HTTP Authentication Extensions for Interactive Clients
draft-ietf-httpauth-extension-06

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HTTPAUTH Working Group                                           Y. Oiwa
Internet-Draft                                               H. Watanabe
Intended status: Experimental                                  H. Takagi
Expires: November 23, 2016                                    ITRI, AIST
                                                              T. Hayashi
                                                                 Lepidum
                                                                 Y. Ioku
                                                              Individual
                                                            May 22, 2016

         HTTP Authentication Extensions for Interactive Clients
                    draft-ietf-httpauth-extension-06

Abstract

   This document specifies extensions of HTTP authentication framework
   for interactive clients.  Recently, fundamental features of HTTP-
   level authentication are insufficient for complex requirements of
   various Web-based applications.  This forces these applications to
   implement their own authentication frameworks using HTML Forms and
   other means, which becomes one of the hurdles against introducing
   secure authentication mechanisms handled jointly by servers and user-
   agent.  The extended framework fills gaps between Web application
   requirements and HTTP authentication provisions to solve the above
   problems, while maintaining compatibility against existing Web and
   non-Web uses of HTTP authentications.

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 November 23, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the

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   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.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Terms for describing authentication protocol flow  . . . .  5
     2.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
   3.  Optional Authentication  . . . . . . . . . . . . . . . . . . .  8
   4.  Authentication-Control header  . . . . . . . . . . . . . . . .  9
     4.1.  Non-ASCII extended header parameters . . . . . . . . . . . 11
     4.2.  Auth-style parameter . . . . . . . . . . . . . . . . . . . 12
     4.3.  Location-when-unauthenticated parameter  . . . . . . . . . 13
     4.4.  No-auth parameter  . . . . . . . . . . . . . . . . . . . . 13
     4.5.  Location-when-logout parameter . . . . . . . . . . . . . . 14
     4.6.  Logout-timeout parameter . . . . . . . . . . . . . . . . . 15
     4.7.  Username parameter . . . . . . . . . . . . . . . . . . . . 15
   5.  Usage examples (informative) . . . . . . . . . . . . . . . . . 16
     5.1.  Example 1: a portal site . . . . . . . . . . . . . . . . . 16
       5.1.1.  Case 1: a simple application . . . . . . . . . . . . . 17
       5.1.2.  Case 2: specific action required on log-out  . . . . . 17
       5.1.3.  Case 3: specific page displayed before log-in  . . . . 18
     5.2.  Example 2: authenticated user-only sites . . . . . . . . . 18
     5.3.  When to use Cookies  . . . . . . . . . . . . . . . . . . . 18
     5.4.  Parallel deployment with Form/Cookie authentications . . . 19
   6.  Methods to extend this protocol  . . . . . . . . . . . . . . . 20
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  (Informative) Applicability of features for each
                messages  . . . . . . . . . . . . . . . . . . . . . . 23
   Appendix B.  (Informative) Draft Change Log  . . . . . . . . . . . 24
     B.1.  Changes in Httpauth WG Revision 06 . . . . . . . . . . . . 24
     B.2.  Changes in Httpauth WG Revision 05 . . . . . . . . . . . . 24
     B.3.  Changes in Httpauth WG revision 04 . . . . . . . . . . . . 24
     B.4.  Changes in Httpauth WG revision 03 . . . . . . . . . . . . 24
     B.5.  Changes in Httpauth WG revision 02 . . . . . . . . . . . . 24
     B.6.  Changes in Httpauth WG revision 01 . . . . . . . . . . . . 24
     B.7.  Changes in Httpauth revision 00 and HttpBis revision 00  . 24
     B.8.  Changes in revision 02 . . . . . . . . . . . . . . . . . . 24
     B.9.  Changes in revision 01 . . . . . . . . . . . . . . . . . . 25
     B.10. Changes in revision 00 . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

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1.  Introduction

   The document proposes several extensions to the current HTTP
   authentication framework, to provide functionality comparable with
   current widely-used form-based Web authentication.  A majority of the
   recent websites on the Internet use custom application-layer
   authentication implementations using Web forms.  The reasons for
   these may vary, but many people believe that the current HTTP Basic
   and Digest authentication methods do not have enough functionality
   (including good user interfaces) to support most realistic Web-based
   applications.  However, this method is very weak against phishing and
   other attacks, because all behavior of the authentication is
   controlled from the server-side application.  This makes it really
   hard to implement any cryptographically strong authentication
   mechanisms into Web systems.  To overcome this problem, we need to
   "modernize" the HTTP authentication framework so that better client-
   controlled secure methods can be used with Web applications.  The
   extensions proposed in this document include:

   o  optional authentication on HTTP (Section 3),

   o  log out from both server and client side (Section 4), and

   o  finer control for redirection depending on authentication status
      (Section 4).

1.1.  Terminology

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

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

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

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2.  Definitions

2.1.  Terms for describing authentication protocol flow

   HTTP Authentication defined in [RFC7235] may involve several pairs of
   HTTP requests/responses.  Throughout this document, the following
   terms are used to categorize those messages: for requests,

   1) A non-authenticating request is a request not attempting any
      authentication: a request without any Authorization header.

   2) An authenticating request is the opposite: a request with an
      Authorization header.

   For responses,

   1) A non-authenticated response is a response which does not involve
      any HTTP authentication.  It does not contain any WWW-Authenticate
      or Authentication-Info header.

      Servers send this response when the requested resource is not
      protected by an HTTP authentication mechanism.  In context of this
      specification, non-authentication-related negative responses (e.g.
      403 and 404) are also considered non-authenticated responses.

      (See note on successfully-authenticated responses below for some
      ambiguous cases.)

   2) An authentication-initializing response is a response which
      requires or allows clients to start authentication attempts.
      Servers send this response when the requested resource is
      protected by HTTP authentication mechanism, and the request meets
      one of the following cases:

      *  The request is a non-authenticating request, or

      *  The request contained an authentication trial directed to a
         protection space (realm) other than the one the server
         expected.

      The server will specify the protection space for authentication in
      this response.

      Upon receiving this response, the client's behavior is further
      divided to two possible cases.

      *  If the client has no prior knowledge on authentication
         credentials (e.g. a user-name and a password) related to the

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         requested protection space, the protocol flow terminates and
         the client will ask the user to provide authentication
         credentials,

      *  On the other hand, if client already has enough authentication
         credentials to the requested protection space, the client will
         automatically send an authenticating request.  Such cases often
         occur when the client did not know beforehand that the current
         request-URL requires authentication.

   3) A successfully-authenticated response is a response for an
      authenticating request meaning that the authentication attempt was
      granted.  (Note: if the authentication scheme used does not use an
      Authentication-Info header, it may be indistinguishable from a
      non-authenticated response.)

   4) An intermediate authenticating response is a response for an
      authenticating request which requires more reaction by the client
      software without involving users.  Such a response is required
      when an authentication scheme requires two or more round-trip
      messages to perform authentication, or when an authentication
      scheme uses some speculative short-cut method (such as uses of
      cached shared secrets) and it failed.

   5) A negatively-authenticated response is a response for an
      authenticating request which means that the authentication attempt
      was declined and can not continue without a different set of
      authentication credentials.  Clients typically erase memory of the
      active credentials and ask the user for other ones.

      Usually the format of these responses are as same as the one for
      authentication-initializing responses.  Client can distinguish
      negatively-authenticated responses from authentication-
      initializing responses by comparing the protection spaces
      contained in the request and in the response.

   Figure 1 shows a state diagram of generic HTTP authentication with
   the above message categorization.  Note that many authentication
   schemes use only a subset of the transitions described on the
   diagram.  Labels in the figure show the abbreviated names of response
   types.

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         ===========                                -----------------
         NEW REQUEST                               ( UNAUTHENTICATED )
         ===========                                -----------------
              |                                            ^ non-auth.
              v                                            | response
   +----------------------+ NO                         +-------------+
   | The requested URI    |--------------------------->| send normal |
   | known to be auth'ed? |           ---------------->|   request   |
   +----------------------+          /                 +-------------+
          YES |                     /             initializing|
              v                    /                          |
     +------------------+ NO      /                           |
     | Can auth-req.(*1)|---------                            |
     | be constructed?  |                                     |
     +------------------+                                     |
          YES |            initializing                       |
              |      ---------------------------------------. |
              |     /                                       v v
              |    |            ----------------    NO  +-----------+
              |    |           ( AUTH-REQUESTED )<------|credentials|
              |    |            ----------------        |   known?  |
              v    |                                    +-----------+
        +-----------+ negative   -------------   negative     |YES
        |   send    |---------->( AUTH-FAILED )<---------,    |
       /| auth-req  |            -------------           |    |
      / +-----------+\                                   |    v
     |             \  \  intermediate                   +-----------+
     |              \  -------------------------------->|   send    |
     |               \                                  | auth-req  |
     | non-auth.      \successful            successful +-----------+
     | response (*2)   \                               /     |    ^
     v                  \                             /      |    |
    -----------------    \       --------------      /       `----'
   ( UNAUTHENTICATED )    ----->( AUTH-SUCCEED )<----    intermediate
    -----------------            --------------

          Figure 1: Generic state diagram for HTTP authentication

   Note: (*1) For example, "Digest" scheme requires server-provided
   nonce to construct client-side challenges.
   (*2) In "Basic" and some others, this cannot be distinguished from a
   successfully-authenticated response.

2.2.  Syntax Notation

   This specification uses an extended BNF syntax defined in [RFC7230].
   The following syntax definitions are quoted from [RFC7230] and

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   [RFC7235]: auth-scheme, quoted-string, auth-param, SP, BWS, header-
   field, and challenge.  It also uses the convention of using header
   names for specifying the syntax of header values.

   Additionally, this specification uses the following syntax
   definitions as a refinement for token and the right-hand-side of
   auth-param in [RFC7235].  (Note: these definitions are consistent
   with those in [I-D.ietf-httpauth-mutual].)

    bare-token        = 1*(%x30-39 / %x41-5A / %x61-7A / "-" / "_")
    extension-token   = "-" bare-token 1*("." bare-token)
    extensive-token   = bare-token / extension-token
    integer           = "0" / (%x31-39 *%x30-39)      ; no leading zeros

               Figure 2: the BNF syntax for common notations

   Extensive-tokens are used in this protocol where the set of
   acceptable tokens may include private extensions.  Any private
   extensions of this protocol MUST use extension-tokens with the format
   "-<token>.<domain-name>", where <domain-name> is a valid (sub-)domain
   name on the Internet owned by the party who defines the extension.

3.  Optional Authentication

   The Optional-WWW-Authenticate header enables a non-mandatory
   authentication, which is not possible under the current HTTP
   authentication mechanism.  In several Web applications, users can
   access the same contents as both a guest user and an authenticated
   user.  In most Web applications, This functionality is implemented
   using HTTP cookies [RFC6265] and custom form-based authentication.
   The new authentication method using this message will provide a
   replacement for these authentication systems.

   Servers MAY send HTTP non-interim responses containing the
   Optional-WWW-Authenticate header as a replacement of a 401 response
   when it the response is authentication-initializing.  The
   Optional-WWW-Authenticate header MUST NOT sent on 401 responses (i.e.
   a usual WWW-Authenticate header MUST be used on 401 responses.)

      HTTP/1.1 200 OK
      Optional-WWW-Authenticate: Basic realm="xxxx"

    Optional-WWW-Authenticate = 1#challenge

         Figure 3: BNF syntax for Optional-WWW-Authenticate header

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   The challenges contained in the Optional-WWW-Authenticate header are
   the same as those for a 401 responses corresponding to the same
   request.  For authentication-related matters, an optional
   authentication request will have the same meaning as a 401 message
   with a corresponding WWW-Authenticate header (as an authentication-
   initializing response).  (The behavior for other matters, such as
   caching, MAY be different between the optional authentication and 401
   messages.)

   A response with an Optional-WWW-Authenticate header SHOULD be
   returned from the server only when the request is either non-
   authenticated or authenticating to a wrong (not the server's
   expected) protection space.  If a response is either an intermediate
   or a negative response to a client's authentication attempt, the
   server MUST respond with a 401 status response with a
   WWW-Authenticate header instead.  Failure to comply with this rule
   will render clients unable to distinguish authentication successes
   and failures.

   The server is NOT RECOMMENDED to include an Optional-WWW-Authenticate
   header in a positive response when a client's authentication attempt
   succeeds.

   Whenever an authentication scheme supports servers sending some
   parameter which gives a hint of the URL space for the corresponding
   protection space for the same realm (e.g. "path" or "domain"),
   servers requesting non-mandatory authentication SHOULD send such
   parameter with the response.  Clients supporting non-mandatory
   authentication MUST recognize the parameter, and MUST send a request
   with an appropriate authentication credential in an Authorization
   header for any URI inside the specified paths.

   Support of this header is OPTIONAL; Clients MAY also choose any set
   of authentication schemes for which optional authentication is
   supported (in other words, its support MAY be scheme-dependent).
   However, some authentication schemes MAY require mandatory/
   recommended support for this header, so that server-side applications
   MAY assume that clients supporting such schemes are likely to support
   the extension as well.

4.  Authentication-Control header

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    Authentication-Control = 1#Auth-Control-Entry
    Auth-Control-Entry     = auth-scheme 1*SP 1#auth-control-param
    auth-control-param     = extensive-token BWS "=" BWS token
                           / extensive-token "*" BWS "=" BWS ext-value
    ext-value              = <see RFC 5987, Section 3.2>

      Figure 4: the BNF syntax for the Authentication-Control header

   The Authentication-Control header provides a more precise control of
   the client behavior for Web applications using an HTTP authentication
   protocol.  This header is supposed to be generated in the application
   layer, as opposed to WWW-Authenticate headers which will usually be
   generated by the Web servers.

   Support of this header is OPTIONAL, and clients MAY choose any subset
   of these parameters to be supported.  The set of supported parameters
   MAY also be authentication scheme-dependent.  However, some
   authentication schemes MAY require mandatory/recommended support for
   some or all of the features provided in this header.

   The Authentication-Control header contains one or more
   "authentication control entries" each of which corresponds to a
   single realm for a specific authentication scheme.  If the
   auth-scheme specified for an entry supports the HTTP "realm" feature,
   that entry MUST contain the "realm" parameter.  If not, the entry
   MUST NOT contain the "realm" parameter.

   Among the multiple entries in the header, the meaningful entries in
   the header are those corresponding to an auth-scheme and a realm (if
   any), for which "the authentication process is being performed, or
   going to be performed".  In more detail,

   (1)  If the response is either an authentication-initializing
        response or a negatively-authenticated response, there may be
        multiple challenges in the WWW-Authenticate header (or the
        Optional-WWW-Authenticate header defined in this extension),
        each of which corresponds to a different scheme and realm.  In
        this case, the client has a choice on the scheme and realm they
        will use to authenticate.  Only the entry in the
        &Authentication-Control; header corresponding to that scheme and
        realm are meaningful.

   (2)  If the response is either an intermediate authenticating
        response or a successfully-authenticated response, the scheme
        and realm given in the Authorization header of the HTTP request
        will determine the currently-ongoing authentication process.
        Only the entry corresponding to that scheme and realm are
        meaningful.

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   The server MAY send an Authentication-Control header containing non-
   meaningful entries.  The client MUST ignore all non-meaningful
   entries it received.

   Each entry contains one or more parameters, each of which is a name-
   value pair.  The name of each parameter MUST be an extensive-token.
   Clients MUST ignore any unknown parameters contained in this header.
   The entries for the same auth-scheme and the realm MUST NOT contain
   duplicated parameters for the same name.

   The type of parameter value depends on the parameter name as defined
   in the following subsections.  Regardless of the type, however, the
   recipients SHOULD accept both quoted and unquoted representations of
   values as defined in HTTP.  If the parameter is defined to have a
   string value, implementations SHOULD send the parameter in a quoted
   form or an ext-value form (see Section 4.1).  If the parameter is
   defined as a token (or similar) or an integer, the value SHOULD
   follow the corresponding ABNF syntax after possible unquoting of the
   quoted-string value (as defined in HTTP), and SHOULD be sent in a
   unquoted form.

   Server-side applications SHOULD be aware that any parameters
   contained in this header MAY be ignored by clients.  Also, even when
   a client accepts this header, users may always be able to circumvent
   the semantics of this header.  Therefore, if this header is used for
   security purposes, its use MUST be limited to providing some non-
   fundamental additional security measures valuable for end-users (such
   as client-side log-out for protecting against console takeover).
   Server-side applications MUST NOT rely on the use of this header for
   protecting server-side resources.

   Note: The header syntax allows servers to specify Authentication-
   Control for multiple authentication schemes, either as multiple
   occurrences of this header or as a combined single header (see
   Section 3.2.2 of [RFC7230] for rationale).  The same care as for
   parsing multiple authentication challenges SHALL be taken.

4.1.  Non-ASCII extended header parameters

   Parameters contained in the Authentication-Control header 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.

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   If a value (except the "realm" header) contains one or more non-ASCII
   characters, the parameter SHOULD be sent using the ext-value syntax
   defined in Section 3.2 of [RFC5987].  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 "username" with value "Renee of France"
   SHOULD be sent as < username="Renee of France" >.  If the value is
   "Ren<e acute>e of France", it SHOULD be sent as < username*=UTF-
   8''Ren%C3%89e%20of%20France > instead.

4.2.  Auth-style parameter

   Authentication-Control: Digest realm="protected space",
           auth-style=modal

   The parameter "auth-style" specifies the server's preferences for
   user interface behavior for user authentication.  This parameter can
   be included in any kind of response, however, it is only meaningful
   for either authentication-initializing or negatively-authenticated
   responses.  The value of this parameter MUST be one of the bare-
   tokens "modal" or "non-modal".  When the Optional-WWW-Authenticate
   header is used, the value of this parameter MUST be disregarded and
   the value "non-modal" is implied.

   The value "modal" means that the server thinks the content of the
   response (body and other content-related headers) is valuable only
   for users refusing the authentication request.  The clients are
   expected to ask the user for a password before processing the
   content.  This behavior is common for most of the current
   implementations of Basic and Digest authentication schemes.

   The value "non-modal" means that the server thinks the content of the
   response (body and other content-related headers) is valuable for
   users before processing an authentication request.  The clients are
   expected to first process the content and then provide users the
   opportunity to perform authentication.

   The default behavior for clients is implementation-dependent, and
   clients MAY choose different defaults for different authentication
   schemes.  The proposed default behavior is "modal" for all
   authentication schemes, but specifications for authentication schemes
   MAY propose a different default.

   The above two different methods of authentication may introduce a
   observable difference of semantics when the response contains state-
   changing side effects; for example, it may change whether Cookie

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   headers [RFC6265] in 401 responses are processed or not.  However,
   the server applications SHOULD NOT depend on both existence and non-
   existence of such side effects.

4.3.  Location-when-unauthenticated parameter

   Authentication-Control: Mutual realm="auth-space-1",
       location-when-unauthenticated="http://www.example.com/login.html"

   The parameter "location-when-unauthenticated" specifies a location
   where any unauthenticated clients should be redirected to.  This
   header may be used, for example, when there is a central login page
   for the entire Web application.  The value of this parameter is a
   string that contains an absolute URL location.  Senders MUST always
   send an absolute URL location.  If a received URL is not absolute,
   the clients SHOULD either ignore it or consider it a relative URL
   from the current location.

   This parameter MAY be used with a 401 response for an authentication-
   initializing response.  It can also be contained, although this is
   NOT RECOMMENDED, in a positive response with an
   Optional-WWW-Authenticate header.  The clients MUST ignore this
   parameter when a response is either successfully-authenticated or
   intermediately-authenticated.  The clients SHOULD ignore this
   parameter when a response is a negatively-authenticated one (the case
   is unlikely to happen, though).

   When a client receives an authentication-initiating response with
   this parameter, if the client has to ask users for authentication
   credentials, the client will treat the entire response as if it were
   a 303 "See Other" response with a Location header that contains the
   value of this parameter (i.e., client will be redirected to the
   specified location with a GET request).  Unlike a normal 303
   response, if the client can process authentication without the user's
   interaction, this parameter MUST be ignored.

4.4.  No-auth parameter

   Authentication-Control: Basic realm="entrance", no-auth=true

   The parameter "no-auth" is a variant of the
   location-when-unauthenticated parameter; it specifies that new
   authentication attempts are not to be performed on this location in
   order to improve the user experience, without specifying the
   redirection on the HTTP level.  This header may be used, for example,
   when there is a central login page for the entire Web application,
   and when an explicit user interaction with the Web content is desired
   before authentications.  The value of this parameter MUST be a token

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   "true".  If the value is incorrect, client MAY ignore this parameter.

   This parameter MAY be used with authentication-initiating responses.
   It can also be contained, although this is NOT RECOMMENDED, in a
   positive response with an Optional-WWW-Authenticate header.  The
   clients MUST ignore this parameter when a response is either
   successfully-authenticated or intermediately-authenticated.  The
   clients SHOULD ignore this parameter when a response is a negatively-
   authenticated one (the case is unlikely to happen, though).

   When a client receives an authentication-initiating response with
   this parameter, if the client has to ask users for authentication
   credentials, the client will ignore the WWW-Authenticate header
   contained in the response and treat the whole response as a normal
   negative 4xx-class response instead of giving the user an opportunity
   to start authentication.  If the client can process authentication
   without the user's interaction, this parameter MUST be ignored.

   This parameter SHOULD NOT be used along with the
   location-when-unauthenticated parameter.  If both were supplied,
   clients MAY choose which one is to be honored.

   This parameter SHOULD NOT be used as a security measure to prevent
   authentication attempts, as it is easily circumvented by users.  This
   parameter SHOULD be used solely for improving user experience of Web
   applications.

4.5.  Location-when-logout parameter

   Authentication-Control: Digest realm="protected space",
       location-when-logout="http://www.example.com/byebye.html"

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

   This parameter MAY be used with successfully-authenticated responses.
   If this parameter is contained in other kinds of responses, the
   clients MUST ignore this parameter.

   When the user requests termination of an authentication period, and
   if the client currently displays a page supplied by a response with
   this parameter, the client will be redirected to the specified
   location by a new GET request (as if it received a 303 response).
   The log-out operation (e.g. erasing memories of user name,
   authentication credential and all related one-time credentials such

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   as nonce or keys) SHOULD occur before processing a redirection.

   When the user requests termination of an authentication period, if
   the client supports this parameter but the server response does not
   contain this parameter, the client's RECOMMENDED behavior is as
   follows: if the request corresponding to the current content was safe
   (e.g.  GET), reload the page without the authentication credential.
   If the request was non-idempotent (e.g.  POST), keep the current
   content as-is and simply forget the authentication status.  The
   client SHOULD NOT replay a non-idempotent request without the user's
   explicit approval.

   Web applications are encouraged to send this parameter with an
   appropriate value for any responses (except those with redirection
   (3XX) statuses) for non-GET requests.

4.6.  Logout-timeout parameter

   Authentication-Control: Basic realm="entrance", logout-timeout=300

   The parameter "logout-timeout", when contained in a successfully-
   authenticated response, means that any authentication credentials and
   state related to the current protection space are to be discarded if
   a time specified in this header (in seconds) has passed since from
   the time this header was received.  The value MUST be an integer.  As
   a special case, the value 0 means that the client is requested to
   immediately log-out from the current authentication space and revert
   to an unauthenticated status.  This does not, however, mean that the
   long-term memories for the passwords and passwords-related details
   (such as the password reminders and auto fill-ins) should be removed.
   If a new timeout value is received for the same authentication space,
   it cancels the previous timeout and sets a new timeout.

4.7.  Username parameter

   Authentication-Control: Basic realm="configuration", username="admin"

   The parameter "username" tells that the only "user name" to be
   accepted by the server is the value given in this parameter.  This
   parameter is particularly useful, for example, for routers and other
   appliances with a Web configuration interface.

   This parameter MAY be used with authentication-initiating responses
   or negatively-authenticated responses requiring another attempt of
   authentication.  The clients MUST ignore this parameter when a
   response is either successfully-authenticated or intermediately-
   authenticated.

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   If the authentication scheme to be used has a syntax limitation on
   the allowed user names (e.g.  Basic and Digest do not allow colons in
   user names), the specified value MUST follow that limitation.
   Clients SHOULD ignore any values which do not conform to such
   limitations.

   Clients MAY still send any authentication requests with other user
   names, possibly in vain.  Servers are not strictly required to reject
   user names other than specified, but doing so will give bad user
   experiences and may confuse users and clients.

   If the used authentication scheme requires a specific style of text
   preparation for the user name (e.g., PRECIS string preparation or
   Unicode normalization), the specified user name SHOULD follow such
   requirements.

5.  Usage examples (informative)

   This section shows some examples for applying this extension to
   typical websites which are using Forms and cookies for managing
   authentication and authorization.  The content of this section is not
   normative and for illustrative purposes only.

   We assume that all features described in the previous sections are
   implemented in clients (Web browsers).  We also assume that browsers
   will have a user interface which allows users to deactivate (log-out
   from) current authentication sessions.  If this assumption is not
   value, the text below provides another approach with de-
   authentication pages used instead of such a UI.

   When not explicitly specified, all settings described below are to be
   applied with Authentication-Control headers, and these can be sent to
   clients regardless of the authentication status (these will be
   silently ignored whenever not effective).

5.1.  Example 1: a portal site

   This subsection provides an example application for a site whose
   structure is somewhat similar to conventional portal sites.  In
   particular, most web pages are available for guest (unauthenticated)
   users, and if authentication is performed, the content of these pages
   is customized for each user.  We assume the site has the following
   kinds of pages currently:

   o  Content pages.

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   o  Pages/mechanism for performing authentication:

      *  There is one page which asks a user name and a password using a
         HTML POST form.

      *  After the authentication attempt, the user will be redirected
         to either the page which is previously displayed before the
         authentication, or some specific page.

   o  A de-authentication (log-out) page.

5.1.1.  Case 1: a simple application

   When such a site does not require specific actions upon log-in and
   log-out, the following simple settings can be used.

   o  Set up an optional authentication to all pages available to
      guests.  Set up an Authentication-Control header with "auth-
      style=non-modal" setting.

   o  If there are pages only available to authenticated users, set up a
      mandatory authentication with "auth-style=non-modal" setting.

   o  No specific pages for authentication are needed.  It will be
      performed automatically, directed by the above setting.

   o  A de-authentication page is also not needed.  If the site has one,
      put "logout-timeout=0" there.

   o  For all pages for POST requests, it is advisable to have
      "location-when-logout=<some page>".

5.1.2.  Case 2: specific action required on log-out

   If the site requires specific actions upon log-out, the following
   settings can be used.

   o  All settings in the Case 1 are applied.

   o  For all pages, set up the Authentication-Control header "location-
      when-logout=<de-authentication page>".

   o  In the de-authentication page, no specific set-up is needed.  If
      there are any direct links to the de-authentication page, put
      "logout-timeout=0".

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5.1.3.  Case 3: specific page displayed before log-in

   If the site needs to display a specific page before log-in actions
   (some announcements, user notices, or even advertisements), the
   following settings can be applied.

   o  Set up an optional authentication to all pages available to
      guests.  Set up an Authentication-Control header with "no-
      auth=true".  Put a link to a specific log-in page in contents.

   o  If there are pages only available to authenticated users, set up a
      mandatory authentication with "location-when-unauthenticated=<the
      log-in page>".

   o  For the specific log-in page, set up a mandatory authentication.

   o  For all pages for POST requests, it is advisable to have
      "location-when-logout=<some page>", too.

   o  De-authentication pages are not needed.  If the site has one, put
      "logout-timeout=0".

5.2.  Example 2: authenticated user-only sites

   If almost all pages in the target site require authentication (e.g.,
   an Internet banking site), or if there are no needs to support both
   unauthenticated and authenticated users on the same resource, the
   settings will become simpler.  The following are an example for such
   a site:

   o  Set up a mandatory authentication to all pages available to
      authenticated users.  Set up an Authentication-Control header with
      "auth-style=non-modal" setting.

   o  Set up a handler for the 401-status which requests users to
      authenticate.

   o  For all pages for POST requests, it is advisable to have
      "location-when-logout=<some page>", too.

   o  De-authentication pages are not needed.  If the site will have
      one, put "logout-timeout=0" there.

5.3.  When to use Cookies

   In the current Web sites using form-based authentications, Cookies
   [RFC6265] are used for managing both authorization and application
   sessions.  Using the extensions in this document, the former features

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   will be provided by using (extended) HTTP authentication/
   authorization mechanisms.  In some cases, there will be ambiguity on
   whether some functions are for authorization management or for
   session management.  The following hints will be helpful for deciding
   which features to use.

   o  If there is a need to serve multiple sessions for a single user
      using multiple browsers concurrently, use a Cookie for
      distinguishing between sessions for the same user.  (C.f. if there
      is a need to distinguish sessions in the same browser, HTML5 Web
      Storage [W3C.REC-webstorage-20130730] features may be used instead
      of Cookies.)

   o  If a web site is currently deploying a session time-out feature,
      consider who benefits from the feature.  In most cases, the main
      requirement for such a feature is to protect users from having
      their consoles and browsers hijacked (i.e. benefits are on the
      users' side).  In such cases, the time-out features provided in
      this extension may be used.  On the other hand, the requirement is
      to protect server's privilege (e.g. when some regulations require
      to limit the time difference between user's two-factor
      authentication and financial transaction commitment; the
      requirement is strictly on the servers' side), that should be
      managed on the server side using Cookies or other session
      management mechanisms.

5.4.  Parallel deployment with Form/Cookie authentications

   In some transition periods, sites may need to support both HTTP-layer
   and form-based authentication.  The following example shows one way
   to achieve that.

   o  If Cookies are used even for HTTP-authenticated users, each
      session determined by Cookies should identify which authentication
      has been used for the session.

   o  First, set up any of the above settings for enabling HTTP-layer
      authentication.

   o  For unauthenticated users, add the following things to the Web
      pages, unless the client supports this extension and HTTP-level
      authentication.

      *  For non-mandatory authenticated pages, put a link to Form-based
         authenticated pages.

      *  For mandatory authenticated pages, either put a link to Form-
         based authenticated pages, or put a HTML-level redirection

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         (using META element) to such pages.

   o  In Form-based authenticated pages, if users are not authenticated,
      the page may have a diversion for HTTP-level authentication by
      "location-when-unauthenticated" setting.

   o  Users are identified to authorization and content customization by
      the following logic.

      *  First, check the result of the HTTP-level authentication.  If
         there is a Cookie session tied to a specific user, both should
         match.

      *  If the user is not authenticated on the HTTP-level, use the
         conventional Form-based method to determine the user.

      *  If there is a Cookie tied to HTTP authentication, but there is
         no corresponding HTTP authentication result, that session will
         be discarded (because it means that authentication is
         deactivated by the corresponding user).

6.  Methods to extend this protocol

   If a private extension to this protocol is implemented, it MUST use
   the extension-param to avoid conflicts with this protocol and other
   future official extensions.

   When bare-tokens are used in this protocol, these MUST be allocated
   by IANA.  Any tokens used for non-private, non-experimental
   parameters are RECOMMENDED to be registered to IANA, regardless of
   the kind of tokens used.

   Extension-tokens MAY be freely used for any non-standard, private,
   and/or experimental uses.  An extension-tokens MUST use the format
   "-<bare-token>.<domain-name>", where <domain-name> is a validly
   registered (sub-)domain name on the Internet owned by the party who
   defines the extensions.  Unknown parameter names are to be ignored
   regardless of whether it is extension-tokens or bare-tokens.

7.  IANA Considerations

   This document defines two new entries for the "Permanent Message
   Header Field Names" registry.

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   +---------------------------+----------+----------------------------+
   | Header Field Name         | Protocol | Specification              |
   +---------------------------+----------+----------------------------+
   | Optional-WWW-Authenticate | http     | Section 3 of this document |
   | Authentication-Control    | http     | Section 4 of this document |
   +---------------------------+----------+----------------------------+

   This document also establishes a registry for HTTP authentication
   control parameters.  The registry manages a case-insensitive ASCII
   strings.  The string MUST follow the extensive-token syntax defined
   in Section 2.2.

   To acquire registered tokens, a specification for the use of such
   tokens MUST be available as a publicly-accessible documents, as
   outlined as "Specification Required" level in [RFC5226].

   Registrations for authentication algorithms are required to include a
   description of the control extension.  New registrations are advised
   to provide the following information:

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

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

   The initial content of this registry is as follows:

     +-------------------------------+------------------------------+
     | Token                         | Specification                |
     +-------------------------------+------------------------------+
     | auth-style                    | Section 4.2 of this document |
     | location-when-unauthenticated | Section 4.3 of this document |
     | no-auth                       | Section 4.4 of this document |
     | location-when-logout          | Section 4.5 of this document |
     | logout-timeout                | Section 4.6 of this document |
     | username                      | Section 4.7 of this document |
     +-------------------------------+------------------------------+

8.  Security Considerations

   The purpose of the log-out timeout feature in the Authentication-
   control header is to protect users of clients from impersonation
   caused by an attacker having access to the same console.  The server
   application implementer SHOULD be aware that the directive may always
   be ignored by either malicious clients or clients not supporting this
   extension.  If the purpose of introducing a timeout for an
   authentication period is to protect server-side resources, this

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   protection MUST be implemented by other means such as HTTP Cookies
   [RFC6265].

   All parameters in Authentication-Control header SHOULD NOT be used
   for any security-enforcement purposes.  Server-side applications MUST
   always consider that the header may be either ignored by clients or
   even bypassed by users.

   The "username" parameter may reveal sensitive information about the
   HTTP server and its configurations, useful for security attacks.  The
   use of the "username" parameter SHOULD be limited to cases where the
   all of the following conditions are met:

   (1)  the valid user name is pre-configured and not modifiable (such
        as root, admin or similar ones);

   (2)  the valid user name for such an appliance is publicly known (for
        example, written in a manual); and

   (3)  either the valid user name for the server is easily guessable by
        other means (for example, from the model number shown in an
        unauthenticated page), or the server is only accessible from
        limited networks.

   Most importantly, the "username" parameter SHOULD NOT be used in any
   case when the valid user names are configured by users or
   administrators.

9.  References

9.1.  Normative References

   [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>.

   [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>.

   [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>.

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   [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>.

9.2.  Informative References

   [I-D.ietf-httpauth-mutual]
              Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "Mutual Authentication Protocol for
              HTTP", draft-ietf-httpauth-mutual-07 (work in progress),
              January 2016.

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

   [W3C.REC-webstorage-20130730]
              Hickson, I., "Web Storage", World Wide Web Consortium
              Recommendation REC-webstorage-20130730, July 2013,
              <http://www.w3.org/TR/2013/REC-webstorage-20130730>.

Appendix A.  (Informative) Applicability of features for each messages

   This section provides a cross-reference table showing the
   applicability of the features provided in this specification to each
   kind of responses described in Section 2.1.  The table provided in
   this section is for informative purposes only.

        +-------------------+-------+----------+-----------+------+
        |                   | init. | success. | intermed. | neg. |
        +-------------------+-------+----------+-----------+------+
        | Optional auth.    | O     | n        | N         | N    |
        | auth-style        | O     | -        | -         | O    |
        | loc.-when-unauth. | O     | I        | I         | i    |
        | no-auth           | O     | I        | I         | i    |
        | loc.-when-logout  | -     | O        | -         | -    |
        | logout-timeout    | -     | O        | -         | -    |
        | username          | O     | -        | -         | O    |
        +-------------------+-------+----------+-----------+------+

   Legends:

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   O = MAY contain; n = SHOULD NOT contain; N = MUST NOT contain
   i = SHOULD be ignored; I = MUST be ignored;
   - = meaningless (to be ignored)

Appendix B.  (Informative) Draft Change Log

   [To be removed on final publication]

B.1.  Changes in Httpauth WG Revision 06

   o  Several comments from reviewers are reflected to the text.

B.2.  Changes in Httpauth WG Revision 05

   o  Authors' addresses updated.

B.3.  Changes in Httpauth WG revision 04

   o  IANA consideration section added.

B.4.  Changes in Httpauth WG revision 03

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

B.5.  Changes in Httpauth WG revision 02

   o  Added realm parameter.

   o  Added username parameter.  We acknowledge Michael Sweet's proposal
      for including this to the Basic authentication.

B.6.  Changes in Httpauth WG revision 01

   o  Clarification on peers' responsibility about handling of relative
      URLs.

   o  Automatic reloading should be allowed only on safe methods, not
      always on idempotent methods.

B.7.  Changes in Httpauth revision 00 and HttpBis revision 00

   None.

B.8.  Changes in revision 02

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   o  Added usage examples.

B.9.  Changes in revision 01

   o  Syntax notations and parsing semantics changed to match httpbis
      style.

B.10.  Changes in revision 00

   o  Separated from HTTP Mutual authentication proposal (-09).

   o  Adopting httpbis works as a referencing point to HTTP.

   o  Generalized, now applicable for all HTTP authentication schemes.

   o  Added "no-auth" and "auth-style" parameters.

   o  Loosened standardization requirements for parameter-name tokens
      registration.

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: mutual-auth-contact-ml@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

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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

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

   Yuichi Ioku
   Individual

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