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rfc:rfc8120

Internet Engineering Task Force (IETF) Y. Oiwa Request for Comments: 8120 H. Watanabe Category: Experimental H. Takagi ISSN: 2070-1721 ITRI, AIST

                                                              K. Maeda
                                                Individual Contributor
                                                            T. Hayashi
                                                               Lepidum
                                                               Y. Ioku
                                                Individual Contributor
                                                            April 2017
              Mutual Authentication Protocol for HTTP

Abstract

 This document specifies an authentication scheme for the Hypertext
 Transfer Protocol (HTTP) that is referred to as either the Mutual
 authentication scheme or the Mutual authentication protocol.  This
 scheme provides true mutual authentication between an HTTP client and
 an HTTP server using password-based authentication.  Unlike the Basic
 and Digest authentication schemes, the Mutual authentication scheme
 specified in this document assures the user that the server truly
 knows the user's encrypted password.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8120.

Oiwa, et al. Experimental [Page 1] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

Copyright Notice

 Copyright (c) 2017 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.

Table of Contents

 1. Introduction ....................................................3
    1.1. Terminology ................................................5
    1.2. Document Structure and Related Documents ...................6
 2. Protocol Overview ...............................................6
    2.1. Messages ...................................................7
    2.2. Typical Flows of the Protocol ..............................8
    2.3. Alternative Flows .........................................10
 3. Message Syntax .................................................12
    3.1. Non-ASCII Extended Header Parameters ......................12
    3.2. Values ....................................................13
         3.2.1. Tokens .............................................13
         3.2.2. Strings ............................................14
         3.2.3. Numbers ............................................14
 4. Messages .......................................................15
    4.1. 401-INIT and 401-STALE ....................................16
    4.2. req-KEX-C1 ................................................19
    4.3. 401-KEX-S1 ................................................19
    4.4. req-VFY-C .................................................20
    4.5. 200-VFY-S .................................................21
 5. Authentication Realms ..........................................21
    5.1. Resolving Ambiguities .....................................23
 6. Session Management .............................................24
 7. Host Validation Methods ........................................26
    7.1. Applicability Notes .......................................27
    7.2. Notes on "tls-unique" .....................................28
 8. Authentication Extensions ......................................28
 9. String Preparation .............................................29
 10. Decision Procedure for Clients ................................29
    10.1. General Principles and Requirements ......................29
    10.2. State Machine for the Client (Informative) ...............31

Oiwa, et al. Experimental [Page 2] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 11. Decision Procedure for Servers ................................36
 12. Authentication Algorithms .....................................39
    12.1. Support Functions and Notations ..........................39
    12.2. Default Functions for Algorithms .........................41
 13. Application Channel Binding ...................................42
 14. Application for Proxy Authentication ..........................42
 15. Methods to Extend This Protocol ...............................43
 16. IANA Considerations ...........................................44
    16.1. Addition to HTTP Authentication Schemes Registry .........44
    16.2. Registry for Authentication Algorithms ...................44
    16.3. Registry for Validation Methods ..........................45
 17. Security Considerations .......................................46
    17.1. Security Properties ......................................46
    17.2. Secrecy of Credentials ...................................46
    17.3. Denial-of-Service Attacks on Servers .....................47
         17.3.1. Online Active Password Attacks ....................47
    17.4. Communicating the Status of Mutual Authentication
          with Users ...............................................48
    17.5. Implementation Considerations ............................48
    17.6. Usage Considerations .....................................49
 18. References ....................................................49
    18.1. Normative References .....................................49
    18.2. Informative References ...................................51
 Authors' Addresses ................................................53

1. Introduction

 This document specifies an authentication scheme for the Hypertext
 Transfer Protocol (HTTP) that is referred to as either the Mutual
 authentication scheme or the Mutual authentication protocol.  This
 scheme provides true mutual authentication between an HTTP client and
 an HTTP server using just a simple password as a credential.
 Password-stealing attacks are one of the most critical threats for
 Web systems.  Plain-text password authentication techniques (Basic
 authentication and Web-form-based authentication) have been widely
 used for a long time.  When these techniques are used with plain HTTP
 protocols, it is trivially easy for attackers to sniff the password
 credentials on the wire.
 The Digest authentication scheme [RFC7616] uses SHA-256 and
 SHA-512/256 (formerly SHA-1 and MD5) hash algorithms to hide the raw
 user password from network sniffers.  However, if the number of
 possible candidate users' passwords is not enough, newer and more
 powerful computers can compute possible hash values for billions of
 password candidates and compare these with the sniffed values to find
 out the correct password.  This kind of attack is called an offline
 password dictionary attack; the search capacity of these newer

Oiwa, et al. Experimental [Page 3] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 computers reduces the effectiveness of users' memorable passwords,
 thereby threatening the effectiveness of such hash-based password
 protections.
 Transport Layer Security (TLS) [RFC5246] provides strong
 cryptographic protection against the network-based sniffing of
 passwords and other communication contents.  If TLS is correctly used
 by both server operators and client users, passwords and other
 credentials will not be available to any outside attackers.  However,
 there is a pitfall related to TLS deployment on Web systems: if the
 users are fraudulently routed to a "wrong Website" via some kind of
 social engineering attack (e.g., phishing) and tricked into
 performing authentication on that site, the credentials will be sent
 to the attacker's server and trivially leaked.  Attacks such as
 phishing have become a serious threat.  In current Web system
 deployments, TLS certificates will be issued to almost any users of
 the Internet (including malicious attackers).  Although those
 certificates include several levels of the "validation results" (such
 as corporate names) of the issued entities, the task of "checking"
 those validation results is left to the users of Web browsers, still
 leaving open the possibility of such social engineering attacks.
 Another way to avoid such threats is to avoid password-based
 authentication and use some kinds of pre-deployed strong secret keys
 (on either the client side or the server side) for authentications.
 Several federated authentication frameworks, as well as HTTP
 Origin-Bound Authentication (HOBA) [RFC7486], are proposed and
 deployed on real Web systems to satisfy those needs.  However, a type
 of authentication based on "human-memorable secrets" (i.e.,
 passwords) is still required in several scenarios, such as
 initialization, key deployment to new clients, or recovery of secret
 accounts with lost cryptographic keys.
 The Mutual authentication protocol, as proposed in this document, is
 a strong cryptographic solution for password authentications.  It
 mainly provides the following two key features:
 o  No password information at all is exchanged in the communications.
    When the server and the user fail to authenticate with each other,
    the protocol will not reveal even the tiniest bit of information
    about the user's password.  This prevents any kind of offline
    password dictionary attacks, even with the existence of phishing
    attacks.
 o  To successfully authenticate, the server, as well as client users,
    must own the valid registered credentials (authentication secret).
    This means that a phishing attacker cannot trick users into
    thinking that it is an "authentic" server.  (It should be

Oiwa, et al. Experimental [Page 4] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

    pointed out that this is not true for Basic and Digest
    authentication; for example, servers using Basic authentication
    can answer "YES" to any clients without actually checking
    authentication at all.)  Client users can ascertain whether or not
    the communicating peer is truly "the server" that registered their
    account beforehand.  In other words, it provides "true" mutual
    authentication between servers and clients.
 Given the information above, the proposed protocol can serve as a
 strong alternative to the Basic, Digest, and Web-form-based
 authentication schemes and also as a strong companion to the
 non-password-based authentication frameworks.
 The proposed protocol will serve in the same way as does existing
 Basic or Digest authentication: it meets the requirements for new
 authentication schemes for HTTP, as described in Section 5.1.2 of
 [RFC7235].  Additionally, to communicate authentication results more
 reliably between the server and the client user, it suggests that Web
 browsers have some "secure" way of displaying the authentication
 results.  Having such a user interface in future browsers will
 greatly reduce the risk of impersonation by various kinds of social
 engineering attacks, in a manner similar to that of the
 "green padlock" for Extended Validation TLS certificates.
 Technically, the authentication scheme proposed in this document is a
 general framework for using password-based authenticated key exchange
 (PAKE) and similar stronger cryptographic primitives with HTTP.  The
 two key features shown above correspond to the nature of PAKE.

1.1. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].
 This document distinguishes the terms "client" and "user" in the
 following way: a "client" is an entity that understands and
 implements HTTP and the specified authentication protocol -- usually
 computer software; a "user" is typically a human being who wants to
 access data resources using a "client".
 The term "natural numbers" refers to the non-negative integers
 (including zero) throughout this document.

Oiwa, et al. Experimental [Page 5] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

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

1.2. Document Structure and Related Documents

 The entire document is organized as follows:
 o  Section 2 presents an overview of the protocol design.
 o  Sections 3 through 11 define a general framework of the Mutual
    authentication protocol.  This framework is independent of
    specific cryptographic primitives.
 o  Section 12 describes properties needed for cryptographic
    algorithms used with this protocol framework and defines a few
    functions that will be shared among such cryptographic algorithms.
 o  Sections 13 through 15 contain general normative and informative
    information about the protocol.
 o  Sections 16 and 17 describe IANA considerations and security
    considerations, respectively.
 In addition, we will refer to the following two companion documents,
 as they are related to this specification:
 o  [RFC8121] defines cryptographic primitives that can be used with
    this protocol framework.
 o  [RFC8053] defines small but useful extensions to the current HTTP
    authentication framework so that it can support application-level
    semantics of existing Web systems.

2. Protocol Overview

 The protocol, as a whole, is designed as a natural extension to HTTP
 [RFC7230] and uses the framework defined in [RFC7235].  Internally,
 the server and the client will first perform a cryptographic key
 exchange, using the secret password as a "tweak" to the exchange.
 The key exchange will only succeed when the secrets used by both
 peers are correctly related (i.e., generated from the same password).
 Then, both peers will verify the authentication results by confirming
 the sharing of the exchanged key.  This section provides a brief
 outline of the protocol and the exchanged messages.

Oiwa, et al. Experimental [Page 6] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

2.1. Messages

 The authentication protocol uses six kinds of messages to perform
 mutual authentication.  These messages have specific names within
 this specification.
 o  Authentication request messages: used by the servers to request
    that clients start mutual authentication.
  • 401-INIT message: a general message to start the authentication

protocol. It is also used as a message indicating an

       authentication failure.
  • 401-STALE message: a message indicating that the client has to

start a new key exchange.

 o  Authenticated key exchange messages: used by both peers to perform
    authentication and the sharing of a cryptographic secret.
  • req-KEX-C1 message: a message sent from the client.
  • 401-KEX-S1 message: an intermediate response to a req-KEX-C1

message from the server.

 o  Authentication verification messages: used by both peers to verify
    the authentication results.
  • req-VFY-C message: a message used by the client to request that

the server authenticate and authorize the client.

  • 200-VFY-S message: a response used by the server to indicate

that client authentication succeeded. It also contains

       information necessary for the client to check the authenticity
       of the server.
 In addition to the above six kinds of messages, a request or response
 without any HTTP headers related to this specification will be
 hereafter called a "normal request" or "normal response",
 respectively.

Oiwa, et al. Experimental [Page 7] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

2.2. Typical Flows of the Protocol

 In typical cases, client access to a resource protected by the
 Mutual authentication scheme will use the following protocol
 sequence:
        Client                                 Server
          |                                      |
          |  ---- (1) normal request --------->  |
      GET / HTTP/1.1                             |
          |                                      |
          |  <---------------- (2) 401-INIT ---  |
          |            401 Unauthorized          |
          |            WWW-Authenticate: Mutual realm="a realm"
          |                                      |
 [user,   |                                      |
  pass]-->|                                      |
          |  ---- (3) req-KEX-C1 ------------->  |
      GET / HTTP/1.1                             |
      Authorization: Mutual user="john",         |--> [user DB]
                     kc1="...", ...              |<-- [user info]
          |                                      |
          |  <-------------- (4) 401-KEX-S1 ---  |
          |           401 Unauthorized           |
          |           WWW-Authenticate: Mutual sid=..., ks1="...", ...
          |                                      |
      [compute] (5) compute session secret   [compute]
          |                                      |
          |                                      |
          |  ---- (6) req-VFY-C -------------->  |
      GET / HTTP/1.1                             |--> [verify (6)]
      Authorization: Mutual sid=...,             |<-- OK
                     vkc="...", ...              |
          |                                      |
          |  <--------------- (7) 200-VFY-S ---  |
 [verify  |           200 OK                     |
   (7)]<--|           Authentication-Info: Mutual vks="..."
          |                                      |
          v                                      v
   Figure 1: Typical Communication Flow for First Access to Resource
 o  As is typical in general HTTP protocol designs, a client will at
    first request a resource without any authentication attempt (1).
    If the requested resource is protected by the Mutual
    authentication protocol, the server will respond with a message
    requesting authentication (401-INIT) (2).

Oiwa, et al. Experimental [Page 8] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 o  The client processes the body of the message and waits for the
    user to input the username and password.  If the username and
    password are available, the client will send a message with the
    authenticated key exchange (req-KEX-C1) to start the
    authentication (3).
 o  If the server has received a req-KEX-C1 message, the server
    looks up the user's authentication information within its user
    database.  Then, the server creates a new session identifier (sid)
    that will be used to identify sets of the messages that follow it
    and responds with a message containing a server-side authenticated
    key exchange value (401-KEX-S1) (4).
 o  At this point (5), both peers calculate a shared "session secret"
    using the exchanged values in the key exchange messages.  Only
    when both the server and the client have used secret credentials
    generated from the same password will the session secret values
    match.  This session secret will be used for access authentication
    of every individual request/response pair after this point.
 o  The client will send a request with a client-side authentication
    verification value (req-VFY-C) (6), calculated from the
    client-generated session secret.  The server will check the
    validity of the verification value using its own version of the
    session secret.
 o  If the authentication verification value from the client was
    correct, then the client definitely owns the credential based on
    the expected password (i.e., the client authentication succeeded).
    The server will respond with a successful message (200-VFY-S) (7).
    Unlike the usual one-way authentication (e.g., HTTP Basic
    authentication or POP APOP authentication [RFC1939]), this message
    also contains a server-side authentication verification value.
    When the client's verification value is incorrect (e.g., because
    the user-supplied password was incorrect), the server will respond
    with a 401-INIT message (the same message as the message used
    in (2)) instead.
 o  The client MUST first check the validity of the server-side
    authentication verification value contained in the message (7).
    If the value was equal to the expected value, server
    authentication succeeded.

Oiwa, et al. Experimental [Page 9] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

    If it is not the expected value or the message does not contain
    the authentication verification value, then the mutual
    authentication has been broken for some unexpected reason.  The
    client MUST NOT process any body or header values contained in the
    HTTP response in this case.  (Note: This case should not happen
    between a correctly implemented server and client without any
    active attacks; such a scenario could be caused by either a
    man-in-the-middle attack or incorrect implementation.)

2.3. Alternative Flows

 As shown above, the typical flow for a first authentication request
 requires three request-response pairs.  To reduce protocol overhead,
 the protocol enables several shortcut flows that require fewer
 messages.
 o  Case A: If the client knows that the resource is likely to require
    authentication, the client MAY omit the first unauthenticated
    request (1) and immediately send a key exchange (req-KEX-C1)
    message.  This will reduce the number of round trips by one.
 o  Case B: If both the client and the server previously shared a
    session secret associated with a valid sid, the client MAY
    directly send a req-VFY-C message using the existing sid and
    corresponding session secret.  This will further reduce the number
    of round trips by one.
    The server MAY have thrown out the corresponding session from the
    session table.  If so, the server will respond with a 401-STALE
    message, indicating that a new key exchange is required.  The
    client SHOULD try again to construct a req-KEX-C1 message in
    this case.

Oiwa, et al. Experimental [Page 10] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 Figure 2 depicts the shortcut flows described above.  When using
 appropriate settings and implementations, most of the requests to
 resources are expected to meet both criteria; thus, only one
 round trip of request/response will be required.
   Case A: Omit first request
           (2 round trips)
      Client            Server
      |                      |
      | --- req-KEX-C1 ----> |
      |                      |
      | <---- 401-KEX-S1 --- |
      |                      |
      | ---- req-VFY-C ----> |
      |                      |
      | <----- 200-VFY-S --- |
      |                      |
   Case B: Reuse session secret (re-authentication)
       (B-1) key available        (B-2) key expired
             (1 round trip)             (3 round trips)
      Client            Server   Client              Server
      |                      |   |                        |
      | ---- req-VFY-C ----> |   | --- req-VFY-C -------> |
      |                      |   |                        |
      | <----- 200-VFY-S --- |   | <------- 401-STALE --- |
      |                      |   |                        |
                                 | --- req-KEX-C1 ------> |
                                 |                        |
                                 | <------ 401-KEX-S1 --- |
                                 |                        |
                                 | --- req-VFY-C -------> |
                                 |                        |
                                 | <------- 200-VFY-S --- |
                                 |                        |
             Figure 2: Several Alternative Protocol Flows
 For more details, see Sections 10 and 11.

Oiwa, et al. Experimental [Page 11] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

3. Message Syntax

 Throughout this specification, the syntax is denoted in the extended
 augmented BNF syntax as defined in [RFC7230] and [RFC5234].  The
 following elements are used in this document per [RFC5234],
 [RFC7230], and [RFC7235]: DIGIT, ALPHA, SP, auth-scheme,
 quoted-string, auth-param, header-field, token, challenge, and
 credentials.
 The Mutual authentication protocol uses three headers:
 WWW-Authenticate (usually in responses with a 401 status code),
 Authorization (in requests), and Authentication-Info (in responses
 other than a 401 status code).  These headers follow the frameworks
 described in [RFC7235] and [RFC7615].  See Section 4 for more details
 regarding these headers.
 The framework in [RFC7235] defines the syntax for the headers
 WWW-Authenticate and Authorization as the syntax elements "challenge"
 and "credentials", respectively.  The auth-scheme element contained
 in those headers MUST be set to "Mutual" when using the protocol
 specified in this document.  The syntax for "challenge" and
 "credentials" to be used with the "Mutual" auth-scheme SHALL be
 name-value pairs (#auth-param), not the "token68" parameter defined
 in [RFC7235].
 The Authentication-Info header used in this protocol SHALL follow the
 syntax defined in [RFC7615].
 In HTTP, the WWW-Authenticate header may contain two or more
 challenges.  Client implementations SHOULD be aware of, and be
 capable of correctly handling, those cases.

3.1. Non-ASCII Extended Header Parameters

 All of the parameters contained in the above three headers, except
 for the "realm" field, MAY be extended to ISO 10646-1 values using
 the framework described in [RFC5987].  All servers and clients MUST
 be capable of receiving and sending values encoded per the syntax
 specified in [RFC5987].
 If a value to be sent contains only ASCII characters, the field MUST
 be sent using plain syntax as defined in RFC 7235.  The syntax as
 extended by RFC 5987 MUST NOT be used in this case.
 If a value (except for the "realm" header) contains one or more
 non-ASCII characters, the parameter SHOULD be sent using the syntax
 defined in Section 3.2 of [RFC5987] as "ext-parameter".  Such a
 parameter MUST have a charset value of "UTF-8", and the language

Oiwa, et al. Experimental [Page 12] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 value MUST always be omitted (have an empty value).  The same
 parameter MUST NOT be sent more than once, regardless of the
 syntax used.
 For example, a parameter "user" with the value "Renee of France"
 SHOULD be sent as < user="Renee of France" >.  If the value is
 "Ren<e acute>e of France", it SHOULD be sent as
 < user*=UTF-8''Ren%C3%89e%20of%20France > instead.
 [RFC7235] requires that the "realm" parameter be in its plain form
 (not as an extended "realm*" parameter), so the syntax specified in
 RFC 5987 MUST NOT be used for this parameter.

3.2. Values

 The parameter values contained in challenges or credentials MUST be
 parsed in strict conformance with HTTP semantics (especially the
 unquoting of string parameter values).  In this protocol, those
 values are further categorized into the following value types:
 tokens (bare-token and extensive-token), string, integer,
 hex-fixed-number, and base64-fixed-number.
 For clarity, it is RECOMMENDED that implementations use the canonical
 representations specified in the following subsections for sending
 values.  However, recipients MUST accept both quoted and unquoted
 representations interchangeably, as specified in HTTP.

3.2.1. Tokens

 For sustaining both security and extensibility at the same time, this
 protocol defines a stricter sub-syntax for the "token" to be used.
 Extensive-token values SHOULD use the following syntax (after the
 parsing of HTTP values):
    bare-token           = bare-token-lead-char *bare-token-char
    bare-token-lead-char = %x30-39 / %x41-5A / %x61-7A
    bare-token-char      = %x30-39 / %x41-5A / %x61-7A / "-" / "_"
    extension-token      = "-" bare-token 1*("." bare-token)
    extensive-token      = bare-token / extension-token
                 Figure 3: BNF Syntax for Token Values
 The tokens (bare-token and extension-token) are case insensitive.
 Senders SHOULD send these in lower case, and receivers MUST accept
 both upper and lower cases.  When tokens are used as (partial) inputs
 to any hash functions or other mathematical functions, they MUST
 always be used in lower case.

Oiwa, et al. Experimental [Page 13] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 Extensive-tokens are used in this protocol where the set of
 acceptable tokens may include non-standard extensions.  Any extension
 of this protocol MAY use either the bare-tokens allocated by IANA
 (see the procedure described in Section 16) or extension-tokens with
 the format "-<bare-token>.<domain-name>", where <domain-name> is a
 valid (sub)domain name on the Internet owned by the party who defines
 the extension.
 Bare-tokens and extensive-tokens are also used for parameter names,
 in the unquoted form.  Requirements for using the extension-token for
 the parameter names are the same as those described in the previous
 paragraph.
 The canonical format for bare-tokens and extensive-tokens is the
 unquoted representation.

3.2.2. Strings

 All character strings MUST be encoded to octet strings using UTF-8
 encoding [RFC3629] for the Unicode character set [Unicode].  Such
 strings MUST NOT contain any leading Byte Order Marks (BOMs) (also
 known as ZERO WIDTH NO-BREAK SPACE, U+FEFF, or EF BB BF).  It is
 RECOMMENDED that both peers reject any invalid UTF-8 sequences that
 might cause decoding ambiguities (e.g., containing <"> in the second
 or subsequent bytes of the UTF-8 encoded characters).
 If strings represent a domain name or URI that contains non-ASCII
 characters, the host parts SHOULD be encoded as they (the parts) are
 used in the HTTP protocol layer (e.g., in a Host: header); per
 current standards, the A-label as defined in [RFC5890] will be used.
 Lowercase ASCII characters SHOULD be used.
 The canonical format for strings is quoted-string (as it may contain
 equals signs ("="), plus signs ("+"), and slashes ("/")), unless the
 parameter containing the string value will use extended syntax as
 defined in [RFC5987].  (Per [RFC5987], an extended parameter will
 have an unquoted encoded value.)

3.2.3. Numbers

 The following syntax definitions provide a syntax for numeric values:
  integer             = "0" / (%x31-39 *DIGIT)     ; no leading zeros
  hex-fixed-number    = 1*(2(DIGIT / %x41-46 / %x61-66))
  base64-fixed-number = 1*( ALPHA / DIGIT / "+" / "/" ) 0*2"="
                   Figure 4: BNF Syntax for Numbers

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 The syntax definition of the integers only allows representations
 that do not contain leading zeros.
 A number represented as a hex-fixed-number MUST include an even
 number of hexadecimal digits (i.e., multiples of eight bits).  Those
 values are case insensitive and SHOULD be sent in lower case.  When
 these values are generated from any cryptographic values, they MUST
 have their "natural length"; if they are generated from a hash
 function, their lengths correspond to the hash size; if they
 represent elements of a mathematical set (or group), their lengths
 SHALL be the shortest lengths that represent all the elements in the
 set.  For example, the results of the SHA-256 hash function will be
 represented by 64 digits, and any elements in a 2048-bit prime field
 (modulo a 2048-bit integer) will be represented by 512 digits,
 regardless of how many zeros appear in front of such representations.
 Session identifiers and other non-cryptographically generated values
 are represented in any (even) length determined by the side that
 generates it first, and the same length MUST be used in all
 communications by both peers.
 The numbers represented as base64-fixed-number SHALL be generated as
 follows: first, the number is converted to a big-endian radix-256
 binary representation as an octet string.  The length of the
 representation is determined in the same way as the technique
 mentioned above.  Then, the string is encoded using base64 encoding
 (described in Section 4 of [RFC4648]) without any spaces and
 newlines.  Implementations decoding base64-fixed-number SHOULD reject
 any input data with invalid characters, excess or insufficient
 padding, or non-canonical pad bits (see Sections 3.1 through 3.5 of
 [RFC4648]).
 The canonical format for integer and hex-fixed-number is unquoted
 tokens, and the canonical format for base64-fixed-number is
 quoted-string.

4. Messages

 In this section, we define the six kinds of messages in the
 authentication protocol, along with the formats and requirements of
 the headers for each type of message.
 To determine under what circumstances each message is expected to be
 sent, see Sections 10 and 11.
 In the descriptions below, the types of allowable values for each
 header parameter are shown in parentheses 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

Oiwa, et al. Experimental [Page 15] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 determined by the value of the "algorithm" parameter.  The parameters
 marked "mandatory" SHALL be contained in the message.  The parameters
 marked "non-mandatory" MAY be either contained in the message or
 omitted from it.  Each parameter SHALL appear in each header exactly
 once at most.
 All credentials and challenges MAY contain any parameters not
 explicitly specified in the following sections.  Recipients that
 do not understand such parameters MUST silently ignore them.
 However, all credentials and challenges MUST meet the following
 criteria:
 o  For responses, the parameters "reason", any "ks#" (where "#"
    stands for any decimal integer), and "vks" are mutually exclusive;
    any challenges MUST NOT contain two or more parameters among them.
    They MUST NOT contain any "kc#" or "vkc" parameters.
 o  For requests, the parameters "kc#" (where "#" stands for any
    decimal integer) and "vkc" are mutually exclusive; any challenges
    MUST NOT contain two or more parameters among them.  They MUST NOT
    contain any "ks#" or "vks" parameters.
 Every message defined in this section contains a "version" field to
 detect any future revisions of the protocol that are incompatible.
 Implementations of the protocol described in this specification MUST
 always send a token "1" to represent the version number.  Recipients
 MUST reject messages that contain any other value for the version,
 unless another specification defines specific behavior for that
 version.

4.1. 401-INIT and 401-STALE

 Every 401-INIT or 401-STALE message SHALL be a valid HTTP 401
 (Unauthorized) status message (or some other 4xx status message, if
 appropriate) containing one and only one (hereafter not explicitly
 noted) WWW-Authenticate header containing a "reason" parameter in the
 challenge.  The challenge SHALL contain all of the parameters marked
 "mandatory" below and MAY contain those marked "non-mandatory".
 version:
    (mandatory extensive-token) should be the token "1".
 algorithm:
    (mandatory extensive-token) specifies the authentication algorithm
    to be used.  The value MUST be one of the tokens specified in
    [RFC8121] or another supplemental specification.

Oiwa, et al. Experimental [Page 16] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 validation:
    (mandatory extensive-token) specifies the method of host
    validation.  The value MUST be one of the tokens described in
    Section 7 or the tokens specified in another supplemental
    specification.
 auth-scope:
    (non-mandatory string) specifies the authentication scope, i.e.,
    the set of hosts for which the authentication credentials are
    valid.  It MUST be one of the strings described in Section 5.  If
    the value is omitted, it is assumed to be the "single-server type"
    domain as described in Section 5.
 realm:
    (mandatory string) is a string representing the name of the
    authentication realm inside the authentication scope.  As
    specified in [RFC7235], this value MUST always be sent in the
    quoted-string form, and an encoding as specified in [RFC5987]
    MUST NOT be used.
    The realm value sent from the server SHOULD be an ASCII string.
    Clients MAY treat any non-ASCII value received in this field as a
    binary blob, an NFC-normalized UTF-8 string ("NFC" stands for
    "Normalization Form C"), or an error.
 reason:
    (mandatory extensive-token) SHALL be an extensive-token that
    describes the possible reason for the failed authentication or
    authorization.  Both servers and clients SHALL understand and
    support the following three tokens:
  • initial: Authentication was not attempted because there was no

Authorization header in the corresponding request.

  • stale-session: The provided sid in the request was either

unknown to the server or expired in the server.

  • auth-failed: The authentication trial failed for some reason,

possibly because of a bad authentication credential.

Oiwa, et al. Experimental [Page 17] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

    Implementations MAY support the following tokens or any
    extensive-tokens defined outside of this specification.  If
    clients receive any unknown tokens, they SHOULD treat them as if
    they were "auth-failed" or "initial".
  • reauth-needed: The server-side application requires a new

authentication trial, regardless of the current status.

  • invalid-parameters: The server did not attempt authentication

because some parameters were not acceptable.

  • internal-error: The server did not attempt authentication

because there are some problems on the server side.

  • user-unknown: This is a special case of auth-failed; it

suggests that the provided username is invalid. Due to

       security implications, the use of this parameter is
       NOT RECOMMENDED, except for special-purpose applications where
       it would make sense to do so.
  • invalid-credential: This is another special case of

auth-failed; it suggests that the provided username was valid

       but authentication still failed.  For security reasons, the use
       of this parameter is NOT RECOMMENDED.
  • authz-failed: Authentication was successful, but access to the

specified resource is not authorized to the specific

       authenticated user.  (It might be used along with either a
       401 (Unauthorized) or 403 (Forbidden) status code to indicate
       that the authentication result is one of the existing reasons
       for the failed authorization.)
    It is RECOMMENDED that the reason for failure be recorded to some
    type of diagnostic log, shown to the client user immediately, or
    both.  It will be helpful to find out later whether the reason for
    the failure is technical or caused by user error.
 The algorithm specified in this header will determine the types
 (among those defined in Section 3) and the values for K_c1, K_s1,
 VK_c, and VK_s.
 Among these messages, any messages with the "reason" parameter value
 "stale-session" will be called "401-STALE" messages hereafter,
 because these messages have a special meaning in the protocol flow.
 Messages with any other "reason" parameters will be called "401-INIT"
 messages.

Oiwa, et al. Experimental [Page 18] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

4.2. req-KEX-C1

 Every req-KEX-C1 message SHALL be a valid HTTP request message
 containing an Authorization header with a credential containing a
 "kc1" parameter.
 The credential SHALL contain the parameters with the following names:
 version:
    (mandatory, extensive-token) should be the token "1".
 algorithm, validation, auth-scope, realm:
    MUST be the same values as those received from the server.
 user:
    (mandatory, string) is the UTF-8 encoded name of the user.  The
    string SHOULD be prepared according to the method presented in
    Section 9.
 kc1:
    (mandatory, algorithm-determined) is the client-side key exchange
    value K_c1, which is specified by the algorithm that is used.

4.3. 401-KEX-S1

 Every 401-KEX-S1 message SHALL be a valid HTTP 401 (Unauthorized)
 status response message containing a WWW-Authenticate header with a
 challenge containing a "ks1" parameter.
 The challenge SHALL contain the parameters with the following names:
 version:
    (mandatory, extensive-token) should be the token "1".
 algorithm, validation, auth-scope, realm:
    MUST be the same values as those received from the client.
 sid:
    (mandatory, hex-fixed-number) MUST be a session identifier, which
    is a random integer.  The sid SHOULD have uniqueness of at least
    80 bits or the square of the maximum estimated transactions
    concurrently available in the session table, whichever is larger.
    See Section 6 for more details.
 ks1:
    (mandatory, algorithm-determined) is the server-side key exchange
    value K_s1, which is specified by the algorithm.

Oiwa, et al. Experimental [Page 19] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 nc-max:
    (mandatory, integer) is the maximum value of nonce numbers that
    the server accepts.
 nc-window:
    (mandatory, integer) is the number of available nonce number slots
    that the server will accept.  It is RECOMMENDED that the value of
    the "nc-window" parameter be 128 or more.
 time:
    (mandatory, integer) represents the suggested time (in seconds)
    that the client can reuse the session represented by the sid.  It
    is RECOMMENDED that the time be set to at least 60 (seconds).
    However, the server is not required to guarantee that the session
    represented by the sid will be available (e.g., alive, usable) for
    the time specified in this parameter.
 path:
    (non-mandatory, string) specifies to 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 that specified
    in the "domain" parameter [RFC7616] for Digest authentications.
    All path elements contained in the "path" parameter MUST be inside
    the specified auth-scope; if not, clients SHOULD ignore such
    elements.  For better performance, it is important that clients
    recognize and use this parameter.

4.4. req-VFY-C

 Every req-VFY-C message SHALL be a valid HTTP request message
 containing an Authorization header with a credential containing a
 "vkc" parameter.
 The parameters contained in the header are as follows:
 version:
    (mandatory, extensive-token) should be the token "1".
 algorithm, validation, auth-scope, realm:
    MUST be the same values as those 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.

Oiwa, et al. Experimental [Page 20] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 nc:
    (mandatory, integer) is a nonce request number that is unique
    among the requests sharing the same sid.  The values of the nonce
    numbers SHOULD satisfy the properties outlined in Section 6.
 vkc:
    (mandatory, algorithm-determined) is the client-side
    authentication verification value VK_c, which is specified by the
    algorithm.

4.5. 200-VFY-S

 Every 200-VFY-S message SHALL be a valid HTTP message that does not
 have a 401 (Unauthorized) status code and SHALL contain an
 Authentication-Info header with a "vks" parameter.
 The parameters contained in the header are as follows:
 version:
    (mandatory, extensive-token) should be the token "1".
 sid:
    (mandatory, hex-fixed-number) MUST be the value received from the
    client.
 vks:
    (mandatory, algorithm-determined) is the server-side
    authentication verification value VK_s, which is specified by the
    algorithm.
 The header MUST be sent before the content body; it MUST NOT be sent
 in the trailer of a chunked-encoded response.  If a "100 (Continue)"
 [RFC7231] response is sent from the server, the Authentication-Info
 header SHOULD be included in that response instead of the final
 response.

5. Authentication Realms

 In this protocol, an authentication realm is defined as a set of
 resources (URIs) for which the same set of usernames and passwords is
 valid.  If the server requests authentication for an authentication
 realm for which the client is already authenticated, the client will
 automatically perform the authentication using the already-known
 credentials.  However, for different authentication realms, clients
 MUST NOT automatically reuse usernames and passwords for another
 realm.

Oiwa, et al. Experimental [Page 21] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 As is the case for the Basic and Digest access authentication
 protocols, the Mutual authentication protocol supports multiple,
 separate protection spaces to be set up inside each host.
 Furthermore, the protocol allows a single authentication realm to
 span several hosts within the same Internet domain.
 Each authentication realm is defined and distinguished by the triple
 of an authentication algorithm, an authentication scope, and a
 "realm" parameter.  However, it is NOT RECOMMENDED that server
 operators use the same pair of an authentication scope and a realm
 with different authentication algorithms.
 The "realm" parameter is a string as defined in Section 4.
 Authentication scopes are described in the remainder of this section.
 An authentication scope specifies the range of hosts spanned by the
 authentication realm.  In this protocol, it MUST be one of the
 following kinds of strings:
 o  Single-server type: A string in the format "<scheme>://<host>" or
    "<scheme>://<host>:<port>", where <scheme>, <host>, and <port> are
    the corresponding URI parts of the request URI.  If the default
    port (i.e., 80 for HTTP and 443 for HTTPS) is used for the
    underlying HTTP communications, the port part MUST be omitted,
    regardless of whether it was present in the request URI.  In all
    other cases, the port part MUST be present, and it MUST NOT
    contain leading zeros.  Use this format when authentication is
    only valid for a specific protocol (such as HTTPS).  This format
    is equivalent to the ASCII serialization of a Web origin, as
    presented in Section 6.2 of [RFC6454].
 o  Single-host type: The "host" part of the requested URI.  This is
    the default value.  Authentication realms within this kind of
    authentication scope will span several protocols (e.g., HTTP and
    HTTPS) and ports but will not span different hosts.
 o  Wildcard-domain type: A string in the format "*.<domain-postfix>",
    where <domain-postfix> is either the host part of the requested
    URI or any domain in which the requested host is included (this
    means that the specification "*.example.com" is valid for all of
    hosts "www.example.com", "web.example.com",
    "www.sales.example.com", and "example.com").  The domain-postfix
    sent by the servers MUST be equal to or included in a valid
    Internet domain assigned to a specific organization; if clients
    know, via some means such as a blacklist for HTTP cookies
    [RFC6265], that the specified domain is not to be assigned to any
    specific organization (e.g., "*.com" or "*.jp"), it is RECOMMENDED
    that clients reject the authentication request.

Oiwa, et al. Experimental [Page 22] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 In the above specifications, every "scheme", "host", and "domain"
 MUST be in lower case, and any internationalized domain names beyond
 the ASCII character set SHALL be represented in the way they are sent
 in the underlying HTTP protocol, represented in lowercase characters,
 i.e., these domain names SHALL be in the form of LDH ("letters,
 digits, hyphen") labels as defined in the Internationalized Domain
 Names for Applications (IDNA) specification [RFC5890].  A "port" MUST
 be given in shortest unsigned decimal number notation.  Not obeying
 these requirements will cause valid authentication attempts to fail.

5.1. Resolving Ambiguities

 In the above definitions of authentication scopes, several scopes may
 overlap each other.  If a client has already been authenticated to
 several realms applicable to the same server, the client may have
 multiple lists of the "path" parameters received with the
 "401-KEX-S1" message (see Section 4).  If these path lists have any
 overlap, a single URI may belong to multiple possible candidate
 realms to which the client can be authenticated.  In such cases,
 clients face an ambiguous choice regarding which credentials to send
 for a new request (see Steps 3 and 4 of the decision procedure
 presented in Section 10).
 In such cases, a client MAY freely send requests that belong to any
 of these candidate realms, or it MAY simply send an unauthenticated
 request and see for which realm the server requests an
 authentication.  It is RECOMMENDED that server operators provide
 properly configured "path" parameters (more precisely, disjoint path
 sets for each realm) for clients so that such ambiguities will not
 occur.
 The following procedure is one possible tactic for resolving
 ambiguities in such cases:
 o  If the client has previously sent a request to the same URI and it
    remembers the authentication realm requested by the 401-INIT
    message at that time, use that realm.
 o  In other cases, use one of the authentication realms representing
    the most-specific authentication scopes.  The list of possible
    domain specifications shown above is given from most specific to
    least specific.
    If there are several choices with different wildcard-domain
    specifications, the one that has the longest domain-postfix has
    priority over those with shorter domain-postfixes.

Oiwa, et al. Experimental [Page 23] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 o  If there are realms with the same authentication scope, there is
    no defined priority; the client MAY choose any one of the possible
    choices.

6. Session Management

 In the Mutual authentication protocol, a session represented by
 an sid is set up using four messages (first request, 401-INIT,
 req-KEX-C1, and 401-KEX-S1), after which a session secret (z)
 associated with the session is established.  After mutually
 establishing a session secret, this session, along with the secret,
 can be used for one or more requests for resources protected by the
 same realm on the same server.  Note that session management is only
 an inside detail of the protocol and usually not visible to normal
 users.  If a session expires, the client and server SHOULD
 automatically re-establish another session without informing
 the user.
 Sessions and session identifiers are local to each server (defined by
 scheme, host, and port), even if an authentication scope covers
 multiple servers; clients MUST establish separate sessions for each
 port of a host to be accessed.  Furthermore, sessions and identifiers
 are also local to each authentication realm, even if they are
 provided by the same server.  The same session identifiers provided
 either from different servers or for different realms MUST be treated
 as being independent of each other.
 The server SHOULD accept at least one req-VFY-C request for each
 session if the request reaches the server in a time window specified
 by the "timeout" parameter in the 401-KEX-S1 message and if there are
 no emergent reasons (such as flooding attacks) to forget the session.
 After that, the server MAY discard any session at any time and MAY
 send 401-STALE messages for any further req-VFY-C requests received
 for that session.
 The client MAY send two or more requests using a single session
 specified by the sid.  However, for all such requests, each value of
 the nonce number (in the "nc" parameter) MUST satisfy the following
 conditions:
 o  It is a natural number.
 o  The same nonce number was not sent within the same session.
 o  It is not larger than the nc-max value that was sent from the
    server in the session represented by the sid.

Oiwa, et al. Experimental [Page 24] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 o  It is larger than (largest-nc - nc-window), where largest-nc is
    the largest value of nc that was previously sent in the session
    and nc-window is the value of the "nc-window" parameter that was
    received from the server for the session.
 The last condition allows servers to reject any nonce numbers that
 are "significantly" smaller than the "current" value (defined by the
 value of nc-window) of the nonce number used in the session involved.
 In other words, servers MAY treat such nonce numbers as "already
 received".  This restriction enables servers to implement
 duplicate-nonce detection in a constant amount of memory for each
 session.
 Servers MUST check for duplication of the received nonce numbers, and
 if any duplication is detected, the server MUST discard the session
 and respond with a 401-STALE message, as outlined in Section 11.  The
 server MAY also reject other invalid nonce numbers (such as those
 above the nc-max limit) by sending a 401-STALE message.
 For example, assume that the nc-window value of the current session
 is 128 and nc-max is 400, and that the client has already used the
 following nonce numbers: {1-120, 122, 124, 130-238, 255-360,
 363-372}.  The nonce number that can then be used for the next
 request is a number from the following set: {245-254, 361, 362,
 373-400}.  The values {0, 121, 123, 125-129, 239-244} MAY be rejected
 by the server because they are not above the current "window limit"
 (244 = 372 - 128).
 Typically, clients can ensure the above property by using a
 monotonically increasing integer counter that counts from zero up to
 the value of nc-max.
 The values of the nonce numbers and any nonce-related values MUST
 always be treated as natural numbers within an infinite range.
 Implementations that use fixed-width integer representations,
 fixed-precision floating-point numbers, or similar representations
 SHOULD NOT reject any larger values that overflow such representative
 limits and MUST NOT silently truncate them using any modulus-like
 rounding operation (e.g., by mod 2^32).  Instead, the whole protocol
 is carefully designed so that recipients MAY replace any such
 overflowing values (e.g., 2^80) with some reasonably large maximum
 representative integer (e.g., 2^31 - 1 or others).

Oiwa, et al. Experimental [Page 25] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

7. Host Validation Methods

 The "validation method" specifies a method to "relate" (or "bind")
 the mutual authentication processed by this protocol with other
 authentications already performed in the underlying layers and to
 prevent man-in-the-middle attacks.  It determines the value vh that
 is an input to the authentication protocols.
 When HTTPS or another possible secure transport is used, this
 corresponds to the idea of "channel binding" as described in
 [RFC5929].  Even when HTTP is used, similar, but somewhat limited,
 "binding" is performed to prevent a malicious server from trying to
 authenticate itself to another server as a valid user by forwarding
 the received credentials.
 The valid tokens for the "validation" parameter and corresponding
 values of vh are as follows:
 host:
    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
    server-side resource currently being accessed.  The scheme and
    host are in lower case, and the port is listed in shortest decimal
    notation.  Even if the request URI does not have a port part, vh
    will include the default port number.
 tls-server-end-point:
    TLS endpoint (certificate) validation.  The value vh will be the
    octet string of the hash value of the server's public key
    certificate used in the underlying TLS [RFC5246] connection,
    processed as specified in Section 4.1 of [RFC5929].
 tls-unique:
    TLS shared-key validation.  The value vh will be the
    channel-binding material derived from the Finished messages,
    as defined in Section 3.1 of [RFC5929].  (Note: See Section 7.2
    for some security-related notes regarding this validation method.)
 If HTTP is used on a non-encrypted channel (TCP and the Stream
 Control Transmission Protocol (SCTP), for example), the validation
 type MUST be "host".  If HTTP/TLS [RFC2818] (HTTPS) is used with a
 server certificate, the validation type MUST be
 "tls-server-end-point".  If HTTP/TLS is used with an anonymous
 Diffie-Hellman key exchange, the validation type MUST be "tls-unique"
 (see the note below).

Oiwa, et al. Experimental [Page 26] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 If the validation type "tls-server-end-point" is used, the server
 certificate provided in the TLS connection MUST be verified at least
 to make sure that the server actually owns the corresponding private
 key.  (Note: This verification is automatic in some RSA-based key
 exchanges but is NOT automatic in Diffie-Hellman-based key exchanges
 with separate exchanges for server verification.)
 Clients MUST validate this parameter upon receipt of 401-INIT
 messages.
 Note: The protocol defines two variants of validation on the TLS
 connections.  The "tls-unique" method is technically more secure.
 However, there are some situations where "tls-server-end-point" is
 preferable:
 o  When TLS accelerating proxies are used.  In this case, it is
    difficult for the authenticating server to acquire the TLS key
    information that is used between the client and the proxy.  This
    is not the case for client-side "tunneling" proxies using the HTTP
    CONNECT method.
 o  When a black-box implementation of the TLS protocol is used on
    either peer.

7.1. Applicability Notes

 When the client is a Web browser with any scripting capabilities
 (support of dynamic contents), the underlying TLS channel used with
 HTTP/TLS MUST provide server identity verification.  This means that
 (1) anonymous Diffie-Hellman key exchange cipher suites MUST NOT be
 used and (2) verification of the server certificate provided by the
 server MUST be performed.  This is to prevent loading identity-
 unauthenticated scripts or dynamic contents, which are referenced
 from the authenticated page.
 For other systems, when the underlying TLS channel used with HTTP/TLS
 does not perform server identity verification, the client SHOULD
 ensure that all responses are validated using the Mutual
 authentication protocol, regardless of the existence of 401-INIT
 responses.

Oiwa, et al. Experimental [Page 27] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

7.2. Notes on "tls-unique"

 As described in the interoperability note in Section 3.1 of
 [RFC5929], the "tls-unique" verification value will be changed by
 possible TLS renegotiation, causing an interoperability problem.  TLS
 renegotiations are used in several HTTPS server implementations for
 enforcing some security properties (such as cryptographic strength)
 for some specific responses.
 If an implementation supports the "tls-unique" verification method,
 the following precautions SHOULD be taken:
 o  Both peers must be aware that the vh values used for vkc (in
    req-VFY-C messages) and vks (in 200-VFY-S messages) may be
    different.  These values MUST be retrieved from underlying TLS
    libraries each time they are used.
 o  After calculating the values vh and vkc to send a req-VFY-C
    request, clients SHOULD NOT initiate TLS renegotiation until the
    end of the corresponding response header is received.  An
    exception is that clients can and SHOULD perform TLS renegotiation
    as a response to the server's request for TLS renegotiation,
    before receipt of the beginning of the response header.
 Also, implementers MUST take care of session resumption attacks
 regarding "tls-unique" channel-binding mechanisms and master secrets.
 As a mitigation, the TLS extension defined in [RFC7627] SHOULD be
 used when "tls-unique" host verification is to be used.

8. Authentication Extensions

 It is RECOMMENDED that interactive clients (e.g., Web browsers)
 supporting this protocol support non-mandatory authentication and the
 Authentication-Control header defined in [RFC8053], except for the
 "auth-style" parameter.  This specification also proposes (but does
 not mandate) that the default "auth-style" be "non-modal".  Web
 applications SHOULD, however, consider the security impacts of the
 behavior of clients that do not support these headers.
 Authentication-initializing messages with the
 Optional-WWW-Authenticate header are used only where the 401-INIT
 response is valid.  It will not replace other 401-type messages such
 as 401-STALE and 401-KEX-S1.  That is, the "reason" field of such a
 message MUST be "initial" (or any extensive-tokens NOT defined in
 Section 4.1).

Oiwa, et al. Experimental [Page 28] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

9. String Preparation

 For interoperability reasons, it is important that usernames and
 passwords used in this protocol be binary-comparable, regardless of
 the user's input methods and/or environments.  To ensure this, the
 following preparation SHOULD be performed:
 o  Usernames received from users SHOULD be prepared using the
    "UsernameCasePreserved" profile defined in Section 3.3 of
    [RFC7613].
 o  Passwords received from users SHOULD be prepared using the
    "OpaqueString" profile defined in Section 4.2 of [RFC7613].
 In both cases, it is the sender's duty to correctly prepare the
 character strings.  If any non-prepared character string is received
 from the other peer of the communication, the behavior of its
 recipient is not defined; the recipient MAY either accept or reject
 such input.
 Server applications SHOULD also prepare usernames and passwords
 accordingly upon registration of user credentials.
 In addition, binary-based "interfaces" of implementations MAY require
 and assume that the string is already prepared accordingly; when a
 string is already stored as a binary Unicode string form,
 implementations MAY omit preparation and Unicode normalization
 (performing UTF-8 encoding only) before using it.  When a string is
 already stored as an octet blob, implementations MAY send it as is.

10. Decision Procedure for Clients

10.1. General Principles and Requirements

 To securely implement the protocol, the client must be careful about
 accepting the authenticated responses from the server.  This also
 holds true for the reception of a "normal response" (a response that
 does not contain mutual-authentication-related headers) from HTTP
 servers.

Oiwa, et al. Experimental [Page 29] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 Per typical HTTP authentication, a single user-level request may
 result in the exchange of two or more HTTP requests and responses in
 sequence.  The following normative rules MUST be followed by the
 clients implementing this protocol:
 o  Any kind of "normal response" MUST only be accepted for the very
    first request in the sequence.  Any "normal response" returned for
    the second or subsequent requests in the sequence SHALL be
    considered invalid.
 o  By the same principle, if any response is related to an
    authentication realm that is different from that of the client's
    request (for example, a 401-INIT message requesting authentication
    on another realm), it MUST only be accepted for the very first
    request in the sequence.  Such a response returned for a second or
    subsequent request in the sequence SHALL be considered invalid.
 o  A req-KEX-C1 message MAY be sent as either an initial request or a
    response to a 401-INIT or 401-STALE message.  However, to avoid
    infinite loops of messages, the req-KEX-C1 message SHOULD NOT be
    sent more than once in the sequence for a single authentication
    realm.  A 401-KEX-S1 response MUST be accepted only when the
    corresponding request is req-KEX-C1.
 o  A req-VFY-C message MAY be sent if there is a valid session secret
    shared between the client and the server, as established by
    req-KEX-C1 and 401-KEX-S1 messages.  If any response with a
    401 status code is returned for such a message, the corresponding
    session secret SHOULD be discarded as unusable.
    In particular, upon the reception of a 401-STALE response, the
    client SHOULD try to establish a new session by sending a
    req-KEX-C1 message, but only once within the request/response
    sequence.
 o  A 200-VFY-S message MUST be accepted only as a response to a
    req-VFY-C message and nothing else.  The VK_s values of such
    response messages MUST always be checked against the correct
    value, and if it is incorrect, the whole response SHOULD be
    considered invalid.

Oiwa, et al. Experimental [Page 30] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 The final status of the client request following the message exchange
 sequence shall be determined as follows:
 o  AUTH-SUCCEED: A 200-VFY-S message with the correct VK_s value was
    returned in response to the req-VFY-C request in the sequence.
 o  AUTH-REQUIRED: Two cases exist:
  • A 401-INIT message was returned from the server, and the client

does not know how to authenticate to the given authentication

       realm.
  • A 401-INIT response was returned for a req-VFY-C (or

req-KEX-C1) message, which means that the user-supplied

       authentication credentials were not accepted.
 o  UNAUTHENTICATED: A "normal response" is returned for an initial
    request of any kind in the sequence.
 Any kind of response (including a "normal response") other than those
 explicitly allowed in the above rules SHOULD be interpreted as a
 fatal communication error.  In such cases, the clients MUST NOT
 process any data (the response body and other content-related
 headers) sent from the server.  However, to handle exceptional error
 cases, clients MAY accept a message without an Authentication-Info
 header if it has a Server Error (5xx) status code.  In such cases,
 they SHOULD be careful about processing the body of the content
 (ignoring it is still RECOMMENDED, as it may possibly be forged by
 intermediate attackers), and the client will then have a status of
 "UNAUTHENTICATED".
 If a request is a sub-request for a resource included in another
 resource (e.g., embedded images, style sheets, frames), clients MAY
 treat an AUTH-REQUESTED status the same way they would treat an
 UNAUTHENTICATED status.  In other words, the client MAY ignore the
 server's request to start authentication with new credentials via
 sub-requests.

10.2. State Machine for the Client (Informative)

 The following state machine describes the possible request-response
 sequences derived from the above normative rules.  If implementers
 are not quite sure of the security consequences of the above rules,
 we strongly advise that the decision procedure below be followed.  In
 particular, clients SHOULD NOT accept "normal responses" unless
 explicitly allowed in the rules.  The labels in the steps below are

Oiwa, et al. Experimental [Page 31] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 for informational purposes only.  Action entries within each step are
 checked in top-to-bottom order, and the first clause satisfied is to
 be followed.
 Step 1 (step_new_request):
     If the client software needs to access a new Web resource, check
     to see whether the resource is expected to be inside some
     authentication realm for which the user has already been
     authenticated via the Mutual authentication scheme.  If yes,
     go to Step 2.  Otherwise, go to Step 5.
 Step 2:
     Check to see whether there is an available sid for the expected
     authentication realm.  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 a 401-INIT message is received with a different

authentication realm than expected, go to Step 6.

  • If a 401-STALE message is received, go to Step 9.
  • If a 401-INIT message is received, go to Step 13.
  • If a 200-VFY-S message is received, go to Step 14.
  • If a "normal response" is received, go to Step 11.
 Step 4 (step_send_kex1_1):
     Send a req-KEX-C1 request.
  • If a 401-INIT message is received with a different

authentication realm than expected, go to Step 6.

  • If a 401-KEX-S1 message is received, go to Step 10.
  • If a 401-INIT message is received with the same authentication

realm, go to Step 13 (see Note 1).

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

Oiwa, et al. Experimental [Page 32] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 Step 5 (step_send_normal_1):
     Send a request without any mutual-authentication headers.
  • If a 401-INIT message is received, go to Step 6.
  • If a "normal response" is received, go to Step 11.
 Step 6 (step_rcvd_init):
     Check to see whether the user's password for the requested
     authentication realm is known.  If yes, go to Step 7.  Otherwise,
     go to Step 12.
 Step 7:
     Check to see whether there is an available sid for the expected
     authentication realm.  If there is one, go to Step 8.  Otherwise,
     go to Step 9.
 Step 8 (step_send_vfy):
     Send a req-VFY-C request.
  • If a 401-STALE message is received, go to Step 9.
  • If a 401-INIT message is received, go to Step 13.
  • If a 200-VFY-S message is received, go to Step 14.
 Step 9 (step_send_kex1):
     Send a req-KEX-C1 request.
  • If a 401-KEX-S1 message is received, go to Step 10.
  • If a 401-INIT message is received, go to Step 13 (see Note 1).
 Step 10 (step_rcvd_kex1):
     Send a req-VFY-C request.
  • If a 401-INIT message is received, go to Step 13.
  • If a 200-VFY-S message is received, go to Step 14.
 Step 11 (step_rcvd_normal):
     The requested resource is out of the authenticated area.  The
     client will be in the "UNAUTHENTICATED" status.  If the response
     contains a request for authentication other than Mutual
     authentication, it MAY be handled normally.

Oiwa, et al. Experimental [Page 33] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 Step 12 (step_rcvd_init_unknown):
     The requested resource requires Mutual authentication, and the
     user is not yet authenticated.  The client will be in the
     "AUTH-REQUESTED" status; it is RECOMMENDED that the client
     process the content sent from the server and ask the user for a
     username and password.  When those are supplied by the user,
     go to Step 9.
 Step 13 (step_rcvd_init_failed):
     The authentication failed for some reason, possibly because the
     password or username is invalid for the authenticated resource.
     Forget the user-provided credentials for the authentication
     realm, and go to Step 12.
 Step 14 (step_rcvd_vfy):
     The received message is the 200-VFY-S message, which always
     contains a "vks" field.  Check the validity of the received VK_s
     value.  If it is equal to the expected value, then the mutual
     authentication succeeded.  The client will be in the
     "AUTH-SUCCEED" status.
     An unexpected value is interpreted as a fatal communication
     error.
     If a user explicitly asks to log out (via the user interface),
     the client MUST forget the user's password, go to Step 5, and
     reload the current resource without an authentication header.
 Note 1:  These transitions MAY be accepted by clients, but it is
     NOT RECOMMENDED that servers initiate them.
 Figure 5 shows an informative diagram of the client state.

Oiwa, et al. Experimental [Page 34] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

       ===========                                  -(11)------------
       NEW REQUEST                                 ( UNAUTHENTICATED )
       ===========                                  -----------------
            |                                              ^ normal
            v                                              | response
 +(1)-------------------+ NO                         +(5)----------+
 | The requested URI    |--------------------------->| send normal |
 | known to be auth'ed? |                            |   request   |
 +----------------------+                            +-------------+
        YES |   401-INIT                            401-INIT|
            |   with a different realm                      |
            |          -----------------------------------. |
            |         /                                   v v
            |        |       -(12)------------    NO  +(6)--------+
            |        |      ( AUTH-REQUESTED  )<------| user/pass |
            |        |       -----------------        |   known?  |
            |        |                                +-----------+
            |        |                                      |YES
            v        |                                      v
      +(2)--------+  |                                +(7)--------+
      | session   |  |                                | session   | NO
  NO /| available?|  |                                | available?|\
    / +-----------+  |                                +-----------+ |
   /        |YES     |                                      |YES    |
  |         |       /|                                      |       |
  |         v      / |  401-                   401-         v       |
  |   +(3)--------+  |  INIT --(13)----------  INIT   +(8)--------+ |
  |   |   send    |--+----->/ AUTH-REQUESTED \<-------|   send    | |
  |  /| req-VFY-C |  |      \forget password /        | req-VFY-C | |
   \/ +-----------+ /        ----------------        /+-----------+ |
   /\           \ \/                 ^ 401-INIT     |     |401-     |
  |  ------      \/\  401-STALE      |              |     | STALE  /
  |        \     /\ -----------------+--------------+---. |       /
  |         |   /  \                 |              |   | |      /
  |         v  /    | 401-           |       401-   |   v v     v
  |   +(4)--------+ | KEX-S1   +(10)-------+ KEX-S1 | +(9)--------+
  |   |   send    |-|--------->|   send    |<-------+-|   send    |
  | --| req-KEX-C1| |          | req-VFY-C |        | | req-KEX-C1|
  |/  +-----------+ |          +-----------+        | +-----------+
  |                 |200-VFY-S      |      200-VFY-S|       ^
  |normal           |               |200-VFY-S     /        |
  |response         |               v             / ==================
  v                  \         -(14)---------    /  USER/PASS INPUTTED
  -(11)------------   ------->( AUTH-SUCCEED )<--   ==================
 ( UNAUTHENTICATED )           --------------
  -----------------
                  Figure 5: State Diagram for Clients

Oiwa, et al. Experimental [Page 35] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

11. Decision Procedure for Servers

 Each server SHOULD have a table of session states.  This table need
 not be persistent over the long term; it MAY be cleared upon server
 restart, reboot, or for other reasons.  Each entry in the table
 SHOULD contain at least the following information:
 o  The session identifier, which is the value of the "sid" parameter.
 o  The algorithm used.
 o  The authentication realm.
 o  The state of the protocol: one of "key exchanging",
    "authenticated", "rejected", or "inactive".
 o  The username received from the client.
 o  A boolean flag indicating 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 value between (largest-nc - nc-window + 1)

and max_nc, a boolean flag indicating whether or not a request

       with the corresponding nc has been received.
 The table MAY contain other information.
 Servers SHOULD respond to the client requests according to the
 following procedure (see Note 1 below regarding 401-INIT messages
 with a plus sign):
 o  When the server receives a "normal request":
  • If the requested resource is not protected by the Mutual

authentication, send a "normal response".

  • If the resource is protected by the Mutual authentication, send

a 401-INIT response.

Oiwa, et al. Experimental [Page 36] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 o  When the server receives a req-KEX-C1 request:
  • If the requested resource is not protected by the Mutual

authentication, send a "normal response".

  • If the authentication realm specified in the req-KEX-C1 request

is not the expected realm, send a 401-INIT response.

  • If the server cannot validate the parameter "kc1", send a

401-INIT (+) response.

  • If the received username is either invalid, unknown, or

unacceptable, create a new session, mark it as a "fake"

       session, compute a random value as K_s1, and send a fake
       401-KEX-S1 response (see Note 2).
  • Otherwise, create a new session, compute K_s1, and send a

401-KEX-S1 response. The created session is marked as not

       fake, and its largest-nc value is initialized to zero.
    The created session is in the "key exchanging" state.
 o  When the server receives a req-VFY-C request:
  • If the requested resource is not protected by the Mutual

authentication, send a "normal response".

  • If the authentication realm specified in the req-VFY-C request

is not the expected realm, send a 401-INIT response.

    If none of the above holds true, the server will look up the
    session corresponding to the received sid and the authentication
    realm.
  • If the session corresponding to the received sid could not be

found or it is in the "inactive" state, send a 401-STALE

       response.
  • If the session is in the "rejected" state, send either a

401-INIT (+) response or a 401-STALE message.

  • If the nc value in the request is larger than the "nc-max"

parameter sent from the server or it is not larger than

       (largest-nc - nc-window) (when in the "authenticated" state),
       the server MAY (but is not REQUIRED to; see Note 3) send a
       401-STALE message.  The session is changed to the "inactive"
       state if the 401-STALE message was sent.

Oiwa, et al. Experimental [Page 37] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

  • 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 is changed to the
       "inactive" state.
  • If the session is a "fake" session or the received vkc is

incorrect, then send a 401-INIT (+) response. If the session

       is in the "key exchanging" state, it MUST be changed to the
       "rejected" state; otherwise, it MAY be either changed to the
       "rejected" state or kept in the previous state.
  • Otherwise, send a 200-VFY-S response. If the session was in

the "key exchanging" state, the session SHOULD be changed to

       the "authenticated" state.  The maximum nc and nc flags of the
       state MUST be updated appropriately.
 At any time, the server MAY change any state entries with both the
 "rejected" and "authenticated" states to the "inactive" state and MAY
 discard any "inactive" states from the table.  Entries with the "key
 exchanging" state SHOULD be kept unless there is an emergency
 situation such as a server reboot or a table capacity overflow.
 Note 1: In relation to, and following the specification of, the
 optional authentication defined in [RFC8053], the 401-INIT messages
 marked with plus signs cannot be replaced with a successful response
 with an Optional-WWW-Authenticate header.  Every other 401-INIT can
 be a response with an Optional-WWW-Authenticate header.
 Note 2: The server SHOULD NOT send a 401-INIT response in this case,
 because it will leak the information to the client that the specified
 username will not be accepted.  Instead, postpone it until the
 response to the next req-VFY-C request.
 Note 3: If the request is not rejected in this clause, the server
 will be required, in the next step, to determine whether the same nc
 value was previously received from the client.  If that is
 impossible, the server MUST send a 401-STALE response in this step.
 If the server does not remember the whole history of the nc values
 received from the client, the server MUST send a 401-STALE message in
 this clause.

Oiwa, et al. Experimental [Page 38] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

12. Authentication Algorithms

 Cryptographic authentication algorithms that are used with this
 protocol will be defined separately.  The algorithm definition MUST
 at least provide definitions for the following functions:
 o  The server-side authentication credential J, derived from the
    client-side authentication credential pi.
 o  Key exchange values K_c1, K_s1 (exchanged on the wire) and
    S_c1, S_s1 (kept secret in each peer).
 o  Shared session secret (z), to be computed by both server and
    client.
 o  A hash function H to be used with the protocol, along with its
    output size hSize.
 o  The value nIterPi, the number of iterations for the key derivation
    operation.
 Specifications for cryptographic algorithms used with this framework
 MUST specify whether those algorithms will (1) use the default
 functions defined below for values pi, VK_c, and VK_s or (2) define
 their own comparable functions.
 All algorithms used with this protocol SHOULD provide secure mutual
 authentication between clients and servers and generate a
 cryptographically strong shared secret value (z) that is equally
 strong or stronger than the hash function H.  If any passwords (or
 passphrases or any equivalents, i.e., weak secrets) are involved,
 these SHOULD NOT be guessable from any data transmitted in the
 protocol, even if an attacker (either an eavesdropper or an active
 server) knows the possible thoroughly searchable candidate list of
 passwords.  Furthermore, it is RECOMMENDED that the function J for
 deriving the server-side authentication credential J(pi) be one-way,
 if possible, so that pi cannot be easily computed from J(pi).

12.1. Support Functions and Notations

 In this section, we define several support functions and notations to
 be shared by several algorithm definitions.
 The integers in the specification are in decimal, or in hexadecimal
 when prefixed with "0x".

Oiwa, et al. Experimental [Page 39] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 The function octet(i) generates an octet string containing a single
 octet of value i.  The operator "|", when applied to octet strings,
 denotes the concatenation of two operands.
 The function VI encodes natural numbers into octet strings in the
 following manner: numbers are represented as big-endian radix-128
 strings, where each digit is represented by an octet within the range
 0x80-0xff, except for the last digit, which is represented by an
 octet within the range 0x00-0x7f.  The first octet MUST NOT be 0x80.
 For example, VI(i) = octet(i) for i < 128, and
 VI(i) = octet(0x80 + (i >> 7)) | octet(i & 127) for 128 <= i < 16384.
 This encoding is the same as the encoding used for the subcomponents
 of object identifiers in ASN.1 encoding [ITU.X690.2015] and is
 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 in the following
 manner:
 VS(s) = VI(length(s)) | s
 where length(s) is a number of octets (not characters) in s.
 Some examples:
    VI(0) = "\000" (in C string notation)
    VI(100) = "d"
    VI(10000) = "\316\020"
    VI(1000000) = "\275\204@"
    VS("") = "\000"
    VS("Tea") = "\003Tea"
    VS("Caf<e acute>" [in UTF-8]) = "\005Caf\303\251"
    VS([10000 "a"s]) = "\316\020aaaaa..." (10002 octets)
 (Note: Unlike the colon-separated format used in the Basic and Digest
 HTTP authentication schemes, 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.)

Oiwa, et al. Experimental [Page 40] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 The function OCTETS converts an integer into the corresponding
 radix-256 big-endian octet string having its natural length.  See
 Section 3.2.3 for the definition of "natural length".
 The function INT converts an octet string into a natural number,
 where the input string is treated as being in radix-256 big-endian
 notation.  The identity INT(OCTETS(n)) = n always holds for any
 natural number n.

12.2. Default Functions for Algorithms

 The functions defined in this section are common default functions
 among authentication algorithms.
 The client-side password-based (credential) pi used by this
 authentication is a natural number derived in the following manner:
    pi = INT(PBKDF2(HMAC_H, password, VS(algorithm) | VS(auth-scope) |
    VS(realm) | VS(username), nIterPi, hSize / 8))
 where
 o  PBKDF2 is the password-based key derivation function defined in
    [RFC8018],
 o  HMAC_H is the Hashed Message Authentication Code (HMAC) function,
    defined in [RFC2104], composed from the hash function H, and
 o  hSize is the output size of hash H in bits.
 The values of algorithm, realm, and auth-scope are taken from the
 values contained in the 401-INIT message.  If the password comes from
 user input, it SHOULD first be prepared according to the method
 presented in Section 9.  Then, the password SHALL be encoded as a
 UTF-8 string.
 The values VK_c and VK_s are derived via the following equations:
    VK_c = INT(H(octet(4) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) |
    VI(nc) | VS(vh)))
    VK_s = INT(H(octet(3) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) |
    VI(nc) | VS(vh)))

Oiwa, et al. Experimental [Page 41] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

13. Application Channel Binding

 Applications and upper-layer communication protocols may need
 authentication binding to the HTTP-layer authenticated user.  Such
 applications MAY use the following values as a standard shared
 secret.
 These values are parameterized with an optional octet string (t),
 which may be arbitrarily chosen by each application or protocol.  If
 there is no appropriate value to be specified, use an empty string
 for t.
 For applications requiring binding to either an authenticated user or
 a shared-key session (to ensure that the requesting client is
 authenticated), the following value b_1 MAY be used:
    b_1 = H(H(octet(6) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) |
    VI(0) | VS(vh)) | VS(t))
 For applications requiring binding to a specific request (to ensure
 that the payload data is generated for the exact HTTP request), the
 following value b_2 MAY be used:
    b_2 = H(H(octet(7) | OCTETS(K_c1) | OCTETS(K_s1) | OCTETS(z) |
    VI(nc) | VS(vh)) | VS(t))
 Note: Channel bindings to lower-layer transports (TCP and TLS) are
 defined in Section 7.

14. Application for Proxy Authentication

 The authentication scheme defined in the previous sections can be
 applied (with modifications) to proxy authentication.  In such cases,
 the following alterations MUST be applied:
 o  The 407 (Proxy Authentication Required) status code is to be sent
    and recognized in places where the 401 status code is used,
 o  The Proxy-Authenticate header is to be used in places where the
    WWW-Authenticate header is used,
 o  The Proxy-Authorization header is to be used in places where the
    Authorization header is used,
 o  The Proxy-Authentication-Info header is to be used in places where
    the Authentication-Info header is used,

Oiwa, et al. Experimental [Page 42] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 o  The "auth-scope" parameter is fixed to the hostname of the proxy,
    which means that it covers all requests processed by the specific
    proxy,
 o  The limitation for the paths contained in the "path" parameter of
    401-KEX-S1 messages is disregarded,
 o  The omission of the "path" parameter of 401-KEX-S1 messages means
    that the authentication realm will potentially cover all requests
    processed by the proxy,
 o  The scheme, hostname, and port of the proxy are used for host
    validation tokens, and
 o  Authentication extensions defined in [RFC8053] are not applicable.

15. Methods to Extend This Protocol

 If a private extension to this protocol is implemented, it MUST use
 the extension-tokens defined in Section 3 to avoid conflicts with
 this protocol and other extensions.  (Standardized extensions, as
 well as extensions that are in the process of being standardized, MAY
 use either bare-tokens or extension-tokens.)
 Specifications defining authentication algorithms MAY use other
 representations for the parameters "kc1", "ks1", "vkc", and "vks";
 replace those parameter names; and/or add parameters to the messages
 containing those parameters in supplemental specifications, provided
 that syntactic and semantic requirements in Section 3 of this
 document, [RFC7230], and [RFC7235] are satisfied.  Any parameters
 starting with "kc", "ks", "vkc", or "vks" and followed by decimal
 natural numbers (e.g., kc2, ks0, vkc1, vks3) are reserved for this
 purpose.  If those specifications use names other than those
 mentioned above, it is RECOMMENDED that extension-tokens be used to
 avoid any parameter-name conflicts with future extensions to this
 protocol.
 Extension-tokens MAY be freely used for any non-standard, private,
 and/or experimental uses for those parameters provided that the
 domain part in the token is used in the manner defined in Section 3.

Oiwa, et al. Experimental [Page 43] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

16. IANA Considerations

16.1. Addition to HTTP Authentication Schemes Registry

 IANA has added the following entry to the "HTTP Authentication
 Schemes" registry:
 o  Authentication Scheme Name: Mutual
 o  Reference: RFC 8120

16.2. Registry for Authentication Algorithms

 This document establishes the "HTTP Mutual Authentication Algorithms"
 registry.  The registry manages case-insensitive ASCII strings.  The
 strings MUST follow the extensive-token syntax defined in Section 3.
 When bare-tokens are used for the authentication-algorithm parameter,
 they MUST be allocated by IANA.  To acquire registered tokens, the
 usage of such tokens MUST be reviewed by a Designated Expert, as
 outlined in [RFC5226].
 Registrations for an authentication algorithm are required to include
 descriptions of the authentication algorithms.  Reviewers assigned by
 the IESG are advised to examine minimum security requirements and
 consistency of the key exchange algorithm descriptions.
 It is advised that new registrations provide the following
 information:
 o  Token: A token used in HTTP headers for identifying the algorithm.
 o  Description: A brief description of the algorithm.
 o  Specification: A reference for a specification defining the
    algorithm.
 [RFC8121] defines the initial contents of this registry.

Oiwa, et al. Experimental [Page 44] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

16.3. Registry for Validation Methods

 This document establishes the "HTTP Mutual Authentication Host
 Validation Methods" registry.  The registry manages case-insensitive
 ASCII strings.  The strings MUST follow the extensive-token syntax
 defined in Section 3.
 When bare-tokens are used for the validation parameter, they MUST be
 allocated by IANA.  To acquire registered tokens, the usage of such
 tokens MUST be reviewed by a Designated Expert, as outlined in
 [RFC5226].
 Registrations for a validation method are required to include a
 description of the validation method.  Reviewers assigned by the IESG
 are advised to examine its use-case requirements and any security
 consequences related to its introduction.
 It is advised that new registrations provide the following
 information:
 o  Token: A token used in HTTP headers for identifying the method.
 o  Description: A brief description of the method.
 o  Specification: A reference for a specification defining the
    method.
 The initial contents of this registry are as follows:
 +----------------------+------------------------+----------------+
 | Token                | Description            | Reference      |
 +----------------------+------------------------+----------------+
 | host                 | Hostname verification  | RFC 8120,      |
 |                      | only                   | Section 7      |
 |                      |                        |                |
 | tls-server-end-point | TLS certificate-based  | RFC 8120,      |
 |                      |                        | Section 7      |
 |                      |                        |                |
 | tls-unique           | TLS unique key-based   | RFC 8120,      |
 |                      |                        | Section 7      |
 +----------------------+------------------------+----------------+

Oiwa, et al. Experimental [Page 45] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

17. Security Considerations

17.1. Security Properties

 o  The protocol is secure against passive eavesdropping and replay
    attacks.  However, the protocol relies on transport security
    (including DNS integrity) for data secrecy and integrity.
    HTTP/TLS SHOULD be used where transport security is not assured
    and/or data confidentiality is important.
 o  When used with HTTP/TLS, if TLS server certificates are reliably
    verified, the protocol provides true protection against active
    man-in-the-middle attacks.
 o  Even if the server certificate is not used or is unreliable, the
    protocol provides protection against active man-in-the-middle
    attacks for each HTTP request/response pair.  However, in such
    cases, JavaScript or similar scripts that are not authenticated by
    this authentication mechanism can affect mutually authenticated
    contents to circumvent the protection.  This is why this protocol
    stipulates that valid TLS server certificates MUST be shown from
    the server to the client (Section 7).

17.2. Secrecy of Credentials

 The client-side password credential MUST always be kept secret and
 SHOULD NOT be used for any other (possibly insecure) authentication
 purposes.  Loss of control of the credential will directly affect the
 control of the corresponding server-side account.
 The use of a client-side credential with THIS authentication scheme
 is always safe, even if the connected server peer is not trustworthy
 (e.g., a phishing scenario).  However, if it is used with other
 authentication schemes (such as Web forms) and the recipient is
 rogue, the result will be obvious.
 It is also important that the server-side password credential (J) be
 kept secret.  If it is stolen and the client's choice of password is
 not strong, anyone who is aware of the server-side password
 credential can employ an offline dictionary attack to search for the
 client's password.  However, if the client has chosen a strong
 password so that an attacker cannot guess the client's password from
 dictionary candidates, the client is still well protected from any
 attacks.

Oiwa, et al. Experimental [Page 46] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 The shared session secret (z) MUST be kept secret inside the
 server/client software; if it is lost and the session is still
 active, session hijacking will result.  After the session expires,
 the key is of no value to attackers.

17.3. Denial-of-Service Attacks on Servers

 The protocol requires a server-side table of active sessions, which
 may become a critical point for server resource consumption.  For
 proper operation, the protocol requires that at least one key
 verification request be processed for each session identifier.  After
 that, servers MAY discard sessions internally at any time without
 causing any operational problems for clients.  Clients will then
 silently re-establish a new session.
 However, if a malicious client sends too many requests for key
 exchanges (req-KEX-C1 messages) only, resource starvation might
 occur.  In such critical situations, servers MAY discard any kind of
 existing sessions, regardless of their statuses.  One way to mitigate
 such attacks is that servers MAY set number and time limits for
 unverified, pending key exchange requests (in the "key exchanging"
 state).
 This is a common weakness of authentication protocols with almost any
 kind of negotiations or states, including the Digest authentication
 scheme and most cookie-based authentication implementations.
 However, regarding resource consumption, the situation for the
 Mutual authentication scheme is slightly better than that for Digest,
 because HTTP requests without any kind of authentication requests
 will not generate any kind of sessions.  Session identifiers are only
 generated after a client starts a key negotiation, so that simple
 clients such as Web crawlers will not accidentally consume
 server-side resources for session management.

17.3.1. Online Active Password Attacks

 Although the protocol provides very strong protection against offline
 dictionary attacks from eavesdropped traffic, the protocol, by its
 nature, cannot prevent active password attacks in which an attacker
 sends so many authentication trial requests for every possible
 password.
 Possible countermeasures for preventing such attacks may be the
 rate-limiting of password authentication trials, statistics-based
 intrusion-detection measures, or similar protection schemes.  If the
 server operators assume that the passwords of users are not strong
 enough, it may be desirable to introduce such ad hoc countermeasures.

Oiwa, et al. Experimental [Page 47] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

17.4. Communicating the Status of Mutual Authentication with Users

 This protocol is designed with two goals in mind.  The first goal is
 simply to provide a secure alternative to existing Basic and Digest
 authentication schemes.  The second goal is to provide users with a
 way to detect forged rogue servers imitating (e.g., via a phishing
 attack) a user's registered account on a server.
 For this protocol to effectively work as a countermeasure against
 such attacks, it is very important that end users of clients be
 notified of the result of mutual authentication performed by this
 protocol, especially the three states "AUTH-SUCCEED",
 "AUTH-REQUIRED", and "UNAUTHENTICATED" as defined in Section 10.  The
 design of secure user interfaces for HTTP interactive clients is out
 of scope for this document, but if possible, having some kind of UI
 indication for the three states above will be desirable from the
 standpoint of providing user security.
 Of course, in such cases, the user interfaces for requesting
 passwords for this authentication shall be protected against
 imitation (for example, by other insecure password input fields, such
 as forms).  If the passwords are known to malicious attackers outside
 of the protocol, the protocol cannot work as an effective security
 measure.

17.5. Implementation Considerations

 o  To securely implement the protocol, the Authentication-Info
    headers in the 200-VFY-S messages MUST always be validated by the
    client.  If the validation fails, the client MUST NOT process any
    content sent with the message, including other headers and the
    body part.  Non-compliance with this requirement will allow
    phishing attacks.
 o  For HTTP/TLS communications, when a Web form is submitted from
    mutually authenticated pages via the "tls-server-end-point"
    validation method to a URI that is protected by the same realm
    (so indicated by the "path" parameter), if the server certificate
    has been changed since the pages were received, it is RECOMMENDED
    that the peer be revalidated using a req-KEX-C1 message with an
    "Expect: 100-continue" header.  The same applies when the page is
    received via the "tls-unique" validation method and when the TLS
    session has expired.
 o  For better protection against possible password database stealing,
    server-side storage of user passwords should contain the values
    encrypted by the one-way function J(pi) instead of the real
    passwords or those hashed by pi.

Oiwa, et al. Experimental [Page 48] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 o  If TLS 1.2 [RFC5246] is used for underlying HTTP/TLS
    communications, follow the best practices specified in [RFC7525].

17.6. Usage Considerations

 o  The usernames inputted by a user may be sent automatically to any
    servers sharing the same auth-scope.  This means that when a
    host-type auth-scope is used for authentication on an HTTPS site
    and an HTTP server on the same host requests the Mutual
    authentication scheme within the same realm, the client will send
    the username in clear text.  If usernames have to be kept secret
    (protected from eavesdroppers), the server must use the
    full-scheme-type "auth-scope" parameter and HTTPS.  Passwords, on
    the other hand, are not exposed to eavesdroppers, even in HTTP
    requests.
 o  If the server provides several ways to store server-side password
    secrets in the password database, it is desirable, for purposes of
    better security, to store the values encrypted by using the
    one-way function J(pi) instead of the real passwords or those
    hashed by pi.

18. References

18.1. Normative References

 [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
            Keyed-Hashing for Message Authentication", RFC 2104,
            DOI 10.17487/RFC2104, February 1997,
            <http://www.rfc-editor.org/info/rfc2104>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of
            ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
            November 2003, <http://www.rfc-editor.org/info/rfc3629>.
 [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
            <http://www.rfc-editor.org/info/rfc4648>.
 [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
            Syntax Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.

Oiwa, et al. Experimental [Page 49] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [RFC5987]  Reschke, J., "Character Set and Language Encoding for
            Hypertext Transfer Protocol (HTTP) Header Field
            Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
            <http://www.rfc-editor.org/info/rfc5987>.
 [RFC7230]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
            Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, DOI 10.17487/RFC7230, June 2014,
            <http://www.rfc-editor.org/info/rfc7230>.
 [RFC7235]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
            Transfer Protocol (HTTP/1.1): Authentication", RFC 7235,
            DOI 10.17487/RFC7235, June 2014,
            <http://www.rfc-editor.org/info/rfc7235>.
 [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
            Enforcement, and Comparison of Internationalized Strings
            Representing Usernames and Passwords", RFC 7613,
            DOI 10.17487/RFC7613, August 2015,
            <http://www.rfc-editor.org/info/rfc7613>.
 [RFC7615]  Reschke, J., "HTTP Authentication-Info and
            Proxy-Authentication-Info Response Header Fields",
            RFC 7615, DOI 10.17487/RFC7615, September 2015,
            <http://www.rfc-editor.org/info/rfc7615>.
 [RFC8018]  Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
            Password-Based Cryptography Specification Version 2.1",
            RFC 8018, DOI 10.17487/RFC8018, January 2017,
            <http://www.rfc-editor.org/info/rfc8018>.
 [RFC8053]  Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
            T., and Y. Ioku, "HTTP Authentication Extensions for
            Interactive Clients", RFC 8053, DOI 10.17487/RFC8053,
            January 2017, <http://www.rfc-editor.org/info/rfc8053>.
 [Unicode]  The Unicode Consortium, "The Unicode Standard",
            <http://www.unicode.org/versions/latest/>.

Oiwa, et al. Experimental [Page 50] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

18.2. Informative References

 [ITU.X690.2015]
            International Telecommunication 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, ISO/IEC 8825-1, August 2015,
            <https://www.itu.int/rec/T-REC-X.690/>.
 [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
            STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
            <http://www.rfc-editor.org/info/rfc1939>.
 [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
            DOI 10.17487/RFC2818, May 2000,
            <http://www.rfc-editor.org/info/rfc2818>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, DOI 10.17487/RFC5890, August 2010,
            <http://www.rfc-editor.org/info/rfc5890>.
 [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
            for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
            <http://www.rfc-editor.org/info/rfc5929>.
 [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
            DOI 10.17487/RFC6265, April 2011,
            <http://www.rfc-editor.org/info/rfc6265>.
 [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
            DOI 10.17487/RFC6454, December 2011,
            <http://www.rfc-editor.org/info/rfc6454>.
 [RFC7231]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
            Transfer Protocol (HTTP/1.1): Semantics and Content",
            RFC 7231, DOI 10.17487/RFC7231, June 2014,
            <http://www.rfc-editor.org/info/rfc7231>.

Oiwa, et al. Experimental [Page 51] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

 [RFC7486]  Farrell, S., Hoffman, P., and M. Thomas, "HTTP
            Origin-Bound Authentication (HOBA)", RFC 7486,
            DOI 10.17487/RFC7486, March 2015,
            <http://www.rfc-editor.org/info/rfc7486>.
 [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
            "Recommendations for Secure Use of Transport Layer
            Security (TLS) and Datagram Transport Layer Security
            (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
            May 2015, <http://www.rfc-editor.org/info/rfc7525>.
 [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
            Digest Access Authentication", RFC 7616,
            DOI 10.17487/RFC7616, September 2015,
            <http://www.rfc-editor.org/info/rfc7616>.
 [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
            Langley, A., and M. Ray, "Transport Layer Security (TLS)
            Session Hash and Extended Master Secret Extension",
            RFC 7627, DOI 10.17487/RFC7627, September 2015,
            <http://www.rfc-editor.org/info/rfc7627>.
 [RFC8121]  Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
            T., and Y. Ioku, "Mutual Authentication Protocol for HTTP:
            Cryptographic Algorithms Based on the Key Agreement
            Mechanism 3 (KAM3)", RFC 8121, DOI 10.17487/RFC8121,
            April 2017, <http://www.rfc-editor.org/info/rfc8121>.

Oiwa, et al. Experimental [Page 52] RFC 8120 Mutual Authentication Protocol for HTTP April 2017

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
 Japan
 Email: y.oiwa@aist.go.jp
 Hajime Watanabe
 National Institute of Advanced Industrial Science and Technology
 Information Technology Research Institute
 Tsukuba Central 1
 1-1-1 Umezono
 Tsukuba-shi, Ibaraki
 Japan
 Email: h-watanabe@aist.go.jp
 Hiromitsu Takagi
 National Institute of Advanced Industrial Science and Technology
 Information Technology Research Institute
 Tsukuba Central 1
 1-1-1 Umezono
 Tsukuba-shi, Ibaraki
 Japan
 Email: takagi.hiromitsu@aist.go.jp
 Kaoru Maeda
 Individual Contributor
 Email: kaorumaeda.ml@gmail.com
 Tatsuya Hayashi
 Lepidum Co. Ltd.
 Village Sasazuka 3, Suite #602
 1-30-3 Sasazuka
 Shibuya-ku, Tokyo
 Japan
 Email: hayashi@lepidum.co.jp
 Yuichi Ioku
 Individual Contributor
 Email: mutual-work@ioku.org

Oiwa, et al. Experimental [Page 53]

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