HTTPAUTH Working Group                                           Y. Oiwa
Internet-Draft                                               H. Watanabe
Intended status: Experimental                                  H. Takagi
Expires: May 18, 2017                                         ITRI, AIST
                                                                K. Maeda
                                                              T. Hayashi
                                                                 Lepidum
                                                                 Y. Ioku
                                                              Individual
                                                       November 14, 2016


                Mutual Authentication Protocol for HTTP
                     draft-ietf-httpauth-mutual-11

Abstract

   This document specifies a mutual authentication scheme for the
   Hypertext Transfer Protocol (HTTP).  This scheme provides true mutual
   authentication between an HTTP client and an HTTP server using
   password-based authentication.  Unlike the Basic and Digest
   authentication schemes, the Mutual authentication scheme 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 May 18, 2017.

Copyright Notice

   Copyright (c) 2016 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  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Document Structure and Related Documents . . . . . . . . .  6
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Messages Overview  . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Typical Flows of the Protocol  . . . . . . . . . . . . . .  8
     2.3.  Alternative Flows  . . . . . . . . . . . . . . . . . . . . 10
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Non-ASCII extended header parameters . . . . . . . . . . . 12
     3.2.  Values . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.2.1.  Tokens . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.2.2.  Strings  . . . . . . . . . . . . . . . . . . . . . . . 14
       3.2.3.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . 14
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.1.  401-INIT and 401-STALE . . . . . . . . . . . . . . . . . . 16
     4.2.  req-KEX-C1 . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.3.  401-KEX-S1 . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.4.  req-VFY-C  . . . . . . . . . . . . . . . . . . . . . . . . 20
     4.5.  200-VFY-S  . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Authentication Realms  . . . . . . . . . . . . . . . . . . . . 21
     5.1.  Resolving Ambiguities  . . . . . . . . . . . . . . . . . . 22
   6.  Session Management . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Host Validation Methods  . . . . . . . . . . . . . . . . . . . 25
     7.1.  Applicability notes  . . . . . . . . . . . . . . . . . . . 26
     7.2.  Notes on tls-unique  . . . . . . . . . . . . . . . . . . . 27
   8.  Authentication Extensions  . . . . . . . . . . . . . . . . . . 27
   9.  String Preparation . . . . . . . . . . . . . . . . . . . . . . 28
   10. Decision Procedure for Clients . . . . . . . . . . . . . . . . 28
     10.1. General Principles and Requirements  . . . . . . . . . . . 28
     10.2. State machine for the client (informative) . . . . . . . . 30
   11. Decision Procedure for Servers . . . . . . . . . . . . . . . . 35
   12. Authentication Algorithms  . . . . . . . . . . . . . . . . . . 37
     12.1. Support Functions and Notations  . . . . . . . . . . . . . 38
     12.2. Default Functions for Algorithms . . . . . . . . . . . . . 39
   13. Application Channel Binding  . . . . . . . . . . . . . . . . . 40
   14. Application for Proxy Authentication . . . . . . . . . . . . . 41



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   15. Methods to Extend This Protocol  . . . . . . . . . . . . . . . 42
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
     16.1. Registry for Authentication Algorithms . . . . . . . . . . 42
     16.2. Registry for Validation Methods  . . . . . . . . . . . . . 43
   17. Security Considerations  . . . . . . . . . . . . . . . . . . . 44
     17.1. Security Properties  . . . . . . . . . . . . . . . . . . . 44
     17.2. Secrecy of Credentials . . . . . . . . . . . . . . . . . . 44
     17.3. Denial-of-service Attacks to Servers . . . . . . . . . . . 45
       17.3.1. On-line Active Password Attacks  . . . . . . . . . . . 45
     17.4. Communicating the status of mutual authentication with
           users  . . . . . . . . . . . . . . . . . . . . . . . . . . 45
     17.5. Implementation Considerations  . . . . . . . . . . . . . . 46
     17.6. Usage Considerations . . . . . . . . . . . . . . . . . . . 47
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     18.2. Informative References . . . . . . . . . . . . . . . . . . 48
   Appendix A.  (Informative) Draft Change Log  . . . . . . . . . . . 50
     A.1.  Changes in Httpauth WG Revision 11 . . . . . . . . . . . . 50
     A.2.  Changes in Httpauth WG Revision 10 . . . . . . . . . . . . 50
     A.3.  Changes in Httpauth WG Revision 09 . . . . . . . . . . . . 50
     A.4.  Changes in Httpauth WG Revision 08 . . . . . . . . . . . . 50
     A.5.  Changes in Httpauth WG Revision 07 . . . . . . . . . . . . 51
     A.6.  Changes in Httpauth WG Revision 06 . . . . . . . . . . . . 51
     A.7.  Changes in Httpauth WG Revision 05 . . . . . . . . . . . . 51
     A.8.  Changes in Httpauth WG Revision 04 . . . . . . . . . . . . 51
     A.9.  Changes in Httpauth WG Revision 03 . . . . . . . . . . . . 51
     A.10. Changes in Httpauth WG Revision 02 . . . . . . . . . . . . 51
     A.11. Changes in Httpauth WG Revision 01 . . . . . . . . . . . . 52
     A.12. Changes in Httpauth Revision 00  . . . . . . . . . . . . . 52
     A.13. Changes in HttpBis Revision 00 . . . . . . . . . . . . . . 52
     A.14. Changes in Revision 12 . . . . . . . . . . . . . . . . . . 52
     A.15. Changes in Revision 11 . . . . . . . . . . . . . . . . . . 52
     A.16. Changes in Revision 10 . . . . . . . . . . . . . . . . . . 53
     A.17. Changes in Revision 09 . . . . . . . . . . . . . . . . . . 54
     A.18. Changes in Revision 08 . . . . . . . . . . . . . . . . . . 54
     A.19. Changes in Revision 07 . . . . . . . . . . . . . . . . . . 54
     A.20. Changes in Revision 06 . . . . . . . . . . . . . . . . . . 55
     A.21. Changes in Revision 05 . . . . . . . . . . . . . . . . . . 55
     A.22. Changes in Revision 04 . . . . . . . . . . . . . . . . . . 55
     A.23. Changes in Revision 03 . . . . . . . . . . . . . . . . . . 55
     A.24. Changes in Revision 02 . . . . . . . . . . . . . . . . . . 56
     A.25. Changes in Revision 01 . . . . . . . . . . . . . . . . . . 56
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 56








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1.  Introduction

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

   Password-stealing attacks are one of most critical threats for Web
   systems.  For a long time, plain-text password authentications (Basic
   and Web form-based) are widely used (and are in use now).  When these
   are used with plain HTTP protocols, it is trivially easy for
   attackers to sniff the password credentials on the wire.

   Digest authentication scheme [RFC7616] uses a SHA-2 (formerly SHA-1
   and MD5) hash algorithms to hide the raw user password from the
   sniffing.  However, if the number of possible candidates of users'
   password is not enough, recent powerful computers can compute
   possible hash values for billions of password candidates, and compare
   these with the sniffed values to find out the correct password.  This
   kind of attack is called "offline password dictionary attacks";
   recently, the size of possible search space by computers is quite
   competing with possibility of user's memorable passwords, threatening
   the effectiveness of such hash-based password protections.

   TLS [RFC5246] provides a strong cryptographic protection against the
   network-based sniffing of passwords and other communication contents.
   If TLS is correctly used by both server operators and client users,
   passwords and other credentials will not be available for any outside
   attackers.  However, there is a pit-hole in the TLS deployment on the
   Web systems; if the users are forged into a "wrong website" by some
   kind of social attacks and tridked to perform authentication on that
   site, the credentials will be sent to the attacker's server and
   trivially leaked.  Such attacks are called "Phishing", and becoming a
   real threats in these days.  In the curent Web system deployment, TLS
   certificates will be issued to almost any users of Internet
   (including malicious attackers).  Although those certificate includes
   several levels of the "validation results" (such as corporate names)
   of the issued entities, task of "checking" those validation results
   are left to the users of Web browsers, still leaving the possibility
   of such social attacks.

   Another direction to avoid such threats is to avoid password-based
   authentication and use some kind of pre-deployed strong secret keys
   (either on client side or on server-side) for authentications.
   Several federated authentication framework as well as HOBA [RFC7486]
   are proposed and deployed on the real Web systems to satisfy those
   needs.  However, a kind of authentication based on "human-memorable



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   secret" (i.e. passwords) is still required on several situations
   within those systems, such is initialization, key deployment to new
   clients, or recovery of secret accounts with lost cryptographic keys.

   The Mutual authentication protocol proposed in this document is a
   strong cryptographic solution for password authentications.  It
   mainly provides the two key features:

   o  No password information, at all, is exchanged in the
      communications.  When the server and the user fails to
      authenticate with each other, the protocol will not reveal the
      tiniest bit of information about the user's password.  This
      prevents any kind of off-line password dictionary attacks, even
      with the existence of Phishing attacks.

   o  To successfully authenticate, the server must own the valid
      registered credentials (authentication secret), as well as client
      users.  (Non-intuitively, this is not true for Basic and Digest
      authentication.  For example, servers for Basic authentications
      can answer "YES" to any clients, without actually checking
      authentication at all.)  This means that phishing attackers cannot
      forge users that they are the "authentic" servers.  Client users
      can assert whether the communicating peer is "the server" who have
      registered their account beforehand.  In other words, it provides
      "true" mutual authentication between servers and clients.

   Given these, the proposed protocol can serve as a strong alternative
   to the Basic, Digest, and web-form-based authentications, and also as
   a strong companion to the non-password-based authentication
   frameworks.

   The proposed protocol will serve in the same way as existing Basic/
   Digest authentication: it meets the requirement for new
   authentication scheme for HTTP as described in Section 5.1.2 of
   [RFC7235].  Additionally, to communiate authentication results more
   reliably between the server and the client user, it suggests for Web
   browsers to have some "secure" way of displaying the authentication
   results.  Having such an user interface in future browser will
   greatly reduce the risk of impersonation by kinds of social attacks,
   similarly in the manner of the "green padlock" for extended
   verification TLS certificates.

   Technically, the authentication scheme proposed in this document is a
   general framework for using password-based authenticated key exchange
   (PAKE) and similar stronger cryptographic primitives with HTTP.  The
   two key features shown above are corresponding to the nature of PAKE.





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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 both the input (domain) and the output
   (codomain) of hash functions to be octet strings.  When a natural
   number output for a hash function is required, it will be written as
   INT(H(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.

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

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

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



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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
   provides a brief outline of the protocol and the exchanged messages.

2.1.  Messages Overview

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

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

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

      *  401-STALE message: a message indicating that client has to
         start a new key exchange.

   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: an intermediate response to a req-KEX-C1
         message from the server.

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

      *  req-VFY-C message: a message used by the client, requesting the
         server authenticate and authorize the client.

      *  200-VFY-S message: a response used by the server to indicate
         the successful client-authentication.  It also contains
         information necessary for the client to check the authenticity
         of the server.

   In addition to the above, either a request or a response without any



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   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 scheme will use the following protocol
   sequence.

          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



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      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).

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

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

   o  The client will send a request with a client-side authentication
      verification value (req-VFY-C) (6), calculated from the client-
      generated session secret.  The server will check the validity of
      the verification value using its own version of the 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, server authentication
      succeeded.

      If it is not the value expected, or if the message does not



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      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
      the HTTP response in this case.  (Note: This case should not
      happen between a correctly implemented server and client without
      any active attacks.  The possible cause of such a case might be
      either a man-in-the-middle attack or an incorrect implementation.)

2.3.  Alternative Flows

   As shown above, the typical flow for a first authentication 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 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.

      The server MAY have thrown out the corresponding session from the
      session table.  If so, 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 criteria, and thus only one
   round-trip of request/response will be required.















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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 protocol flows

   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 [RFC7615].  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 [RFC7615].

   In HTTP, the WWW-Authenticate header may contain two or more
   challenges.  Client implementations SHOULD be aware of and be capable
   of handling 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 using plain RFC 7235 syntax.  The syntax as 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 a parameter MUST
   have a charset value of "UTF-8", and the language value MUST always
   be omitted (have an empty value).  The same parameter MUST NOT be
   sent more than once, regardless of the used syntax.

   For example, a parameter "user" with value "Renee of 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 the realm parameter to be in its plain form (not
   as an 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.  However, recipients MUST 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.
   Extensive-token values SHOULD use the following syntax (after HTTP
   value parsing):

    bare-token           = bare-token-lead-char *bare-token-char
    bare-token-lead-char = %x30-39 / %x41-5A / %x61-7A
    bare-token-char      = %x30-39 / %x41-5A / %x61-7A / "-" / "_"
    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, they 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 extension
   of this protocol MAY use either the bare-tokens allocated by IANA
   (under the procedure described in Section 16), or extension-tokens
   with the format "-<bare-token>.<domain-name>", where <domain-name> is
   a valid (sub-)domain name on the Internet owned by the party who
   defines the extension.

   Bare-tokens and extensive-tokens are also used for parameter names,
   in the unquoted form.  Requirements for using the extension-token for
   the parameter names are the same as the previous paragraph.

   The canonical format for bare-tokens and extensive-tokens is the



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   unquoted representation.

3.2.2.  Strings

   All character strings MUST be encoded to octet strings using the
   UTF-8 encoding [RFC3629] for the Unicode character set [Unicode].
   Such strings MUST NOT contain any leading BOM markers (also known as
   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 is 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].  (An
   [RFC5987] extended parameter will have an unquoted encoded value, as
   defined therein.)

3.2.3.  Numbers

   The following syntax definitions give a syntax for numeric 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 numbers

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

   A number represented as a hex-fixed-number MUST include an even
   number of hexadecimal digits (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 MUST
   have their "natural length"; if these values are generated from a
   hash function, these lengths correspond to the hash size; if these
   are representing elements of a mathematical set (or group), these
   lengths SHALL be the shortest for representing all the elements in
   the set.  For example, the results of the SHA-256 hash function will
   be represented by 64 digits, and any elements in a 2048-bit prime
   field (modulo a 2048-bit integer) will be represented by 512 digits,



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   regardless of how much zeros appear in front of such representations.
   Session-identifiers and other non-cryptographically generated values
   are represented in any (even) length determined by the side that
   generates it first, and the same length MUST be used throughout 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 (described in
   Section 4 of [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 in what circumstances each message is 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 header exactly once at
   most.

   All credentials and challenges MAY contain any parameters not
   explicitly specified in the following sections.  Recipients that 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 integer), and "vks" are mutually exclusive; any
      challenge MUST NOT contain two or more parameters among them.
      They MUST NOT contain any "kc#" or "vkc" parameters.



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   o  For requests, the parameters "kc#" (where # stands for any decimal
      integer), and "vkc" are mutually exclusive and any challenge
      MUST NOT contain two or more parameters among them.  They MUST NOT
      contain any "ks#" or "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
   "1", and recipients MUST reject messages that contain any other value
   as a version, unless another specification defines a behavior for
   that version.

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 status if sensible)
   containing one and only one (hereafter not explicitly noted)
   "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 "1".

   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 another
                  supplemental specification.

   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
                  another supplemental specification.

   auth-scope:    (non-mandatory string) specifies the authentication
                  scope, 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
                  scope.  As specified in [RFC7235], this value MUST
                  always be sent in the quoted-string form, and an
                  [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



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                  received in this field as a binary blob, an NFC-
                  normalized UTF-8 string, or an error.

   reason:        (mandatory extensive-token) SHALL be an extensive-
                  token that 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 for
                     some reason, possibly with a bad authentication
                     credential.

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

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

                  *  invalid-parameters: the server did not attempt
                     authentication because some parameters were not
                     acceptable.

                  *  internal-error: the server did not attempt
                     authentication because there are some troubles on
                     the server-side.

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

                  *  invalid-credential: ditto, suggesting that the
                     provided user name was valid but authentication
                     still failed.  The use of this parameter is



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                     NOT RECOMMENDED for security reasons.

                  *  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 a 401 or 403 status to
                     indicate that the authentication result is one of
                     the existing reasons for the failed authorization.)

                  It is RECOMMENDED to record the reasons to a kind of
                  diagnostic log, for an example, or shown to the client
                  user immediately.  It will be helpful to find out
                  later that the reason of the failed authentication is
                  either technical reasons of user errors.

   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:

   version:       (mandatory, extensive-token) should be the token "1".

   algorithm, validation, auth-scope, realm:  MUST be the same values as
                  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.







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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 "1".

   algorithm, validation, auth-scope, realm:  MUST be the same values as
                  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 maximum 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 maximum 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.

   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 for
                  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 [RFC7616] for Digest authentications.  All
                  path elements contained in the parameter MUST be
                  inside the specified auth-scope; if not, clients
                  SHOULD ignore such elements.  For better performance,



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                  recognition of this parameter by clients is 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 "1".

   algorithm, validation, auth-scope, realm:  MUST be the same values as
                  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.

4.5.  200-VFY-S

   Every 200-VFY-S message SHALL be a valid HTTP message that does not
   have a 401 (Authentication Required) status code and SHALL contain 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 "1".

   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



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   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.  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 credentials.  However, for different authentication
   realms, clients MUST NOT automatically reuse user names and passwords
   for another realm.

   Just like in the Basic and Digest access authentication protocols,
   the Mutual authentication protocol supports multiple, separate
   protection spaces to be set up inside each host.  Furthermore, the
   protocol allows a single authentication realm to span over several
   hosts within the same Internet domain.

   Each authentication realm is defined and distinguished by the triple
   of an "authentication algorithm", an "authentication scope", and a
   "realm" parameter.  However, server operators are NOT RECOMMENDED to
   use the same pair of an authentication scope and a realm with
   different authentication algorithms.

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

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

   o  Single-server type: A string in the 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 all
      other cases, the port part MUST be present, and it MUST NOT
      contain leading zeros.  Use this format when authentication is
      only valid for a 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 scope will span over several protocols (e.g., http



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

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

5.1.  Resolving Ambiguities

   In the above definitions of authentication scopes, several scopes may
   overlap each other.  If a client has already been authenticated to
   several realms applicable to the same server, the client may have a
   multiple lists 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 in deciding which credentials to send for a new request (in
   steps 3 and 4 of the decision procedure presented in Section 10).

   In such cases, a client MAY send request which belong to any of these
   candidate realms freely, or it MAY simply send an unauthenticated
   request and see for which realm the server requests 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 is one possible tactic for resolving
   ambiguity in such cases.





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

   o  In other cases, use one of the authentication realms representing
      the most-specific authentication scopes.  The list of possible
      domain specifications shown above is given from most specific to
      least specific.

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

   o  If there are realms with the same authentication scope, 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 four messages (first request, 401-INIT,
   req-KEX-C1 and 401-KEX-S1), after which a "session secret" (z)
   associated with the session is established.  After mutually
   establishing a session secret, this session, along with the secret,
   can be used for one or more requests for resources protected by the
   same realm on 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
   user.

   Sessions and session identifiers are local to each server (defined by
   scheme, host, and port), even if an authentication scope covers
   multiple servers; 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 by the same server.  The same session identifiers provided
   either from different servers or for different realms MUST be treated
   as independent or each other.

   The server SHOULD accept at least one req-VFY-C request for each
   session, if the request reaches the server in a time window specified
   by the timeout parameter in the 401-KEX-S1 message, and there are no
   emergent reasons (such as flooding attacks) to forget the session.
   After that, the server MAY discard any session at any time and MAY
   send 401-STALE messages for any further req-VFY-C requests received
   for that session.



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   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 largest value of nc which was previously sent in the session,
      and nc-window is the value of the nc-window parameter that was
      received from the server for 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 duplicate
   nonce detection in a constant amount of memory for each session.

   Servers MUST check for duplication of the received nonce numbers, and
   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 the 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 up to
   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-point numbers, or similar representations
   SHOULD NOT reject any larger values which overflow such
   representative limits, and MUST NOT silently truncate them using any



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   modulus-like rounding operation (e.g., by mod 2^32).  Instead, the
   whole protocol is carefully designed so that recipients MAY replace
   any such overflowing values (e.g. 2^80) with some reasonably-large
   maximum 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 determines 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
   itself 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:

   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
                  server-side resource currently being accessed.  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] connection, processed as
                  specified in Section 4.1 of [RFC5929].


   tls-unique:    TLS shared-key validation: The value vh 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 when
                  using this validation method.)

   If HTTP is used on a non-encrypted channel (TCP and SCTP, for



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   example), the validation type MUST be "host".  If HTTP/TLS [RFC2818]
   (HTTPS) is used with a server certificate, the validation type MUST
   be "tls-server-end-point".  If HTTP/TLS is used with an anonymous
   Diffie-Hellman key exchange, the validation type MUST be "tls-unique"
   (see the note below).

   If the validation type "tls-server-end-point" is used, the server
   certificate provided in the TLS connection MUST be verified at least
   to make sure that the server actually owns the corresponding private
   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 receipt of 401-INIT
   messages.

   Note: The protocol defines two variants of validation on the TLS
   connections.  The "tls-unique" method is technically 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 the HTTP CONNECT method.

   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
   (dynamic contents support), the underlying TLS channel used with
   HTTP/TLS MUST provide server identity verification.  This means (1)
   anonymous Diffie-Hellman key exchange cipher suites MUST NOT be used,
   and (2) verification of the server certificate provided by the server
   MUST be performed.  This is to prevent loading identity-
   unauthenticated scripts or dynamic contents, which are referenced
   from the authenticated page.

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






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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 the "tls-unique" verification method,
   the following caution SHOULD be taken:

   o  Both peers must be aware that the vh values 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
      they are used.

   o  After calculating the values 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.  An
      exception is that clients can and SHOULD perform TLS re-
      negotiation as a response to the server's request for TLS
      renegotiation, before receipt of the beginning of the response
      header.

   Also, implementers MUST take care of session resumption attacks
   regarding tls-unique channel binding mechanisms and master secrets.
   As a mitigation, a TLS extension defined in [RFC7627] 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 for the "auth-style" parameter.
   This specification also proposes (however, does not mandate) the
   default "auth-style" 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 the 401-INIT
   response is valid.  It will not replace other 401-type messages such
   as 401-STALE and 401-KEX-S1.  That is, the reason field of such a
   message MUST be "initial" (or any extensive-tokens NOT defined in
   Section 4.1).



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9.  String Preparation

   It is important for interoperability that user names and passwords
   used in this protocol are 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
      [RFC7613].

   o  Passwords received from users SHOULD be prepared using the
      "OpaqueString" profile defined in Section 4.2 of [RFC7613].

   In both cases, it is the sender's duty to correctly prepare the
   character strings.  If any non-prepared character string is received
   from the other peer of the communication, the behavior of its
   recipient is not defined; the recipient MAY either accept or reject
   such input.

   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; when a
   string is already stored as a binary Unicode string form,
   implementations MAY omit preparation and Unicode normalization
   (performing UTF-8 encoding only) before using it.  When a string is
   already stored as an octet blob, implementations MAY send it as is.


10.  Decision Procedure for Clients

10.1.  General Principles and Requirements

   To securely implement the protocol, the client must be careful about
   accepting the authenticated responses from the server.  This also
   holds true for the reception of a "normal response" (a response which
   does not contain Mutual authentication-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
   clients implementing this protocol:

   o  Any kind of a "normal response" MUST only be accepted for the very
      first request in the sequence.  Any "normal response" returned for



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      the second or later requests in the sequence SHALL be considered
      invalid.

   o  In the same principle, if any response is related to an
      authentication realm which is different from that of the client's
      request (for example, a 401-INIT message requesting authentication
      on another realm), it MUST only be accepted for the very first
      request in the sequence.  Such a response returned for a 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 or 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 secret
      shared between the client and the server, established by
      req-KEX-C1 and 401-KEX-S1.  If any response with 401 status is
      returned for such a message, the corresponding session secret
      SHOULD be discarded as unusable.
      Especially, upon the reception of a 401-STALE response, 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 values of such response
      messages MUST always be checked against the correct value, and if
      it is incorrect, the whole response SHOULD be considered invalid.

   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 was
      returned in response to the req-VFY-C request in the sequence.

   o  AUTH-REQUIRED: Two cases exists.

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

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





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   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 has a Server-Error (5xx) status code.  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
   resource (e.g., embedded images, style sheets, frames etc.), clients
   MAY treat an AUTH-REQUESTED status as the same as an UNAUTHENTICATED
   status.  In other words, the client MAY ignore server's request to
   start authentication with new credentials via sub-requests.

10.2.  State machine for the client (informative)

   The following state machine describes the possible request-response
   sequences derived from the above normative rules.  If implementers
   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 expected
       authentication realm.  If there is one, go to Step 3.  Otherwise,
       go to Step 4.







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   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 a 401-STALE message is received, go to Step 9.

       *  If a 401-INIT message is received, go to Step 13.

       *  If a 200-VFY-S message is received, go to Step 14.

       *  If a normal response is received, go to Step 11.

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

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

       *  If a 401-KEX-S1 message is received, go to Step 10.

       *  If a 401-INIT message is received with the same authentication
          realm, go to Step 13 (see Note 1).

       *  If a normal response is received, go to Step 11.

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

       *  If a 401-INIT message is received, go to Step 6.

       *  If a normal response is received, go to Step 11.

   Step 6 (step_rcvd_init):
       Check whether the user's password for the requested
       authentication realm is known.  If yes, go to Step 7.  Otherwise,
       go to Step 12.

   Step 7:
       Check whether there is an available sid for the expected
       authentication realm.  If there is one, go to Step 8.  Otherwise,
       go to Step 9.

   Step 8 (step_send_vfy):
       Send a req-VFY-C request.





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       *  If a 401-STALE message is received, go to Step 9.

       *  If a 401-INIT message is received, go to Step 13.

       *  If a 200-VFY-S message is received, go to Step 14.

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

       *  If a 401-KEX-S1 message is received, go to Step 10.

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

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

       *  If a 401-INIT message is received, go to Step 13.

       *  If a 200-VFY-S message is received, go to Step 14.

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

   Step 12 (step_rcvd_init_unknown):
       The requested resource requires 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 the 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 user-provided credentials for the authentication realm
       and go to Step 12.

   Step 14 (step_rcvd_vfy):
       The received message is the 200-VFY-S message, which always
       contains 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.



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       If a user explicitly requests to log out (via the user
       interface), 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 are
       NOT RECOMMENDED for servers to initiate.

   Figure 5 shows an informative diagram of the client 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 the long term; it MAY be cleared upon server
   restart, reboot, or for other reasons.  Each entry in the table
   SHOULD contain at least the following information:

   o  The session identifier, which is 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  A boolean flag representing 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 boolean 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 a
   plus sign)

   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.  (See Note 2.)

      *  Otherwise, create a new session, compute K_s1 and send a
         401-KEX-S1 response.  The created session is marked as not
         fake, and its largest-nc is initialized to zero.

      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 look up 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 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 is not REQUIRED to; See Note 3) send a
         401-STALE message.  The session is changed to the "inactive"
         state if so did.





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      *  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 it changed to the
         "inactive" state.

      *  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 MUST be changed to the
         "rejected" state; otherwise, it MAY either be changed to the
         "rejected" state 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 MUST be updated appropriately.

   At any time, the server MAY change any state entries with both the
   "rejected" and "authenticated" states to the "inactive" status, and
   MAY discard any "inactive" states from the table.  Entries with the
   "key exchanging" state 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 pluses cannot 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 name will not be accepted.  Instead, postpone it to the response
   for the next req-VFY-C request.

   Note 3: The next case implies that, when the request is not rejected
   in this clause, the server must be able to determine whether the same
   nc value was previously received from the client.  If the server does
   not remember a whole history of the nc values received from the
   client, the server MUST send a 401-STALE message on this clause.


12.  Authentication Algorithms

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

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



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

   o  Shared session secret z, to be computed by both server and client.

   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 values pi, VK_c, and VK_s; or, these will define their own
   versions for these.

   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 J for deriving
   server-side authentication credential J(pi) 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(i) generates an octet string containing a single
   octet of value i.  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 as big-endian radix-128
   strings, where each digit is represented by an octet within the range
   0x80-0xff except the last digit, which is represented by a octet
   within the range 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



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   ASN.1 encoding [ITU.X690.1994], and available as a "w" conversion in
   the "pack" function of several scripting languages.

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

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

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

   Some examples:

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

      VI(100) = "d"

      VI(10000) = "\316\020"

      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 being in 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.




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   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, password, VS(algorithm) | VS(auth-scope) |
   VS(realm) | VS(username), nIterPi, hSize / 8)),

   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 in bits.

   The values of algorithm, realm, and auth-scope are taken from the
   values contained in the 401-INIT message.  If the password comes from
   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.

   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 application or protocol.  If
   there is no appropriate value to be specified, use an empty 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)



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   | 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.

   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 authentication.  In such
   cases, the following alterations MUST be applied:

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

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

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

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

   o  The auth-scope parameter is fixed to the host-name of the proxy,
      which means it covers 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.




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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-
   standardized 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 future extensions to 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 used in the manner defined in Section 3.


16.  IANA Considerations

   This document requires an additional entry to the "Hypertext Transfer
   Protocol (HTTP) Authentication Scheme Registry" as follows:

   o  Authentication Scheme Name: "Mutual"

   o  Pointer to specification text: (this document)

   When bare-tokens are used for the authentication-algorithm and
   validation parameters, these MUST be allocated by IANA.  To acquire
   registered tokens, the usage 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 case-insensitive ASCII strings.
   The strings MUST follow the extensive-token syntax defined in
   Section 3.

   Registrations for an authentication algorithm are required to include
   a description of the authentication algorithms.  Reviewers assigned



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   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.

   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 Validation Methods

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

   Registrations for a validation method are required to include a
   description of the validation method.  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 method.

   o  Description: A brief description of the method.

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

   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     |
   +----------------------+----------------------------+---------------+




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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).

17.2.  Secrecy of Credentials

   The client-side password credential MUST be kept secret all the time,
   and SHOULD NOT be used with any other (possibly insecure)
   authentication purpose.  Loss of control of the credential will
   directly affect the control of corresponding server-side account.

   Use of client-side credential with THIS authentication scheme is
   always safe, even if the connected server peer is not trustful
   (condition of Phishing).  However, if it is used with other
   authentication schemes (such as Web forms), and if the recipient is
   rogue, the result will be obvious.

   The server-side password credential (J) is also important to be kept
   secret.  If it is stolen, and if the client's choice of password is
   not strong, the person aware of server-side password credential can
   employ a off-line dictionary attack to search for the client
   password.  However, if the client has chosen a strong password, so
   that the attacker cannot guess the client's password from dictionary
   candidate, the client is still well protected from any attacks.

   The shared session secret (z) MUST be kept secret inside the server/
   client software; if it is lost, and if the session is still active,
   it will lead to session hijacking.  After the session is expired, the
   key is valueless for attackers.



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17.3.  Denial-of-service Attacks to Servers

   The protocol requires a server-side table of active sessions, which
   may become a critical point for 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
   reestablish a new session then.

   However, if a malicious client sends too many requests for 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 their statuses.  One way to mitigate
   such attacks is that servers MAY have a number and a time limit for
   unverified, pending key exchange requests (in the "key exchanging"
   state).

   This is a common weakness of authentication protocols with almost any
   kind of negotiations or states, including Digest authentication
   scheme and most Cookie-based authentication implementations.
   However, regarding the resource consumption, the situation for the
   mutual authentication scheme is a slightly better than for 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.

17.3.1.  On-line Active Password Attacks

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

   Possible countermeasures for preventing such attacks may be rate-
   limiting of 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.

17.4.  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 a user's registered account on a
   server, commonly known as (a part or kind of) Phishing attacks.

   For this protocol to effectively work as some countermeasure to such
   attacks, it is very important that end users of clients be notified
   of the result of the mutual authentication performed by this
   protocol, especially the three states "AUTH-SUCCEED",
   "UNAUTHENTICATED", and "AUTH-REQUIRED" defined in Section 10.  The
   design of secure user interfaces of the HTTP interactive clients is
   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 the
   user's security benefit.

   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 cannot work as an effective security measures.

17.5.  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 stealing,
      server-side storage of user passwords should contain the values
      encrypted by the one-way function J(pi), instead of the real
      passwords or those hashed by pi.

   o  If the TLS 1.2 is used for underlying HTTP/TLS communications,
      follow best practices in [RFC7525].






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17.6.  Usage Considerations

   o  The user names inputted by a user may be sent automatically to any
      servers sharing the same auth-scope.  This means that when a host-
      type auth-scope 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 clear text.  If user names have to be kept secret
      against eavesdropping, the server must use the full-scheme-type
      auth-scope parameter and HTTPS.  Contrarily, passwords are not
      exposed to eavesdroppers even on HTTP requests.

   o  If the server provides several ways for storing server-side
      password secrets in 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 or those hashed by
      pi.


18.  References

18.1.  Normative References

   [I-D.ietf-httpauth-extension]
              Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "HTTP Authentication Extensions for
              Interactive Clients", draft-ietf-httpauth-extension-09
              (work in progress), August 2016.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898, DOI 10.17487/
              RFC2898, September 2000,
              <http://www.rfc-editor.org/info/rfc2898>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
              November 2003, <http://www.rfc-editor.org/info/rfc3629>.




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   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
              <http://www.rfc-editor.org/info/rfc5987>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

   [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 7613,
              DOI 10.17487/RFC7613, August 2015,
              <http://www.rfc-editor.org/info/rfc7613>.

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <http://www.rfc-editor.org/info/rfc7615>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.

18.2.  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:



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              KAM3-based Cryptographic Algorithms",
              draft-ietf-httpauth-mutual-algo-07 (work in progress),
              November 2016.

   [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, DOI 10.17487/RFC1939, May 1996,
              <http://www.rfc-editor.org/info/rfc1939>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/
              RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
              <http://www.rfc-editor.org/info/rfc5929>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <http://www.rfc-editor.org/info/rfc6265>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <http://www.rfc-editor.org/info/rfc6454>.

   [RFC7486]  Farrell, S., Hoffman, P., and M. Thomas, "HTTP Origin-
              Bound Authentication (HOBA)", RFC 7486, DOI 10.17487/
              RFC7486, March 2015,
              <http://www.rfc-editor.org/info/rfc7486>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer



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              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
              May 2015, <http://www.rfc-editor.org/info/rfc7525>.

   [RFC7564]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 7564, DOI 10.17487/RFC7564, May 2015,
              <http://www.rfc-editor.org/info/rfc7564>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616, DOI 10.17487/
              RFC7616, September 2015,
              <http://www.rfc-editor.org/info/rfc7616>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.


Appendix A.  (Informative) Draft Change Log

   [To be removed on final publication]

A.1.  Changes in Httpauth WG Revision 11

   o  Reflecting IESG comments.

A.2.  Changes in Httpauth WG Revision 10

   o  Small rephrasing and a typo fix.

A.3.  Changes in Httpauth WG Revision 09

   o  Reflected AD review comments.

   o  Authors' addresses updated.

A.4.  Changes in Httpauth WG Revision 08

   o  Minor text update, in sync with httpauth-extension.

   o  The version token is raised to "1".






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A.5.  Changes in Httpauth WG Revision 07

   o  Several comments from reviewers are reflected to the text.

   o  The password-hash has been completely dropped.

   o  The version token is raised to "1".

A.6.  Changes in Httpauth WG Revision 06

   o  The auth-domain parameter has been renamed to auth-scope,
      following suggestions on the mailing list.

   o  The digest-md5 password-hash has been dropped, as Digest with MD5
      hash is now obsoleted.

A.7.  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.

A.8.  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.

A.9.  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.)

A.10.  Changes in Httpauth WG Revision 02

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




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

A.11.  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.

   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.

A.12.  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)

A.13.  Changes in HttpBis Revision 00

   None.

A.14.  Changes in Revision 12

   o  Added a reason "authz-failed".

A.15.  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.



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

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

A.16.  Changes in Revision 10

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

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

   o  Provision for application channel binding is added.

   o  Provision for proxy access authentication is added.

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

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

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

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

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






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A.17.  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:

   +-------------------+-----------------+-----------------------------+
   | new name          | old name        | description                 |
   +-------------------+-----------------+-----------------------------+
   | 401-INIT          | 401-B0          | initial response            |
   | 401-STALE         | 401-B0-stale    | session shared secret       |
   |                   |                 | 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              |
   +-------------------+-----------------+-----------------------------+

A.18.  Changes in Revision 08

   o  The English text has been revised.

A.19.  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.





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A.20.  Changes in Revision 06

   o  Integrated Optional Mutual Authentication to the main part.

   o  Clarified the decision procedure for message recognitions.

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

   o  Typos and confusing phrases are fixed.

   o  Several "future considerations" are added.

A.21.  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.

A.22.  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.

A.23.  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.






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A.24.  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.

   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.

A.25.  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 1
   1-1-1 Umezono
   Tsukuba-shi, Ibaraki
   JP

   Email: y.oiwa@aist.go.jp


   Hajime Watanabe
   National Institute of Advanced Industrial Science and Technology
   Information Technology Research Institute
   Tsukuba Central 1
   1-1-1 Umezono
   Tsukuba-shi, Ibaraki
   JP

   Email: h-watanabe@aist.go.jp




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

   Email: takagi.hiromitsu@aist.go.jp


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

   Email: maeda@lepidum.co.jp


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

   Email: hayashi@lepidum.co.jp


   Yuichi Ioku
   Individual

   Email: mutual-work@ioku.org
















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