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Versions: 00

Internet Engineering Task Force                                  Y. Oiwa
Internet-Draft                                               H. Watanabe
Intended status: Experimental                                  H. Takagi
Expires: August 22, 2013                                     RISEC, AIST
                                                               B. Kihara
                                                              T. Hayashi
                                                                 Lepidum
                                                                 Y. Ioku
                                                            Yahoo! Japan
                                                       February 18, 2013


    Common Template for HTTP Message-based Multi-hop Authentication
                draft-oiwa-httpauth-multihop-template-00

Abstract

   This document specifies a common protocol design template for
   authentication on the Hyper-text Transport Protocol (HTTP) involving
   multi-hop message exchanges.  To facilitate advanced authentication
   technologies such as hash-based exchanges, zero-knowledge password
   proof, or public-key authentications on HTTP, a kind of state
   management and key management facilities are required on the general
   HTTP authentication message framework.  Also, to optimize performance
   of such authentication schemes, a well-designed mechanism for key
   caching and re-authentication are needed.  The template defined in
   this document provides a generic foundation for implementing such
   advanced authentication technologies.

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 August 22, 2013.

Copyright Notice




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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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







































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  How to Use This Document . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Document Structure and Related Documents . . . . . . . . .  6
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Messages Overview  . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Typical Flows of the Protocol  . . . . . . . . . . . . . .  7
     2.3.  Alternative Flows  . . . . . . . . . . . . . . . . . . . .  9
   3.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Values . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.1.  Tokens . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.2.  Strings  . . . . . . . . . . . . . . . . . . . . . . . 13
       3.1.3.  Numbers  . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  401-INIT and 401-STALE . . . . . . . . . . . . . . . . . . 15
     4.2.  req-KEX-C1 . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  401-KEX-S1 . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.4.  req-VFY-C  . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.5.  200-VFY-S  . . . . . . . . . . . . . . . . . . . . . . . . 18
   5.  Session Management . . . . . . . . . . . . . . . . . . . . . . 19
   6.  Host Validation Methods  . . . . . . . . . . . . . . . . . . . 21
   7.  Decision Procedure for Clients . . . . . . . . . . . . . . . . 22
   8.  Decision Procedure for Servers . . . . . . . . . . . . . . . . 27
   9.  Applying for Specific Authentication Schemes . . . . . . . . . 29
     9.1.  Default Functions for Algorithms . . . . . . . . . . . . . 30
   10. Application Channel Binding  . . . . . . . . . . . . . . . . . 30
   11. String Preparation . . . . . . . . . . . . . . . . . . . . . . 31
   12. Application for Proxy Authentication . . . . . . . . . . . . . 31
   13. Methods to extend this protocol template . . . . . . . . . . . 32
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 33
     15.1. Security Properties  . . . . . . . . . . . . . . . . . . . 33
     15.2. Denial-of-service Attacks to Servers . . . . . . . . . . . 33
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 34
     16.2. Informative References . . . . . . . . . . . . . . . . . . 35
   Appendix A.  (Normative) Support Functions and Notations . . . . . 35
   Appendix B.  (Informative) Draft Remarks from Authors  . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37










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1.  Introduction

   This document specifies a common protocol design template for
   authentication on the Hyper-text Transport Protocol (HTTP) involving
   multi-hop message exchanges.

   To facilitate advanced authentication technologies such as hash-based
   exchanges, zero-knowledge password proof, or public-key
   authentications on HTTP, a kind of state management and key
   management facilities are required on the general HTTP authentication
   message framework.  Also, to optimize performance of such
   authentication schemes, a well-designed mechanism for key caching and
   re-authentication are needed.

   The template defined in this document provides a generic foundation
   for implementing such advanced authentication technologies.  Such
   generic foundations can reduce cumbersomeness of both designers and
   implementors of such authentication protocols on HTTP.  By using this
   template, protocol designers can easily apply any specific
   authenticated key exchange (or agreement) mechanisms onto HTTP
   protocol and enable authentication session management, shared-key
   based optimized re-authentication.

   The design template provided on this document is mainly designed for
   multi-hop authentication mechanisms which do not use connection-based
   session managements.  Some of existing authentication technologies
   applied on HTTP/1.0 or 1.1 are bound to underlying TCP connection,
   which violates strict definition of HTTP stateless semantics and not
   directly applicable to forthcoming HTTP/2.0.  Retrofitting of such
   existing authentication schemes are out-of-scope of this
   specification (although, an additional specification for such
   retrofitting _may_ be defined on top of this template).

   The template is defined using terminology and representation of
   existing HTTP/1.1, but it can be also directly applied on forthcoming
   HTTP/2.0.

1.1.  How to Use This Document

   This document is only providing a "template" for actual
   implementation of HTTP authentication: by itself only it will be
   useless.  To use this document, there must be a specific definition
   document for each authentication schemes referring to this document.
   In other words, this document and such a specific definitions will
   compose "layers" of protocol definitions, the latter will exist upon
   the former.

   However, for implementors' perspective, the definitions in this



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   document can be implemented as a "base class" for multi-hop
   authentication: such class can be a common bases for "deriving"
   implementations of each authentication schemes, which will avoid
   duplicated implementation of same features and reduce burdens for
   testing such implementations one by one.

   For terminology, this document uses the following three terms for
   referring each "layers" of protocols:

   o  "The authentication template" or "this template" will refer to the
      common protocol template defined in this document.

   o  "Authentication scheme(s)" will refer to a scheme which will
      realize a specific purpose/method of authentication.  Examples of
      these schemes (which do not always depend on "this template") are
      Basic, Digest and others.  Each of them will also correspond to a
      specific "auth-scheme" in the HTTP headers.

   o  "Sub-algorithms" or simply "algorithms" in an authentication
      scheme will refer to variations within a single authentication
      scheme which will provide a small differences of authentication
      properties such as cryptographic strength or others.  Examples of
      them are "auth" and "auth-int" in Digest.  Differences of used
      cryptographic primitives and/or parameters which provides the same
      functionalities except strengths (e.g. key lengths, hash choices
      etc.) will often fall into this category.

1.2.  Terminology

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

   The terms "encouraged" and "advised" are used for suggestions that do
   not constitute "SHOULD"-level requirements.  People MAY freely choose
   not to include the suggested items regarding [RFC2119], but complying
   with those suggestions would be a best practice; it will improve
   security, interoperability, and/or operational performance.

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

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



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

1.3.  Document Structure and Related Documents

   The entire document is organized as follows:

   o  Section 2 presents an overview of the protocol design.

   o  Sections 3 to 8 define a general template for the multi-hop
      authentication protocol.  This template is independent of specific
      cryptographic primitives and authentication schemes.

   o  Section 9 describes requirements for each authentication schemes
      used with this protocol template, and defines a few functions
      which will be shared among such cryptographic algorithms.

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

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


2.  Protocol Overview

   The protocol template, as a whole, is designed as a natural extension
   to the HTTP protocol [I-D.ietf-httpbis-p1-messaging] using a
   framework defined in [I-D.ietf-httpbis-p7-auth].  Internally, the
   server and the client will first perform a cryptographic key
   exchange, defined for each authentication schemes.  The key-exchange
   will derive the same session keys only when the clients and servers
   are agreed with the authentication credentials used.  Then, both
   peers will verify the authentication results by confirming the
   sharing of the exchanged key.  This section describes a brief image
   of the protocol and the exchanged messages.

2.1.  Messages Overview

   The authentication protocol template uses six kinds of messages to
   perform multi-hop authentication.  These messages have specific names
   within this specification.

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





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

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

   o  Authenticated key exchange messages: used by both peers to perform
      authentication and the sharing of a session key (shared secret).

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

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

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

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

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

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

2.2.  Typical Flows of the Protocol

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


















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         Client                                 Server
           |                                      |
           |  ---- (1) normal request --------->  |
       GET / HTTP/1.1                             |
           |                                      |
           |  <---------------- (2) 401-INIT ---  |
           |            401 Authentication Required
           |            WWW-Authenticate: Example realm="a realm"
           |                                      |
  [user,   |                                      |
   cred.]->|                                      |
           |  ---- (3) req-KEX-C1 ------------->  |
       GET / HTTP/1.1                             |
       Authorization: Example user="john",        |--> [user DB]
                      kc1="...", ...              |<-- [user info]
           |                                      |
           |  <-------------- (4) 401-KEX-S1 ---  |
           |           401 Authentication Required
           |           WWW-Authenticate: Example sid=..., ks1="...", ...
           |                                      |
       [compute] (5) compute session secret   [compute]
           |                                      |
           |                                      |
           |  ---- (6) req-VFY-C -------------->  |
       GET / HTTP/1.1                             |--> [verify (6)]
       Authorization: Example sid=...,            |<-- OK
                      vkc="...", ...              |
           |                                      |
           |  <--------------- (7) 200-VFY-S ---  |
  [verify  |           200 OK                     |
    (7)]<--|           Authentication-Info: Example 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 authentication, the server
      will respond with a message requesting authentication (401-INIT)
      (2).

   o  The client processes the body of the message, and waits for the
      user to input the authentication credentials (such as a user name
      and a password).  When the credentials to be used become
      available, the client will send a message with the authenticated
      key exchange (req-KEX-C1) to start the authentication (3).




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

   o  At this point (5), both peers calculate a shared "session secret"
      using the exchanged values in the key exchange messages.  It is
      assumed that underlying authentication protocol will generate the
      same "session secret" on both sides only when the user
      authentication succeeds.  This session secret will be used for the
      actual access authentication after this point.

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

   o  If the authentication verification value from the client was
      correct, it means that the client definitely owns the credentials
      required for authentication. (i.e. the client authentication
      succeeded.)  The server will respond with a successful message
      (200-VFY-S) (7).

      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 response (200-VFY-S) may contain the server-side
      authentication verification value (7).  When the underlying
      authentication mechanism supports bidirectional authentication,
      clients can check server's identity using this information.

2.3.  Alternative Flows

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

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





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

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

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



































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       (A) omit first request
          (2 round trips)

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


       (B) reusing session secret (re-authentication)

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

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

              Figure 2: Several alternative flows on protocol

   For more details, see Sections 7 and 8.


3.  Message Syntax

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



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

   Each authentication scheme using this template SHALL specify a single
   token specific to the underlying scheme (like Basic or Digest).  All
   of the "auth-scheme" contained in all of those headers MUST be that
   token.

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

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

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

3.1.  Values

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

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

3.1.1.  Tokens

   Tokens will have syntax of the "token" defined in HTTP.  The
   canonical format for tokens are unquoted tokens.







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3.1.2.  Strings

   All character strings outside ASCII character sets MUST be encoded
   using the UTF-8 encoding [RFC3629] for the ISO 10646-1 character set
   [ISO.10646-1.1993], without any leading BOM characters.  Both peers
   are RECOMMENDED to reject any invalid UTF-8 sequences that might
   cause decoding ambiguities (e.g., containing <"> in the second or
   later byte of the UTF-8 encoded characters).

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

   The canonical format for strings are quoted-string.

3.1.3.  Numbers

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

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

                   Figure 3: BNF syntax for number types

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

   The numbers represented as a hex-fixed-number MUST include an even
   number of characters (i.e. multiples of eight bits).  Those values
   are case-insensitive, and SHOULD be sent in lower-case.  When these
   values are generated from any cryptographic values, they SHOULD have
   their "natural length": if these are generated from a hash function,
   these lengths SHOULD correspond to the hash size; if these are
   representing elements of a mathematical set (or group), its lengths
   SHOULD be the shortest for representing all the elements in the set.
   For example, any results of SHA-256 hash function will be represented
   by 64 characters, and any elements in 2048-bit prime field (modulo a
   2048-bit integer) will be represented by 512 characters, regardless
   of how much 0's will be appear in front of such representations.
   Session-identifiers and other non-cryptographically generated values
   are represented in any (even) length determined by the side who
   generates it first, and the same length SHALL be used throughout the
   all communications by both peers.



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   The numbers represented as base64-fixed-number SHALL be generated as
   follows: first, the number is converted to a big-endian radix-256
   binary representation as an octet string.  The length of the
   representation is determined in the same way as mentioned above.
   Then, the string is encoded using the Base 64 encoding [RFC4648]
   without any spaces and newlines.  Implementations decoding base64-
   fixed-number SHOULD reject any input data with invalid characters,
   excess/insufficient paddings, or non-canonical pad bits (See Sections
   3.1 to 3.5 of [RFC4648]).

   The canonical format for integer and hex-fixed-number are unquoted
   tokens, and that for base64-fixed-number is quoted-string (as it will
   contain equal, plus signs and slashs).


4.  Messages

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

   To determine which message are expected to be sent, see Sections 7
   and 8.

   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 and the auth-
   scheme to be used.  The parameters marked "mandatory" SHALL be
   contained in the message.  The parameters marked "non-mandatory" MAY
   either be contained or omitted in the message.  Each parameter SHALL
   appear in each headers exactly once at most.

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

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

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



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4.1.  401-INIT and 401-STALE

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

   algorithm:     (mandatory token) specifies the authentication sub-
                  algorithm to be used.  The set of allowed value for
                  this field MUST be specified within each specification
                  for a specific authentication protocol.

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

   validation:    (mandatory token) specifies the method of host
                  validation.  The value MUST be one of the tokens
                  described in Section 6, or the tokens specified in
                  other supplemental specification documentation.

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

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

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

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

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




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

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

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

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

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

                  *  authz-failed: authentication was successful, but
                     access to the specified resource is not authorized
                     to the specific authenticated user.  (It is
                     different from 403 responses which suggest that the
                     reason of inaccessibility is other that
                     authentication.)

   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:

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






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   user:          (non-mandatory, string) is the UTF-8 encoded name of
                  the user.  This field MUST be present unless the
                  authentication scheme defines other means of
                  identifying the authenticating users other than the
                  textual user name.  If this name comes from a user
                  input, client software SHOULD prepare the string using
                  the preparation mechanism defined with each scheme
                  (see Section 11 for more information) before encoding
                  it to UTF-8.

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

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:

   algorithm, realm:  MUST be the same value as it is when received from
                  the client.

   sid:           (mandatory, hex-fixed-number) MUST be a session
                  identifier, which is a random integer.  The sid SHOULD
                  have uniqueness of at least 80 bits or the square of
                  the maximal estimated transactions concurrently
                  available in the session table, whichever is larger.
                  See Section 5 for more details.

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

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

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

   time:          (mandatory, integer) represents the suggested time (in
                  seconds) that the client can reuse the session
                  represented by the sid.  It is RECOMMENDED to be at
                  least 60.  The value of this parameter is, however,



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                  not directly linked to the duration that the server
                  keeps track of the session represented by the sid.

   path:          (non-mandatory, string) specifies which path in the
                  URI space the same authentication is expected to be
                  applied.  The value is a space-separated list of URIs,
                  in the same format as it was specified in domain
                  parameter [RFC2617] for the Digest authentications,
                  and clients are RECOMMENDED to recognize it.  The all
                  path elements contained in the parameter MUST be
                  inside the specified auth-domain: if not, clients
                  SHOULD ignore such elements.

4.4.  req-VFY-C

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

   The parameters contained in the header are as follows:

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

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

   nc:            (mandatory, integer) is a nonce value that is unique
                  among the requests sharing the same sid.  The values
                  of the nonces SHOULD satisfy the properties outlined
                  in Section 5.

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

4.5.  200-VFY-S

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

   The parameters contained in the header are as follows:







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

   algorithm, realm:  MUST be the same value as it is when received from
                  the client.

   vks:           (mandatory, algorithm-determined) is the server-side
                  authentication verification value VK_s, which is
                  specified by the algorithm.  If the algorithm
                  specification do not specify any specific value for
                  this field, the value SHALL the token "0".

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


5.  Session Management

   In this authentication protocol template, a session represented by an
   sid is set up using first four messages (first request, 401-INIT,
   req-KEX-C1 and 401-KEX-S1).  After sharing a session secret, this
   session, along with the secret, can be used for one or more requests
   for resources protected by the same realm in the same server.  Note
   that session management is only an inside detail of the protocol and
   usually not visible to normal users.  If a session expires, the
   client and server SHOULD automatically re-establish another session
   without informing the users.

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

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

   The client MAY send two or more requests using a single session
   specified by the sid.  However, for all such requests, each value of



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   the nonce (in the nc parameter) MUST satisfy the following
   conditions:

   o  It is a natural number.

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

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

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

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

   Servers MUST check for duplication of the received nonces, and if any
   duplication is detected, the server MUST discard the session and
   respond with a 401-STALE message, as outlined in Section 8.  The
   server MAY also reject other invalid nonce values (such as ones above
   the nc-max limit) by sending a 401-STALE message.

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

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

   The values of the nonces and any nonce-related values MUST always be
   treated as natural numbers within an infinite range.  Implementations
   using fixed-width integers or fixed-precision floating numbers MUST
   correctly and carefully handle integer overflows.  Such
   implementations are RECOMMENDED to accept any larger values that
   cannot be represented in the fixed-width integer representations, as
   long as other limits such as internal header-length restrictions are
   not involved.  The protocol is designed carefully so that both the



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   clients and servers can implement the protocol using only fixed-width
   integers, by rounding any overflowed values to the maximum possible
   value.


6.  Host Validation Methods

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

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

   host:          hostname validation: The value vh will be the ASCII
                  string in the following format:
                  "<scheme>://<host>:<port>", where <scheme>, <host>,
                  and <port> are the URI components corresponding to the
                  currently accessing resource.  The scheme and host are
                  in lower-case, and the port is in a shortest decimal
                  representation.  Even if the request-URI does not have
                  a port part, vh will include the default port number.

   tls-cert:      TLS certificate validation: The value vh will be the
                  octet string of the hash value of the public key
                  certificate used in the underlying TLS [RFC5246] (or
                  SSL) connection.  The hash value is defined as the
                  value of the entire signed certificate (specified as
                  "Certificate" in [RFC5280]), hashed by the hash
                  algorithm specified by the authentication algorithm
                  used.

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

   If the HTTP protocol is used on a non-encrypted channel (TCP and
   SCTP, for example), the validation type MUST be "host".  If HTTP/TLS
   [RFC2818] (HTTPS) protocol is used with the server certificates, the
   validation type MUST be "tls-cert".  If HTTP/TLS protocol is used
   without any kind of server certificates, the validation type MUST be
   "tls-key".

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



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

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


7.  Decision Procedure for Clients

   To securely implement the protocol, the user client must be careful
   about accepting the authenticated responses from the server.  This
   also holds true for the reception of "normal responses" from HTTP
   servers.

   Clients SHOULD implement a decision procedure equivalent to the one
   shown below.  (Unless implementers understand what is required for
   the security, they should not alter this.)  In particular, clients
   SHOULD NOT accept "normal responses" unless explicitly allowed below.
   The labels on the steps are for informational purposes only.  Action
   entries within each step are checked in top-to-bottom order, and the
   first clause satisfied SHOULD be taken.

   Step 1 (step_new_request):
       If the client software needs to access a new Web resource, check
       whether the resource is expected to be inside some authentication
       realm for which the user has already been authenticated by the
       authentication scheme.  If yes, go to Step 2.  Otherwise, go to
       Step 5.

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

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

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

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

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




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

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

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

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

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

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

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

   Step 5 (step_send_normal_1):
       Send a request without any authentication headers related to this
       specification.

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

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

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

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

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

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

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

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








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

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

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

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

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

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

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

   Step 12 (step_rcvd_init_unknown):
       The requested resource requires a authentication, and the user is
       not yet authenticated.  The client will be in the "AUTH-
       REQUESTED" status, and is RECOMMENDED to process the content sent
       from the server, and to ask user for any user's authentication
       credentials.  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 used
       authentication credentials are invalid for the authenticated
       resource.  Forget such authentication credentials (or disable,
       whichever appropriate for the specific kind of credentials) for
       the authentication realm and go to Step 12.

   Step 14 (step_rcvd_vfy):
       Check the validity of the received VK_s value.  If it is equal to
       the expected value, it means that the server authentication has
       succeeded.  The client will be in the "AUTH-SUCCEEDED" status.

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

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

   Any kind of response (including a normal response) other than those
   shown in the above procedure SHOULD be interpreted as a fatal
   communication error, and in such cases the clients SHOULD NOT process



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   any data (response body and other content-related headers) sent from
   the server.  However, to handle exceptional error cases, clients MAY
   accept a message without an Authentication-Info header, if it is a
   Server-Error (5xx) status.  The client will be in the
   "UNAUTHENTICATED" status in these cases.

   Figure 4 shows a diagram of the client-side state.












































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         ===========                                  -(11)------------
         NEW REQUEST                                 ( UNAUTHENTICATED )
         ===========                                  -----------------
              |                                              ^ normal
              v                                              | response
   +(1)-------------------+ NO                         +(5)----------+
   | The requested URI    |--------------------------->| send normal |
   | known to be auth'ed? |                            |   request   |
   +----------------------+                            +-------------+
          YES |   401-INIT                            401-INIT|
              |   with a different realm                      |
              |          -----------------------------------. |
              |         /                                   v v
              |        |       -(12)------------    NO  +(6)--------+
              |        |      ( AUTH-REQUESTED  )<------|credentials|
              |        |       -----------------        |   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 credentials/      | 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 4: State diagram for clients



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8.  Decision Procedure for Servers

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

   o  The session identifier, the value of the sid parameter.

   o  The algorithm used.

   o  The authentication realm.

   o  The state of the protocol: one of "key exchanging",
      "authenticated", "rejected", or "inactive".

   o  The user name received from the client

   o  The boolean flag noting whether or not the session is fake.

   o  When the state is "key exchanging", the values of K_c1 and S_s1.

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

      *  The value of the session secret z

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

      *  For each possible nc values between (largest-nc - nc-
         window + 1) and max_nc, a flag whether or not a request with
         the corresponding nc has been received.

   The table MAY contain other information.

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

   o  When the server receives a normal request:

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

      *  If the resource is protected by the authentication, send a
         401-INIT response.

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





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

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

      *  If the server cannot validate the parameter kc1, send a
         401-INIT response.

      *  If the received user name is either invalid, unknown or
         unacceptable, create a new session, mark it a "fake" session,
         compute a random value as K_s1, and send a fake 401-KEX-S1
         response.  (Note: the server SHOULD NOT send a 401-INIT
         response in this case, because it will leak the information to
         the client that the specified user will not be accepted.
         Instead, postpone it to the response for the next req-VFY-C
         request.)

      *  Otherwise, create a new session, compute K_s1 and send a
         401-KEX-S1 response.

      The created session has the "key exchanging" state.

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

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

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

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

      *  If the session corresponding to the received sid could not be
         found, or it is in the "inactive" state, send a 401-STALE
         response.

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

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

      *  If the nc value in the request is larger than the nc-max
         parameter sent from the server, or if it is not larger then



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         (largest-nc - nc-window) (when in "authenticated" status), the
         server MAY (but not REQUIRED to) send a 401-STALE message.  The
         session SHOULD be changed to the "inactive" status if so.

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

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

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


9.  Applying for Specific Authentication Schemes

   Each authentication scheme to use this template MUST at least provide
   a definitions for the following functions:

   o  A token for distinguishing the protocol from any others (like
      Basic or Digest), to be used as "auth-scheme"s.

   o  A set of tokens which will be allowed in the "algorithm" field of
      401-INIT message.

   o  * A string preparation algorithm based on
      [I-D.ietf-precis-framework]. (see Section 11)

   Furthermore, for each sub-algorithm defined by the "algorithm" field,
   the following MUST be defined:

   o  A format for representing fields "kc1", "ks1", "vkc" and "vks".

   o  An algorithm for computing key exchange values K_c1, K_s1.

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

   o  * An algorithm for computing authentication confirmation values
      VK_c, VK_s.  Values derived by these algorithms SHOULD depend on



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      the value of "nc" value used for each re-authenticating requests
      using the same "sid".  It SHOULD also depend on the value of the
      host verification value "vh".

   o  * If possible, an algorithm for computing "application channel
      binding keys" (see Section 10).

   For items marked with asterisks (*), default template functions are
   provided in the following sections.

9.1.  Default Functions for Algorithms

   If there are no specific (such as compatibility) requirements for
   values VK_c, VK_s, schemes MAY use the default functions for
   computing VK_c and VK_s, defined in this section.  Designers of
   specific authentication schemes MAY choose either to use this default
   function or not, depending on the nature and the background settings
   for each authentication schemes to be defined.

   To use this default function, the algorithm specification SHALL
   specify the following values.

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

   The values VK_c and VK_s are derived by the following equation.

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

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

   The definitions of any support functions in the above definitions are
   provided in Appendix A.


10.  Application Channel Binding

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

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



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   The following definitions are assuming that the authentication scheme
   uses the default function shown above for computing VK_c and VK_s.
   If not, the specification for the authentication scheme is encouraged
   to provide an alternative means for this purpose (e.g., either to
   specify the function for z, or to specify functions b_1 and b_2).

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

   b_1 = OCTETS(H(OCTETS(H(octet(6) | OCTETS(K_c1) | OCTETS(K_s1) |
   OCTETS(z) | VI(0) | VS(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 = OCTETS(H(OCTETS(H(octet(7) | OCTETS(K_c1) | OCTETS(K_s1) |
   OCTETS(z) | VI(nc) | VS(vh))) | VS(t))).

   The definitions of any support functions in the above definitions are
   provided in Appendix A.


11.  String Preparation

   For proper internationalization of the protocol to be designed, each
   authentication scheme SHOULD specify algorithms for preparing string
   inputs, unless the underlying protocol does not use any kind of
   human-readable (i.e., possibly-non-ASCII-capable) identifier or
   passwords.

   If some algorithm is suitable for each specific authentication scheme
   in relation to other existing protocols, that one should be used
   (e.g.  [RFC4013] or [I-D.melnikov-precis-saslprepbis] for any SASL-
   related authentication algorithms).

   If there is no specific one to be chosen, schemes may choose the
   following default choice: use [I-D.melnikov-precis-saslprepbis] for
   user-identifiers and passwords-like strings, except that case mapping
   of upper-case and title-case letters will NOT be applied (i.e., the
   string will be left case-sensitive, for keeping compatibility with
   existing HTTP-based authentication mechanisms).


12.  Application for Proxy Authentication

   The authentication scheme defined by using the previous sections can



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   be applied also for proxy authentications.  In such cases, the
   following alterations MUST be applied:

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

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

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

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

   o  The 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.


13.  Methods to extend this protocol template

   The template is designed to have fair amount of flexibility for
   implementing several authentication schemes.  However, if needed,
   specifications defining authentication schemes or authentication
   algorithms MAY define its own representations for the parameters
   "kc1", "ks1", "vkc", and "vks", and/or add parameters to the messages
   containing those parameters in supplemental specifications, provided
   that syntactic and semantic requirements in Section 3,
   [I-D.ietf-httpbis-p1-messaging] and [I-D.ietf-httpbis-p7-auth] are
   satisfied.

   If there is more than two round-trips of messages needed for
   performing authentication, messaged named "req-KEX-C2", "401-KEX-S2",
   "req-KEX-C3" and so on MAY be used between 401-KEX-S1 and req-VFY-C
   messages.  These messages MUST have algorithm, realm, and sid fields
   as the same as req-KEX-C1 and 401-KEX-S1. and they SHOULD have fields
   named "kc2", "ks2", "kc3" and so on, respectively.

   It is RECOMMENDED that 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.  It is strongly
   encouraged that specifications for authentication schemes do not
   rename or remove there fields, as they are important for
   distinguishing message types.



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14.  IANA Considerations

   [TBD]


15.  Security Considerations

15.1.  Security Properties

   o  The protocol template relies on transport security including DNS
      integrity for data secrecy and integrity, regardless of any
      underlying authentication algorithm to be used.  HTTP/TLS SHOULD
      be used where transport security is not assured and/or data
      secrecy is important.

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

15.2.  Denial-of-service Attacks to Servers

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

   However, if a malicious client sends too many requests of key
   exchanges (req-KEX-C1 messages) only, resource starvation might
   occur.  In such critical situations, servers MAY discard any kind of
   existing sessions regardless of these statuses.  One way to mitigate
   such attacks are that servers MAY have a number and a time limits for
   unverified pending key exchange requests (in the "wa received"
   status).

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




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16.  References

16.1.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-21 (work in progress),
              October 2012.

   [I-D.ietf-httpbis-p7-auth]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth-21
              (work in progress), October 2012.

   [I-D.ietf-precis-framework]
              Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation and Comparison of Internationalized Strings in
              Application Protocols", draft-ietf-precis-framework-06
              (work in progress), September 2012.

   [I-D.melnikov-precis-saslprepbis]
              Saint-Andre, P. and A. Melnikov, "Preparation and
              Comparison of Internationalized Strings Representing
              Simple User Names and Passwords",
              draft-melnikov-precis-saslprepbis-04 (work in progress),
              September 2012.

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

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

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

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

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

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






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16.2.  Informative References

   [ISO.10646-1.1993]
              International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Part 1: Architecture and Basic
              Multilingual Plane", ISO Standard 10646-1, May 1993.

   [ITU.X690.1994]
              International Telecommunications Union, "Information
              Technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T Recommendation
              X.690, 1994.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

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

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

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

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


Appendix A.  (Normative) Support Functions and Notations

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

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

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

   The function VI encodes natural numbers into octet strings in the



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   following manner: numbers are represented in big-endian radix-128
   string, where each digit is represented by a octet within 0x80-0xff
   except the last digit represented by a octet within 0x00-0x7f.  The
   first octet MUST NOT be 0x80.  For example, VI(i) = octet(i) for i <
   128, and VI(i) = octet(0x80 + (i >> 7)) | octet(i & 127) for 128 <= i
   < 16384.  This encoding is the same as the one used for the
   subcomponents of object identifiers in the ASN.1 encoding
   [ITU.X690.1994], and available as a "w" conversion in the pack
   function of several scripting languages.

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

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

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

   Some examples:

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

      VI(100) = "d"

      VI(10000) = "\316\020"

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

      VS("") = "\000"

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

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

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

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

   The function OCTETS converts an integer into the corresponding radix-
   256 big-endian octet string having its natural length: See
   Section 3.1.3 for the definition of "natural length".







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Appendix B.  (Informative) Draft Remarks from Authors

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

   o  Whether to keep TLS-key validation or not.

   o  When keeping tls-key validation, whether to use "TLS channel
      binding" [RFC5929] for "tls-key" verification (Section 6).  Note
      that existing TLS implementations should be considered to
      determine this.


Authors' Addresses

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

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


   Hajime Watanabe
   National Institute of Advanced Industrial Science and Technology


   Hiromitsu Takagi
   National Institute of Advanced Industrial Science and Technology


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


   Tatsuya Hayashi
   Lepidum Co. Ltd.







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













































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