

      *  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,the session secret values will
      match.  This session secret will be used for access authentication
      of every individual request 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
      [RFC1939]), 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 without any
      interventions.  Possible cause of such cases might be either a
      man-in-the-middle attack or a mis-implementation.)






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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 (re-authentication)

         (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 10 and 11.


3.  Message Syntax

   Throughout this specification, The syntax is denoted in the extended
   augmented BNF syntax defined in [RFC7230] and [RFC5234].  The
   following elements are quoted from [RFC5234], [RFC7230] and
   [RFC7235]: 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 (usually 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 [RFC7235] and [I-D.ietf-httpbis-auth-info].  The
   detailed meanings for these headers are contained in Section 4.

   The framework in [RFC7235] 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 [RFC7235].

   The Authentication-Info: header used in this protocol SHALL follow
   the syntax defined in [I-D.ietf-httpbis-auth-info].

   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.  Non-ASCII extended header parameters

   All of parameters contained in the above three headers, except the
   "realm" field, MAY be extended to ISO 10646-1 values using the
   framework described in [RFC5987].  All servers and clients MUST be
   capable of receiving and sending values encoded in [RFC5987] syntax.

   If a value to be sent contains only ASCII characters, the field MUST
   be sent in clear using plain RFC 7235 syntax.  The syntax extended by
   RFC 5987 MUST NOT be used in this case.

   If a value (except the "realm" header) contains one or more non-ASCII
   characters, the parameter SHOULD be sent using the syntax defined in
   Section 3.2 of [RFC5987] as "ext-parameter".  Such parameter MUST
   have charset value of "UTF-8", and the language value MUST always be
   omitted (have an empty value).  The same parameter MUST NOT be sent
   twice or more, those in both plain- and extended-syntax.

   For example, a parameter "user" with value "Renee or France" SHOULD
   be sent as < user="Renee of France" >.  If the value is "Ren<e
   acute>e of France", it SHOULD be sent as < user*=UTF-
   8''Ren%C3%89e%20of%20France > instead.

   [RFC7235] requires realm parameter to be exist as its plain form (not
   as extended "realm*" parameter), so RFC 5987 syntax MUST NOT be used
   for this parameter.



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3.2.  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 RECOMMENDED 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.

3.2.1.  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

                   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.






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3.2.2.  Strings

   All character strings MUST be encoded to octet strings using the
   UTF-8 encoding [RFC3629] for the ISO 10646-1 character set
   [ISO.10646-1.1993].  Such strings MUST NOT contain any leading BOM
   characters (ZERO WIDTH NO-BREAK SPACE, U+FEFF or EF BB BF).  Both
   peers are RECOMMENDED to reject any invalid UTF-8 sequences that
   might cause decoding ambiguities (e.g., containing <"> in the second
   or later bytes 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 (as it may contain
   equal signs, plus signs and slashes), unless the parameter containing
   the string value will use extended syntax defined in [RFC5987].
   ([RFC5987] extended parameter will have unquoted encoded value, as
   defined there.)

3.2.3.  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 / "+" / "/" ) 0*2"="

                   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.



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   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 padding, 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 10
   and 11.

   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
   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.




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

   Every message in this section contains a "version" field, to detect
   future incompatible revisions of the protocol.  Implementations of
   the protocol described in this specification MUST always send a token
   "-wg-draft04", and recipients MUST reject messages which contain any
   other value as a version, unless another specification defines a
   behavior for that version.  [[Editorial Note: This token is updated
   on every draft revisions which will affect the wire protocol.  It
   will (shall) be updated to "1" in the final published RFC.]]

4.1.  401-INIT and 401-STALE

   Every 401-INIT or 401-STALE message SHALL be a valid HTTP 401-status
   (Authentication Required) message (or other 4XX statuses if sensible)
   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 "-wg-
                  draft04".

   algorithm:     (mandatory extensive-token) specifies the
                  authentication algorithm to be used.  The value MUST
                  be one of the tokens specified in
                  [I-D.ietf-httpauth-mutual-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.

   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-server" type domain in
                  Section 5.

   realm:         (mandatory string) is a string representing the name
                  of the authentication realm inside the authentication
                  domain.  As specified in [RFC7235], this value MUST
                  always be sent in the quoted-string form, and an



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                  [RFC5987] encoding MUST NOT be used.
                  The realm value sent from the server SHOULD be an
                  ASCII string.  Clients MAY treat any non-ASCII value
                  received in this field as one of a binary blob, an
                  NFC-normalized UTF-8 string, or an error.

   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 desirable.

   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.

                  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".





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                  *  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.

                  *  authz-failed: authentication was successful, but
                     access to the specified resource is not authorized
                     to the specific authenticated user.  (It might be
                     used along with either 401 or 403 status to
                     indicate that the authentication result is one of
                     highly likely reasons for the failed
                     authorization.)

   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 "-wg-
                  draft04".

   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.  The string SHOULD be prepared according to the
                  method presented in Section 9.

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

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 "-wg-
                  draft04".

   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.

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

   nc-window:     (mandatory, integer) the number of available nonce
                  number slots that the server will accept.  The value
                  of the nc-window parameter is RECOMMENDED to be 128 or
                  more.




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   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.
                  The all path elements contained in the parameter MUST
                  be inside the specified auth-domain; if not, clients
                  SHOULD ignore such elements.  For better performance,
                  recognition of this parameter by clients are
                  significantly important.

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 "-wg-
                  draft04".

   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 request number that is
                  unique among the requests sharing the same sid.  The
                  values of the nonce numbers SHOULD satisfy the
                  properties outlined in Section 6.

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







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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 "-wg-
                  draft04".

   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.

   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 MUST NOT automatically reuse the user names 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.



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   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>" or
      "<scheme>://<host>:<port>", where <scheme>, <host>, and <port> are
      the corresponding URI parts of the request URI.  If the default
      port (i.e. 80 for http and 443 for https) is used for the
      underlying HTTP communications, the port part MUST be omitted,
      regardless of whether it was present in the request-URI.  In other
      cases, the port part MUST be present, and it MUST NOT contain
      leading zeros.  Use this when authentication is only valid for
      specific protocol (such as https).  This format is equivalent to
      the ASCII serialization of a Web Origin, presented in Section 6.2
      of [RFC6454].

   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
      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.






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5.1.  Resolving Ambiguities

   In the above definitions of authentication domains, several domains
   will overlap each other.  If a client has already been authenticated
   to several realms applicable to the same server, the client may have
   a multiple list of the "path" parameters received with the
   "401-KEX-S1" message (see Section 4).  If these path lists have any
   overlap, a single URI may belong to multiple possible candidate of
   realms to be authenticated to.  In such cases, clients faces an
   ambiguity on deciding which credentials to be sent for a new request
   (in steps 3 and 4 of the decision procedure presented in Section 10).

   In such cases, clients MAY send requests which belongs to any of
   these candidate realms freely, or it MAY simply send an
   unauthenticated request and see for which realm the server request an
   authentication.  Server operators are RECOMMENDED to provide
   properly-configured "path" parameters (more precisely, disjoint path
   sets for each realms) for clients so that such ambiguities will not
   occur.

   The following procedure are one of the possible tactics for resolving
   ambiguity in such cases.

   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.

   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.


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



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   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 re-establish
   another session without informing the users.

   Sessions and session identifiers are local to each server (defined by
   scheme, host and port), even if an authentication domain covers
   multiple servers; 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 server.  The same session
   identifiers provided either from different servers or for different
   realms MUST 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 number (in the nc parameter) MUST satisfy the following
   conditions:

   o  It is a natural number.

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

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

   o  It is larger than (largest-nc - nc-window), where largest-nc is
      the maximal value of nc which was previously sent in the session,
      and nc-window is the value of the nc-window parameter which was
      received from the server in the session.

   The last condition allows servers to reject any nonce numbers that
   are "significantly" smaller than the "current" value (defined by the
   value of nc-window) of the nonce number used in the session involved.
   In other words, servers MAY treat such nonce numbers 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 nonce numbers, and



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   if any duplication is detected, the server MUST discard the session
   and respond with a 401-STALE message, as outlined in Section 11.  The
   server MAY also reject other invalid nonce numbers (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
   128, nc-max is 400, and that the client has already used the
   following nonce numbers: {1-120, 122, 124, 130-238, 255-360, 363-
   372}.  Then the nonce number that can be used for next request is one
   of the following set: {245-254, 361, 362, 373-400}.  The values {0,
   121, 123, 125-129, 239-244} MAY be rejected by the server, because
   they are not above the current "window limit" (244 = 372 - 128).

   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 nonce numbers and any nonce-related values MUST
   always be treated as natural numbers within an infinite range.
   Implementations which uses fixed-width integer representations,
   fixed-precision floating numbers or similar representations
   SHOULD NOT reject any larger values which overflow such
   representative limits, and MUST NOT silently truncate it using any
   modulus-like rounding operation (e.g. by mod 2^32).  Instead, the
   whole protocol is carefully designed so that recipients MAY replace
   any such overflowed values (e.g. 2^80) with some reasonably-large
   maximal representative integer (e.g. 2^31 - 1 or others).


7.  Host Validation Methods

   The "validation method" specifies a method to "relate" (or "bind")
   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 vh that is
   an input to the authentication protocols.

   When HTTPS or other possible secure transport is used, this
   corresponds to the idea of "channel binding" described in [RFC5929].
   Even when HTTP is used, similar, but somewhat limited, "binding" is
   performed to prevent a malicious server from trying to authenticate
   themselves to another server as a valid user by forwarding the
   received credentials.

   The valid tokens for the validation parameter and corresponding
   values of vh are as follows:





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   host:          host-name validation: The value vh 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 the default port number.

   tls-server-end-point:  TLS endpoint (certificate) validation: The
                  value vh will be the octet string of the hash value of
                  the server's public key certificate used in the
                  underlying TLS [RFC5246] (or SSL) connection,
                  processed as specified in Section 4.1 of [RFC5929].

                  [[Pending editorial issue: a small security issue is
                  pending around here, awaiting analysis and WG
                  discussions for final adoption.]]

   tls-unique:    TLS shared-key validation: The value v will be the
                  channel binding material derived from the Finished
                  messages, as defined in Section 3.1 of [RFC5929].
                  (Note: see Section 7.2 for some security notices for
                  using this validation method.)

   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 "tls-server-end-point".  If HTTP/TLS protocol
   is used with an anonymous Diffie-Hellman key exchange, the validation
   type MUST be "tls-unique" (see the note below).

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

   If the validation type "tls-server-end-point" is used, the server
   certificate provided on TLS connection MUST be verified at least to
   make sure that the server actually owns the corresponding secret key.
   (Note: this verification is automatic in some RSA-based key exchanges
   but NOT automatic in Diffie-Hellman-based key exchanges with separate
   exchange for server verification.)

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

   Note: The protocol defines two variants for validation on the TLS



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   connections.  The "tls-unique" method is more secure.  However, there
   are some situations where tls-server-end-point 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.

7.1.  Applicability notes

   When the client is a Web browser with any scripting capabilities, the
   underlying TLS channel used with HTTP/TLS MUST provide server
   identity verification.  This means (1) the anonymous Diffie-Hellman
   key exchange cipher-suite 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.

7.2.  Notes on tls-unique

   As described in the interoperability note in the above channel
   binding specification, the tls-unique verification value will be
   changed by possible TLS renegotiation, causing an interoperability
   problem.  TLS re-negotiations are used in several HTTPS server
   implementations for enforcing some security properties (such as
   cryptographic strength) for some specific responses.

   If an implementation supports "tls-unique" verification method, the
   following caution SHOULD be taken:

   o  Both peers must be aware that the values vh used for vkc (in
      req-VFY-C) and for vks (in 200-VFY-S) may be different.  These
      values MUST be retrieved from underlying TLS libraries each time
      it is used.

   o  After calculating value vh and vkc to send a req-VFY-C request,
      Clients SHOULD NOT initiate TLS renegotiation until the end of the
      corresponding response header is received.  Exceptionally, Clients
      can and SHOULD perform TLS re-negotiation as a response to
      server's request for TLS renegotiation, occurring before the top



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      of response header.

   Also, implementer MUST take care of session resumption attacks
   regarding tls-unique channel binding mechanisms and master secrets.
   As a mitigation, a TLS extension defined in
   [I-D.ietf-tls-session-hash] SHOULD be used when tls-unique host
   verification is to be used.


8.  Authentication Extensions

   Interactive clients (e.g.  Web browsers) supporting this protocol are
   RECOMMENDED to support non-mandatory authentication and the
   Authentication-Control header defined in
   [I-D.ietf-httpauth-extension], 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 only where 401-INIT
   response is valid.  It will not replace other 401-type messages such
   as 401-STALE and 401-KEX-S1.


9.  String Preparation

   It is important for interoperability that user-names and passwords
   used in this protocol is binary-comparable regardless of the user's
   input methods and/or environments.  To ensure this, the following
   preparation SHOULD be performed:

   o  User-names received from users SHOULD be prepared using the
      "UsernameCasePreserved" profile defined in Section 3.3 of
      [I-D.ietf-precis-saslprepbis].

   o  Passwords received from users SHOULD be prepared using the
      "OpaqueString" profile defined in Section 4.2 of
      [I-D.ietf-precis-saslprepbis].

   In both cases, it is the sender's duty to correctly preparing the
   character strings.  If any non-normalized character string is
   received from the other peer of the communication, recipients MAY
   either use it as a bare UTF-8 string without any preparation, perform
   any appropriate preparations (which may cause authentication
   failure), or reject any ill-prepared inputs from the sender and
   respond as a communication error.



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   Server applications SHOULD also prepare user-names and passwords
   accordingly upon registration of user credentials.

   In addition, binary-based "interfaces" of implementations MAY require
   and assume that the string is already prepared accordingly; in
   detail, when a string is already stored as an binary Unicode string
   form, implementations MAY omit preparation and Unicode normalization
   (performs UTF-8 encoding only) before using it.  When a string is
   already stored as an octet blob, implementations MAY send it as it
   is.


10.  Decision Procedure for Clients

10.1.  General Principles and Requirements

   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.

   As usual in the HTTP authentication, a single user-level request may
   result in exchange of two-or-more HTTP requests and responses in
   sequence.  The following normative rules MUST be followed by the all
   clients implementing this protocol:

   o  Any kinds of "normal responses" MUST only be accepted for the very
      first request in the sequence.  Any "normal responses" returned
      for the second or later request in the sequence SHALL be
      considered invalid.

   o  In the same principle, any responses which refer to, or request
      changing to, the authentication realm different from the client's
      request MUST only be accepted for the very first request in the
      sequence.  Any kind of responses referring to the different realms
      which are returned for the second or later request in the sequence
      SHALL be considered invalid.

   o  A req-KEX-C1 message MAY be sent either as a initial request or as
      a response to 401-INIT, and 401-STALE.  However, it SHOULD NOT be
      sent more than once in the sequence for a single authentication
      realm, to avoid infinite loops of messages.  A 401-KEX-S1 response
      MUST be accepted only when the corresponding request is
      req-KEX-C1.

   o  A req-VFY-C message MAY be sent if there is a valid session key
      shared between the client and the server, established by
      req-KEX-C1 and 401-KEX-S1.  If any response with 401 status is



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      returned for such a message, the corresponding session key SHOULD
      be discarded as unusable.
      Especially, upon the reception of response 401-STALE, the client
      SHOULD try establishing a new session by sending req-KEX-C1, but
      only once within the request/response sequence.

   o  A 200-VFY-S message MUST be accepted only as a response to
      req-VFY-C and nothing else.  The VK_s field of such response
      message MUST always be checked against the correct value, and if
      it is incorrect, the whole response SHOULD be considered invalid.
      Any content, both the content body and the headers, of such an
      invalid response SHOULD be ignored and discarded.

   The final status of the client request following the message exchange
   sequence shall be determined as follows:

   o  AUTH-SUCCEED: A 200-VFY-S message with the correct VK_s value is
      returned to the req-VFY-C request in the sequence.

   o  AUTH-REQUIRED: Two cases exists.

      *  A 401-INIT message is returned from the server, and the client
         does not know how to authenticate to the given authentication
         realm.

      *  A 401-INIT response is returned for req-VFY-C (or req-KEX-C1),
         which means the user-supplied authentication credentials are
         not accepted.

   o  UNAUTHENTICATED: a normal response is returned for an initial
      request of any kind in the sequence.

   Any kind of response (including a normal response) other than those
   explicitly allowed in the above rules SHOULD be interpreted as a
   fatal communication error.  In such cases, the clients MUST NOT
   process any data (the 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.  In such cases, they
   SHOULD be careful about processing the body of the content (ignoring
   it is still RECOMMENDED, as it may possibly be forged by intermediate
   attackers,) and the client will be in the "UNAUTHENTICATED" status
   then.

   If a request is a sub-request for a resource included in another
   resources (e.g., embedded images, style sheets, frames etc.), clients
   MAY treat an AUTH-REQUESTED status as the same as UNAUTHENTICATED
   status.  In other words, the client MAY ignore server's request to



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   start authentication with new credentials via sub-requests.

10.2.  State machine for the client-side (informative)

   The following state machine describes the possible request-response
   sequences derived from the above normative rules.  If implementer are
   not quite sure on the security consequences of the above rules, it is
   strongly advised to follow the decision procedure below.  In
   particular, clients SHOULD NOT accept "normal responses" unless
   explicitly allowed in the rules.  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 is to
   be followed.

   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 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.

   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 401-KEX-S1 message, go to Step 10.




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       *  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 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.

   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.







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   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):
       The received message is the 200-VFY-S message, which SHALL always
       contain a vks field.  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.

   Figure 5 shows an informative 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                            401-INIT|
              |   with a different realm                      |
              |          -----------------------------------. |
              |         /                                   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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11.  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: (See Note 1 below for 401-INIT message with *
   marks)

   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.






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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 a 401-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 2)

      *  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 a 401-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
         "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



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         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.

   Note 1: In relation with, and following the specification of the
   optional authentication defined in [I-D.ietf-httpauth-extension], the
   401-INIT messages marked with the asterisks can not be replaced with
   a successful responses with an Optional-WWW-Authenticate header.
   Every other 401-INIT can be a response with an
   Optional-WWW-Authenticate.

   Note 2: 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.


12.  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.




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   o  A hash function H to be used with the protocol, along with its
      output size hSize.

   o  The number of iterations for password hashing nIterPi, if it uses
      the default password hashing function defined below.

   Specifications for cryptographic algorithms used with this framework
   MUST specify whether these will use the default functions defined
   below for the functions pi, VK_c, and VK_s; or, these will define
   their own versions for these functions.

   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-
   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).

12.1.  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 sub-
   components 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



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   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"

      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)

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

   The function OCTETS converts an integer into the corresponding radix-
   256 big-endian octet string having its natural length: See
   Section 3.2.3 for the definition of "natural length".

   The function INT converts an octet string into a natural number,
   where the input string is treated as a radix-256 big-endian notation.
   The identity INT(OCTETS(n)) = n always holds for any natural number
   n.

12.2.  Default Functions for Algorithms

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

   The client-side password-based (credential) pi used by this
   authentication is a natural number derived in the following manner:

   pi = INT(PBKDF2(HMAC_H, ph(password), VS(algorithm) | VS(auth-domain)
   | VS(realm) | VS(username), nIterPi, hSize / 8)),



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   where

   o  PBKDF2 is the password-based key derivation function defined in
      [RFC2898],

   o  HMAC_H is the HMAC function, defined in [RFC2104], composed from
      the hash function H, and

   o  hSize is the output size of hash H, counted in bits.

   The values of algorithm, realm, and auth-domain are taken from the
   values contained in the 401-INIT message.  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 according to the method presented in Section 9.  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 = INT(H(octet(4) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) |
   VI(nc) | VS(vh)))

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


13.  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 = H(H(octet(6) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) | VI(0)
   | VS(vh)) | 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.



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

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


14.  Application for Proxy Authentication

   The authentication scheme defined by the previous sections can be
   applied (with modifications) 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,

   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 host
      validation tokens, and

   o  Authentication extensions in [I-D.ietf-httpauth-extension] are not
      applicable.


15.  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-



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   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, [RFC7230] and
   [RFC7235] are satisfied.  Any parameters starting with "kc", "ks",
   "vkc" or "vks" and followed by decimal natural numbers (e.g. kc2,
   ks0, vkc1, vks3 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.


16.  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
   reviewed by a designated expert, as outlined in [RFC5226].

16.1.  Registry for Authentication Algorithms

   This document establishes a registry for HTTP Mutual authentication
   algorithms.  The registry manages a case-insensitive ASCII strings.
   The string MUST follow the extensive-token syntax defined in
   Section 3.

   Registrations for authentication algorithms are required to include a
   description of the key exchange algorithms.  Reviewers assigned by
   IESG are advised to examine minimum security requirements and
   consistency of the key exchange algorithm descriptions.

   New registrations are advised to provide the following information:

   o  Token: a token used in HTTP headers for identifying the algorithm.

   o  Description: A brief description of the algorithm.

   o  Specification: A reference for a specification defining the
      algorithm.




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   The initial content of this registry is empty.  [[Editorial Note: A
   separate document [I-D.ietf-httpauth-mutual-algo] will effectively
   define the initial content of the registry.]]

16.2.  Registry for Password Hashes

   This document establishes a registry for HTTP Mutual authentication
   password hashes.  The registry manages a case-insensitive ASCII
   strings.  The string MUST follow the extensive-token syntax defined
   in Section 3.

   Registrations for authentication algorithms are required to include a
   description of the key exchange algorithms.  Reviewers assigned by
   IESG are advised to examine its use-case requirements and security
   consequence of its introduction.

   New registrations are advised to provide the following information:

   o  Token: a token used in HTTP headers for identifying the algorithm.

   o  Description: A brief description of the algorithm.

   o  Specification: A reference for a specification defining the
      algorithm.

   The initial content of this registry is as follows:

    +------------+------------------------------------+---------------+
    | Token      | Description                        | Specification |
    +------------+------------------------------------+---------------+
    | none       | No additional hashing, recommended | Section 4.1   |
    | md5        | MD5-based preprocessing            | Section 4.1   |
    | digest-md5 | Digest-compatible preprocessing    | Section 4.1   |
    | sha1       | SHA1-based preprocessing           | Section 4.1   |
    +------------+------------------------------------+---------------+

16.3.  Registry for Validation Methods

   This document establishes a registry for HTTP Mutual authentication
   host validations.  The registry manages a case-insensitive ASCII
   strings.  The string MUST follow the extensive-token syntax defined
   in Section 3.

   Registrations for authentication algorithms are required to include a
   description of the key exchange algorithms.  Reviewers assigned by
   IESG are advised to examine its use-case requirements and security
   consequence of its introduction.




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   New registrations are advised to provide the following information:

   o  Token: a token used in HTTP headers for identifying the algorithm.

   o  Description: A brief description of the algorithm.

   o  Specification: A reference for a specification defining the
      algorithm.

   The initial content of this registry is as follows:

   +----------------------+----------------------------+---------------+
   | Token                | Description                | Specification |
   +----------------------+----------------------------+---------------+
   | host                 | Host name verification     | Section 7     |
   |                      | only                       |               |
   | tls-server-end-point | TLS certificate-based      | Section 7     |
   | tls-unique           | TLS unique key-based       | Section 7     |
   +----------------------+----------------------------+---------------+


17.  Security Considerations

17.1.  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
      confidentiality 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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17.2.  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 consumption.  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 "key exchanging"
   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.

17.2.1.  On-line Active Password Attacks

   Although the protocol provides very strong protection against off-
   line dictionary attacks from eavesdropped traffics, the protocol, by
   its nature, can not prevent an active password attacks which the
   attackers sends so many authentication trial requests for every
   possible passwords.

   Possible countermeasures for preventing such attacks may be rate-
   limiting of the password authentication trials, statistics-based
   intrusion detection measures or similar protection schemes.  If the
   server operators assume that the passwords of users are not strong
   enough, it may be desirable to introduce such ad-hoc countermeasures.

17.3.  Communicating the status of mutual authentication with users

   This protocol is designed for two goals.  The first goal is just
   providing a secure alternative for existing Basic and Digest
   authentication.  The second goal is to provide users a way to detect



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   forged rogue servers imitating user's registered account on server-
   side, commonly known as (a part or kind of) Phishing attacks.

   For this protocol to effectively work as some countermeasures to such
   attacks, it is very important that end users of clients will be
   notified of the result of mutual authentication performed by this
   protocol, especially the three states "AUTH-SUCCEED",
   "UNAUTHENTICATED" and "AUTH-REQUIRED" defined in Section 10.  The
   design of secure users' interfaces of the HTTP interactive clients
   are out of the scope of this document, but if possible, having some
   kind of UI indication for the three states above will be desirable
   for user's benefits on their security.

   Of course, in such cases, the user interfaces for asking passwords
   for this authentication shall be clearly identifiable against
   imitation by other insecure password input fields (such as forms).
   If the passwords are known to malicious attackers outside of the
   protocol, the protocol can not work as an effective security
   measures.

17.4.  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  For HTTP/TLS communications, when a web form is submitted from
      Mutually-authenticated pages with the "tls-server-end-point"
      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 re-validated using a req-KEX-C1 message with an
      "Expect: 100-continue" header.  The same applies when the page is
      received with the "tls-unique" validation method, and when the TLS
      session has expired.

   o  For better protection against possible password database steal,
      Server-side storage of user passwords are better containing the
      values encrypted by one-way function J(pi), instead of the real
      passwords, those hashed by ph, or pi.








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17.5.  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 and HTTPS.  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 desirable for
      better security to store the values encrypted by using the one-way
      function J(pi), instead of the real passwords, those hashed by ph,
      or pi.


18.  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
   implementer of this protocol and its variants under non-exclusive
   royalty-free manner.  For the details of the patent application and
   its status, please contact the author of this document.

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


19.  References







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19.1.  Normative References

   [I-D.ietf-httpauth-extension]
              Oiwa, Y., Watanabe, H., Takagi, H., Hayashi, T., and Y.
              Ioku, "HTTP Authentication Extensions for Interactive
              Clients", draft-ietf-httpauth-extension-04 (work in
              progress), July 2015.

   [I-D.ietf-httpbis-auth-info]
              Reschke, J., "The Hypertext Transfer Protocol (HTTP)
              Authentication-Info and Proxy- Authentication-Info
              Response Header Fields", draft-ietf-httpbis-auth-info-05
              (work in progress), April 2015.

   [I-D.ietf-precis-saslprepbis]
              Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords",
              draft-ietf-precis-saslprepbis-18 (work in progress),
              May 2015.

   [I-D.ietf-tls-session-hash]
              Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
              A., and M. Ray, "Transport Layer Security (TLS) Session
              Hash and Extended Master Secret Extension",
              draft-ietf-tls-session-hash-05 (work in progress),
              April 2015.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

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

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898, September 2000.

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

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

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security



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              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, August 2010.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230,
              June 2014.

   [RFC7235]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", RFC 7235, June 2014.

19.2.  Informative References

   [I-D.ietf-httpauth-mutual-algo]
              Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "Mutual Authentication Protocol for HTTP:
              KAM3-based Cryptographic Algorithms",
              draft-ietf-httpauth-mutual-algo-03 (work in progress),
              July 2015.

   [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.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [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.



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

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

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

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              December 2011.

   [RFC7564]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 7564, May 2015.


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-unique validation or not.

   o  Whether to introduce password strengthening hashes other than
      PBKDF2 into the function pi().  This requires standardization of
      such other hash algorithms in IETF.

   o  Whether to modify current definition of nIterPi, which is per-
      algorithm defined.  To increase this parameter requires defining a
      new algorithm, possibly with reconsideration for other security
      parameters as well.

   o  Whether to keep ph() function for legacy migration or not.

   o  Adding test vectors for ensuring implementation correctness.

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


Appendix B.  (Informative) Draft Change Log







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B.1.  Changes in Httpauth WG Revision 05

   o  Minimum nonce number window has increased to 128.  (HTTP 2.0
      recommends at least 100 concurrent sessions to exist)

   o  Reference to TLS session hash extension added for tls-unique
      security issues.

   o  Comments in the previous F2F meeting has been reflected to the
      text.

B.2.  Changes in Httpauth WG Revision 04

   o  Merged httpauthprep proposal into general PRECIS Username/Password
      profile.

   o  Adopting RFC 5987 extended syntax for non-ASCII parameter values.

   o  Refer draft-ietf-httpbis-auth-info for Authentication-Info header.
      This results in a different syntax for that header.

B.3.  Changes in Httpauth WG Revision 03

   o  Incompatible change: Single-port type authentication realm label
      has been changed to harmonize with Web Origin.  (That is, the
      default ports (80 and 443) are to be omitted.)

B.4.  Changes in Httpauth WG Revision 02

   o  Major change: introduction of password-strengthening function
      PBKDF2.

   o  Changed Section 10 to adopt "list of requirements" style.  Strict
      definition of state machine is now a derived, informational
      definition.

B.5.  Changes in Httpauth WG Revision 01

   o  Changed "tls-key" verification to "tls-unique" verification, and
      "tls-cert" to "tls-server-end-point", adopting RFC 5929.

   o  Adopted PRECIS framework [RFC7564].

   o  Reverted reservation of "rekey-sid" and "rekey-method" parameters.

   o  Degraded secure UI requirement to application note level, non-
      normative.




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   o  Adjusted levels of several requirements.

   o  Added warning text for handling of exceptional 5XX responses.

   o  Dropped several references for optional authentications, except
      one "Note".

   o  Several textual fixes, improvements and revisions.

B.6.  Changes in Httpauth Revision 00

   o  Changed the version token.

   o  Renamed "verification tokens" to "Host verification tokens" and
      variables "v" to "vh" for clarification.  (Back-ported from
      draft-oiwa-httpauth-multihop-template-00)

B.7.  Changes in HttpBis Revision 00

   None.

B.8.  Changes in Revision 12

   o  Added a reason "authz-failed".

B.9.  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 informative/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.

B.10.  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.




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   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.11.  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.12.  Changes in Revision 08

   o  The English text has been revised.

B.13.  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.14.  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.15.  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.16.  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.17.  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.18.  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.19.  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
   Information Technology Research Institute
   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
   Information Technology Research Institute
   Tsukuba Central 2
   1-1-1 Umezono
   Tsukuba-shi, Ibaraki
   JP


   Hiromitsu Takagi
   National Institute of Advanced Industrial Science and Technology
   Information Technology Research Institute
   Tsukuba Central 2
   1-1-1 Umezono
   Tsukuba-shi, Ibaraki
   JP







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   Kaoru Maeda
   Lepidum Co. Ltd.
   Village Sasazuka 3, Suite #602
   1-30-3 Sasazuka
   Shibuya-ku, Tokyo
   JP


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


   Yuichi Ioku
   Individual

































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