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Versions: (draft-sakimura-oauth-tcse) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 7636

OAuth Working Group                                     N. Sakimura, Ed.
Internet-Draft                                 Nomura Research Institute
Intended status: Standards Track                              J. Bradley
Expires: January 9, 2016                                   Ping Identity
                                                              N. Agarwal
                                                                  Google
                                                            July 8, 2015


          Proof Key for Code Exchange by OAuth Public Clients
                        draft-ietf-oauth-spop-15

Abstract

   OAuth 2.0 public clients utilizing the Authorization Code Grant are
   susceptible to the authorization code interception attack.  This
   specification describes the attack as well as a technique to mitigate
   against the threat through the use of Proof Key for Code Exchange
   (PKCE, pronounced "pixy").

Status of This Memo

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

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

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

   This Internet-Draft will expire on January 9, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Client creates a code verifier  . . . . . . . . . . . . .   7
     4.2.  Client creates the code challenge . . . . . . . . . . . .   8
     4.3.  Client sends the code challenge with the authorization
           request . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Server returns the code . . . . . . . . . . . . . . . . .   9
       4.4.1.  Error Response  . . . . . . . . . . . . . . . . . . .   9
     4.5.  Client sends the Authorization Code and the Code Verifier
           to the token endpoint . . . . . . . . . . . . . . . . . .   9
     4.6.  Server verifies code_verifier before returning the tokens  10
   5.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  OAuth Parameters Registry . . . . . . . . . . . . . . . .  10
     6.2.  PKCE Code Challenge Method Registry . . . . . . . . . . .  11
       6.2.1.  Registration Template . . . . . . . . . . . . . . . .  11
       6.2.2.  Initial Registry Contents . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     7.1.  Entropy of the code_verifier  . . . . . . . . . . . . . .  12
     7.2.  Protection against eavesdroppers  . . . . . . . . . . . .  13
     7.3.  Salting the code_challenge  . . . . . . . . . . . . . . .  13
     7.4.  OAuth security considerations . . . . . . . . . . . . . .  14
     7.5.  TLS security considerations . . . . . . . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Revision History  . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Notes on implementing base64url encoding without
                padding  . . . . . . . . . . . . . . . . . . . . . .  18
   Appendix B.  Example for the S256 code_challenge_method . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   OAuth 2.0 [RFC6749] public clients are susceptible to the
   authorization code interception attack.




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   The attacker thereby intercepts the authorization code returned from
   the authorization endpoint within communication path not protected by
   TLS, such as inter-app communication within the operating system of
   the client.

   Once the attacker has gained access to the authorization code it can
   use it to obtain the access token.

   Figure 1 shows the attack graphically.  In step (1) the native app
   running on the end device, such as a smart phone, issues an OAuth 2.0
   Authorization Request via the browser/operating system.  The
   Redirection Endpoint URI in this case typically uses a custom URI
   scheme.  Step (1) happens through a secure API that cannot be
   intercepted, though it may potentially be observed in advanced attack
   scenarios.  The request then gets forwarded to the OAuth 2.0
   authorization server in step (2).  Because OAuth requires the use of
   TLS, this communication is protected by TLS, and also cannot be
   intercepted.  The authorization server returns the authorization code
   in step (3).  In step (4), the Authorization Code is returned to the
   requester via the Redirection Endpoint URI that was provided in step
   (1).

   A malicious app that has been designed to attack this native app has
   previously registered itself as a handler for the custom URI scheme
   is now able to intercept the Authorization Code in step (4).  This
   allows the attacker to request and obtain an access token in steps
   (5) and (6), respectively.
























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    +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
    | End Device (e.g., Smart Phone) |
    |                                |
    | +-------------+   +----------+ | (6) Access Token  +----------+
    | |Legitimate   |   | Malicious|<--------------------|          |
    | |OAuth 2.0 App|   | App      |-------------------->|          |
    | +-------------+   +----------+ | (5) Authorization |          |
    |        |    ^          ^       |        Grant      |          |
    |        |     \         |       |                   |          |
    |        |      \   (4)  |       |                   |          |
    |    (1) |       \  Authz|       |                   |          |
    |   Authz|        \ Code |       |                   |  Authz   |
    | Request|         \     |       |                   |  Server  |
    |        |          \    |       |                   |          |
    |        |           \   |       |                   |          |
    |        v            \  |       |                   |          |
    | +----------------------------+ |                   |          |
    | |                            | | (3) Authz Code    |          |
    | |     Operating System/      |<--------------------|          |
    | |         Browser            |-------------------->|          |
    | |                            | | (2) Authz Request |          |
    | +----------------------------+ |                   +----------+
    +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+

             Figure 1: Authorization Code Interception Attack.

   A number of pre-conditions need to hold in order for this attack to
   work:

   1) The attacker manages to register a malicious application on the
      client device and registers a custom URI scheme that is also used
      by another application.
      The operating systems must allow a custom URI schemes to be
      registered by multiple applications.
   2) The OAuth 2.0 authorization code grant is used.
   3) The attacker has access to the OAuth 2.0 [RFC6749] client_id and
      client_secret(if provisioned).  All OAuth 2.0 native app client-
      instances use the same client_id.  Secrets provisioned in client
      binary applications cannot be considered confidential.
   4a)  The attacker (via the installed app) is able to observe only the
      responses from the authorization endpoint.  The plain
      code_challenge_method mitigates only this attack.
   4b)  A more sophisticated attack scenario allows the attacker to
      observe requests (in addition to responses) to the authorization
      endpoint.  The attacker is, however, not able to act as a man-in-
      the-middle.  This has been caused by leaking http log information
      in the OS.  To mitigate this the S256 code_challenge_method or




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      cryptographically secure code_challenge_method extension must be
      used.

   While this is a long list of pre-conditions the described attack has
   been observed in the wild and has to be considered in OAuth 2.0
   deployments.
   While the OAuth 2.0 Threat Model Section 4.4.1 [RFC6819] describes
   mitigation techniques they are, unfortunately, not applicable since
   they rely on a per-client instance secret or aper client instance
   redirect URI.

   To mitigate this attack, this extension utilizes a dynamically
   created cryptographically random key called "code verifier".  A
   unique code verifier is created for every authorization request and
   its transformed value, called "code challenge", is sent to the
   authorization server to obtain the authorization code.  The
   authorization code obtained is then sent to the token endpoint with
   the "code verifier" and the server compares it with the previously
   received request code so that it can perform the proof of possession
   of the "code verifier" by the client.  This works as the mitigation
   since the attacker would not know this one-time key, since it is sent
   over TLS and cannot be intercepted.

1.1.  Protocol Flow

                                                 +-------------------+
                                                 |   Authz Server    |
       +--------+                                | +---------------+ |
       |        |--(A)- Authorization Request ---->|               | |
       |        |       + t(code_verifier), t_m  | | Authorization | |
       |        |                                | |    Endpoint   | |
       |        |<-(B)---- Authorization Code -----|               | |
       |        |                                | +---------------+ |
       | Client |                                |                   |
       |        |                                | +---------------+ |
       |        |--(C)-- Access Token Request ---->|               | |
       |        |          + code_verifier       | |    Token      | |
       |        |                                | |   Endpoint    | |
       |        |<-(D)------ Access Token ---------|               | |
       +--------+                                | +---------------+ |
                                                 +-------------------+

                     Figure 2: Abstract Protocol Flow

   This specification adds additional parameters to the OAuth 2.0
   Authorization and Access Token Requests, shown in abstract form in
   Figure 1.




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   A. The client creates and records a secret named the "code_verifier",
      and derives a transformed version "t(code_verifier)" (referred to
      as the "code_challenge") which is sent in the OAuth 2.0
      Authorization Request, along with the transformation method "t_m".
   B. The Authorization Endpoint responds as usual, but records
      "t(code_verifier)" and the transformation method.
   C. The client then sends the authorization code in the Access Token
      Request as usual, but includes the "code_verifier" secret
      generated at (A).
   D. The authorization server transforms "code_verifier" and compares
      it to "t(code_verifier)" from (B).  Access is denied if they are
      not equal.

   An attacker who intercepts the Authorization Grant at (B) is unable
   to redeem it for an Access Token, as they are not in possession of
   the "code_verifier" secret.

2.  Notational Conventions

   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 Key
   words for use in RFCs to Indicate Requirement Levels [RFC2119].  If
   these words are used without being spelled in uppercase then they are
   to be interpreted with their normal natural language meanings.

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

   STRING denotes a sequence of zero or more ASCII [RFC0020] characters.

   OCTETS denotes a sequence of zero or more octets.

   ASCII(STRING) denotes the octets of the ASCII [RFC0020]
   representation of STRING where STRING is a sequence of zero or more
   ASCII characters.

   BASE64URL-ENCODE(OCTETS) denotes the base64url encoding of OCTETS,
   per Section 3 producing a STRING.

   BASE64URL-DECODE(STRING) denotes the base64url decoding of STRING,
   per Section 3, producing a sequence of octets.

   SHA256(OCTETS) denotes a SHA2 256bit hash [RFC6234] of OCTETS.







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3.  Terminology

   In addition to the terms defined in OAuth 2.0 [RFC6749], this
   specification defines the following terms:

   code verifier
      A cryptographically random string that is used to correlate the
      authorization request to the token request.
   code challenge
      A challenge derived from the code verifier that is sent in the
      authorization request, to be verified against later.
   Base64url Encoding
      Base64 encoding using the URL- and filename-safe character set
      defined in Section 5 of [RFC4648], with all trailing '='
      characters omitted (as permitted by Section 3.2 of [RFC4648]) and
      without the inclusion of any line breaks, whitespace, or other
      additional characters.  (See Appendix A for notes on implementing
      base64url encoding without padding.)

3.1.  Abbreviations

   ABNF  Augmented Backus-Naur Form
   Authz  Authorization
   PKCE  Proof Key for Code Exchange
   MITM  Man-in-the-middle
   MTI  Mandatory To Implement

4.  Protocol

4.1.  Client creates a code verifier

   The client first creates a code verifier, "code_verifier", for each
   OAuth 2.0 [RFC6749] Authorization Request, in the following manner:

   code_verifier = high entropy cryptographic random STRING using the
   Unreserved Characters [A-Z] / [a-z] / [0-9] / "-" / "." / "_" / "~"
   from Sec 2.3 of [RFC3986], with a minimum length of 43 characters and
   a maximum length of 128 characters.

   ABNF for "code_verifier" is as follows.

   code-verifier = 43*128unreserved
   unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
   ALPHA = %x41-5A / %x61-7A
   DIGIT = %x30-39

   NOTE: code verifier SHOULD have enough entropy to make it impractical
   to guess the value.  It is RECOMMENDED that the output of a suitable



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   random number generator be used to create a 32-octet sequence.  The
   Octet sequence is then base64url encoded to produce a 43-octet URL
   safe string to use as the code verifier.

4.2.  Client creates the code challenge

   The client then creates a code challenge derived from the code
   verifier by using one of the following transformations on the code
   verifier:

   plain
      code_challenge = code_verifier
   S256
      code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier)))

   If the client is capable of using "S256", it MUST use "S256", as
   "S256" is Mandatory To Implement (MTI) on the server.  Clients are
   permitted to use "plain" only if they cannot support "S256" for some
   technical reason and know via out of band configuration that the
   server supports "plain".

   The plain transformation is for compatibility with existing
   deployments and for constrained environments that can't use the S256
   transformation.

   ABNF for "code_challenge" is as follows.

   code-challenge = 43*128unreserved
   unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
   ALPHA = %x41-5A / %x61-7A
   DIGIT = %x30-39

4.3.  Client sends the code challenge with the authorization request

   The client sends the code challenge as part of the OAuth 2.0
   Authorization Request (Section 4.1.1 of [RFC6749].) using the
   following additional parameters:

   code_challenge  REQUIRED.  Code challenge.

   code_challenge_method  OPTIONAL, defaults to "plain" if not present
      in the request.  Code verifier transformation method, "S256" or
      "plain".








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4.4.  Server returns the code

   When the server issues the authorization code in the authorization
   response, it MUST associate the "code_challenge" and
   "code_challenge_method" values with the authorization code so it can
   be verified later.

   Typically, the "code_challenge" and "code_challenge_method" values
   are stored in encrypted form in the "code" itself, but could
   alternatively be stored on the server, associated with the code.  The
   server MUST NOT include the "code_challenge" value in client requests
   in a form that other entities can extract.

   The exact method that the server uses to associate the
   "code_challenge" with the issued "code" is out of scope for this
   specification.

4.4.1.  Error Response

   If the server requires Proof Key for Code Exchange (PKCE) by OAuth
   Public Clients, and the client does not send the "code_challenge" in
   the request, the authorization endpoint MUST return the authorization
   error response with "error" value set to "invalid_request".  The
   "error_description" or the response of "error_uri" SHOULD explain the
   nature of error, e.g., code challenge required.

   If the server supporting PKCE does not support the requested
   transform, the authorization endpoint MUST return the authorization
   error response with "error" value set to "invalid_request".  The
   "error_description" or the response of "error_uri" SHOULD explain the
   nature of error, e.g., transform algorithm not supported.

4.5.  Client sends the Authorization Code and the Code Verifier to the
      token endpoint

   Upon receipt of the Authorization Code, the client sends the Access
   Token Request to the token endpoint.  In addition to the parameters
   defined in the OAuth 2.0 Access Token Request (Section 4.1.3 of
   [RFC6749]), it sends the following parameter:

   code_verifier  REQUIRED.  Code verifier

   The code_challenge_method is bound to the Authorization Code when the
   Authorization Code is issued.  That is the method that the token
   endpoint MUST use to verify the code_verifier.






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4.6.  Server verifies code_verifier before returning the tokens

   Upon receipt of the request at the Access Token endpoint, the server
   verifies it by calculating the code challenge from received
   "code_verifier" and comparing it with the previously associated
   "code_challenge", after first transforming it according to the
   "code_challenge_method" method specified by the client.

   If the "code_challenge_method" from Section 4.2 was "S256", the
   received "code_verifier" is hashed by SHA-256, then base64url
   encoded, and then compared to the "code_challenge". i.e.,

   BASE64URL-ENCODE(SHA256(ASCII("code_verifier" ))) == "code_challenge"

   If the "code_challenge_method" from Section 4.2 was "plain", they are
   compared directly. i.e.,

   "code_verifier" == "code_challenge".

   If the values are equal, the Access Token endpoint MUST continue
   processing as normal (as defined by OAuth 2.0 [RFC6749]).  If the
   values are not equal, an error response indicating "invalid_grant" as
   described in section 5.2 of [RFC6749] MUST be returned.

5.  Compatibility

   Server implementations of this specification MAY accept OAuth2.0
   Clients that do not implement this extension.  If the "code_verifier"
   is not received from the client in the Authorization Request, servers
   supporting backwards compatibility revert to a normal OAuth 2.0
   [RFC6749] protocol.

   As the OAuth 2.0 [RFC6749] server responses are unchanged by this
   specification, client implementations of this specification do not
   need to know if the server has implemented this specification or not,
   and SHOULD send the additional parameters as defined in Section 3. to
   all servers.

6.  IANA Considerations

   This specification makes a registration request as follows:

6.1.  OAuth Parameters Registry

   This specification registers the following parameters in the IANA
   OAuth Parameters registry defined in OAuth 2.0 [RFC6749].

   o  Parameter name: code_verifier



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   o  Parameter usage location: token request
   o  Change controller: IESG
   o  Specification document(s): this document

   o  Parameter name: code_challenge
   o  Parameter usage location: authorization request
   o  Change controller: IESG
   o  Specification document(s): this document

   o  Parameter name: code_challenge_method
   o  Parameter usage location: authorization request
   o  Change controller: IESG
   o  Specification document(s): this document

6.2.  PKCE Code Challenge Method Registry

   This specification establishes the PKCE Code Challenge Method
   registry.  The new registry should be a sub-registry of OAuth
   Parameters registry.

   Additional code_challenge_method types for use with the authorization
   endpoint are registered using the Specification Required policy
   [RFC5226], which includes review of the request by one or more
   Designated Experts.  The DEs will ensure there is at least a two-week
   review of the request on the oauth-ext-review@ietf.org mailing list,
   and that any discussion on that list converges before they respond to
   the request.  To allow for the allocation of values prior to
   publication, the Designated Expert(s) may approve registration once
   they are satisfied that an acceptable specification will be
   published.

   Registration requests and discussion on the oauth-ext-review@ietf.org
   mailing list should use an appropriate subject, such as "Request for
   PKCE code_challenge_method: example").

   The Designated Expert(s) should consider the discussion on the
   mailing list, as well as the overall security properties of the
   challenge Method when evaluating registration requests.  New methods
   should not disclose the value of the code_verifier in the request to
   the Authorization endpoint.  Denials should include an explanation
   and, if applicable, suggestions as to how to make the request
   successful.

6.2.1.  Registration Template

   Code Challenge Method Parameter Name:
      The name requested (e.g., "example").  Because a core goal of this
      specification is for the resulting representations to be compact,



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      it is RECOMMENDED that the name be short -- not to exceed 8
      characters without a compelling reason to do so.  This name is
      case-sensitive.  Names may not match other registered names in a
      case-insensitive manner unless the Designated Expert(s) state that
      there is a compelling reason to allow an exception in this
      particular case.
   Change Controller:
      For Standards Track RFCs, state "IESG".  For others, give the name
      of the responsible party.  Other details (e.g., postal address,
      email address, home page URI) may also be included.
   Specification Document(s):
      Reference to the document(s) that specify the parameter,
      preferably including URI(s) that can be used to retrieve copies of
      the document(s).  An indication of the relevant sections may also
      be included but is not required.

6.2.2.  Initial Registry Contents

   This specification registers the Code Challenge Method Parameter
   names defined in Section 4.2 in this registry.

   o  Code Challenge Method Parameter Name: "plain"
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.2 of [[ this document ]]

   o  Code Challenge Method Parameter Name: "S256"
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.2 of [[ this document ]]

7.  Security Considerations

7.1.  Entropy of the code_verifier

   The security model relies on the fact that the code verifier is not
   learned or guessed by the attacker.  It is vitally important to
   adhere to this principle.  As such, the code verifier has to be
   created in such a manner that it is cryptographically random and has
   high entropy that it is not practical for the attacker to guess.

   The client SHOULD create a code_verifier with a minimum of 256bits of
   entropy.  This can be done by having a suitable random number
   generator create a 32-octet sequence.  The Octet sequence can then be
   base64url encoded to produce a 43-octet URL safe string to use as a
   code_challenge that has the required entropy.







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7.2.  Protection against eavesdroppers

   Clients MUST NOT downgrade to "plain" after trying "S256" method.
   Servers that support PKCE are required to support "S256", and servers
   that do not support PKCE will simply ignore the unknown
   "code_verifier" OAuth 2.0 (see Section 3.2 of [RFC6749].  Because of
   that, an error when "S256" is presented can only mean that the server
   is faulty or that a MITM attacker is trying a downgrade attack.

   "S256" method protects against eavesdroppers observing or
   intercepting the "code_challenge", because the challenge cannot be
   used without the verifier.  With the "plain" method, there is a
   chance that "code_challenge" will be observed by the attacker on the
   device, or in the http request.  Since the code challenge is the same
   as the code verifier in this case, "plain" method does not protect
   against the eavesdropping of the initial request.

   The use of "S256" protects against disclosure of "code_verifier"
   value to an attacker.

   Because of this, "plain" SHOULD NOT be used, and exists only for
   compatibility with deployed implementations where the request path is
   already protected.  The "plain" method SHOULD NOT be used in new
   implementations, unless they cannot support "S256" for some technical
   reason.

   The "S256" code_challenge_method or other cryptographically secure
   code_challenge_method extension SHOULD be used.  The plain
   code_challenge_method relies on the operating system and transport
   security not to disclose the request to an attacker.

   If the code_challenge_method is plain, and the "code_challenge" is to
   be returned inside authorization "code" to achieve a stateless
   server, it MUST be encrypted in such a manner that only the server
   can decrypt and extract it.

7.3.  Salting the code_challenge

   In order to reduce implementation complexity Salting is not used in
   the production of the code_challenge, as the code_verifier contains
   sufficient entropy to prevent brute force attacks.  Concatenating a
   publicly known value to a code_verifier (containing 256 bits of
   entropy) and then hashing it with SHA256 to produce a code_challenge
   would not increase the number of attempts necessary to brute force a
   valid value for code_verifier.

   While the S256 transformation is like hashing a password there are
   important differences.  Passwords tend to be relatively low entropy



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   words that can be hashed offline and the hash looked up in a
   dictionary.  By concatenating a unique though public value to each
   password prior to hashing, the dictionary space that an attacker
   needs to search is greatly expanded.

   Modern graphics processors now allow attackers to calculate hashes in
   real time faster than they could be looked up from a disk.  This
   eliminates the value of the salt in increasing the complexity of a
   brute force attack for even low entropy passwords.

7.4.  OAuth security considerations

   All the OAuth security analysis presented in [RFC6819] applies so
   readers SHOULD carefully follow it.

7.5.  TLS security considerations

   Curent security considerations can be found in Recommendations for
   Secure Use of TLS and DTLS [BCP195].  This supersedes the TLS version
   recommendations in OAuth 2.0 [RFC6749].

8.  Acknowledgements

   The initial draft of this specification was created by the OpenID AB/
   Connect Working Group of the OpenID Foundation.

   This specification is the work of the OAuth Working Group, which
   includes dozens of active and dedicated participants.  In particular,
   the following individuals contributed ideas, feedback, and wording
   that shaped and formed the final specification:

      Anthony Nadalin, Microsoft
      Axel Nenker, Deutsche Telekom
      Breno de Medeiros, Google
      Brian Campbell, Ping Identity
      Chuck Mortimore, Salesforce
      Dirk Balfanz, Google
      Eduardo Gueiros, Jive Communications
      Hannes Tschonfenig, ARM
      James Manger, Telstra
      John Bradley, Ping Identity
      Justin Richer, MIT Kerberos
      Josh Mandel, Boston Children's Hospital
      Lewis Adam, Motorola Solutions
      Madjid Nakhjiri, Samsung
      Michael B.  Jones, Microsoft
      Nat Sakimura, Nomura Research Institute
      Naveen Agarwal, Google



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      Paul Madsen, Ping Identity
      Phil Hunt, Oracle
      Prateek Mishra, Oracle
      Ryo Ito, mixi
      Scott Tomilson, Ping Identity
      Sergey Beryozkin
      Takamichi Saito
      Torsten Lodderstedt, Deutsche Telekom
      William Denniss, Google

9.  Revision History

   -15

   o  Addressed Barry's IESG comments around IANA Registration
   o  Addressed Barry's IESG comments around Sec 7.2 downgrade attack
   o  fix a typo for William and make a small change to Fig 1.1
      clarifying t_m
   o  more wording changes to sec 7.2 re Barry
   o  made the two SHOULD NOT use plain recommendations consistent.
   o  slightly cleaned up grammer in Sec 7.2

   -14

   o  #38.  Expanded Section 7.2 to explain why plain should not be
      used.
   o  #39.  Modified Section 4.4.1 to discourage the use of plain.
   o  #40.  Modified Intro text to explain the attack better.
   o  #41.  Added explanation that the token request is protected in the
      Last paragraph of the Introduction.
   o  #42.  Sec 4.2: Removed redundant double quotes caused by spanx.
   o  #43.  Sec 4.4: Replaced code with authorization code.
   o  #44.  Sec 4.5: say "code_verifier" rather than "secret"
   o  #45.  Sec 4.4.1: Expanded PKCE.
   o  #46.  Sec 5: SHOULD in para 1 removed.
   o  Added abbreviations section.

   -13

   o  Fix the parameter usage locations for the OAuth Parameters
      Registry per Hannes response.
   o  Clarify for IANA that the new registry is a sub-registry of OAuth
      Parameters registry
   o  aded text on why the code_challenge_method is not sent to the
      token endpoint.

   -12




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   o  clarify that the client secret we are talking about in the
      Introduction is a OAuth 2 client_secret.
   o  Update salting security consideration based on Ben's feedback

   -11

   o  add spanx for plain in sec 4.4 RE Kathleen's comment
   o  Add security consideration on TLS and reference BCP195
   o  Update to make clearer that plain can only be used for backwards
      compatibility and constrained environments

   -10

   o  re #33 specify lower limit to code_verifier in prose
   o  remove base64url decode from draft, all steps now use encode only
   o  Expanded MTI
   o  re #33 change length of 32 octet base64url encoded string back to
      43 octets

   -09

   o  clean up some external references so they don't point at internal
      sections

   -08

   o  changed BASE64URL to BASE64URL-ENCODE to be more consistent with
      appendix A Fixed lowercase base64url in appendix B
   o  Added appendix B as an example of S256 processing
   o  Change reference for unreserved characters to RFC3986 from
      base64URL

   -07

   o  removed unused discovery reference and UTF8
   o  re #32 added ASCII(STRING) to make clear that it is the byte array
      that is being hashed
   o  re #2 Remove discovery requirement section.
   o  updated Acknowledgement
   o  re #32 remove unneeded UTF8(STRING) definition, and define STRING
      for ASCII(STRING)
   o  re #32 remove unneeded utf8 reference from BASE64URL-
      DECODE(STRING) def
   o  resolves #31 unused definition of concatenation
   o  re #30 Update figure text call out the endpoints
   o  re #30 Update figure to call out the endpoints
   o  small wording change to the introduction




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   -06

   o  fix date
   o  replace spop with pkce for registry and other references
   o  re #29 change name again
   o  re #27 removed US-ASCII reference
   o  re #27 updated ABNF for code_verifier
   o  resolves #24 added security consideration for salting
   o  resolves #29 Changed title
   o  updated reference to RFC4634 to RFC6234 re #27
   o  changed reference for US-ASCII to RFC20 re #27
   o  resolves #28 added Acknowledgements
   o  resolves #27 updated ABNF
   o  resolves #26 updated abstract and added Hannes figure

   -05

   o  Added IANA registry for code_challenge_method + fixed some broken
      internal references.

   -04

   o  Added error response to authorization response.

   -03

   o  Added an abstract protocol diagram and explanation

   -02

   o  Copy edits

   -01

   o  Specified exactly two supported transformations
   o  Moved discovery steps to security considerations.
   o  Incorporated readability comments by Eduardo Gueiros.
   o  Changed MUST in 3.1 to SHOULD.

   -00

   o  Initial IETF version.

10.  References







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10.1.  Normative References

   [BCP195]   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, May 2015.

   [RFC0020]  Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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

   [RFC6234]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

   [RFC6749]  Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
              6749, October 2012.

10.2.  Informative References

   [RFC6819]  Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              January 2013.

Appendix A.  Notes on implementing base64url encoding without padding

   This appendix describes how to implement a base64url encoding
   function without padding based upon standard base64 encoding function
   that uses padding.

   To be concrete, example C# code implementing these functions is shown
   below.  Similar code could be used in other languages.




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     static string base64urlencode(byte [] arg)
     {
       string s = Convert.ToBase64String(arg); // Regular base64 encoder
       s = s.Split('=')[0]; // Remove any trailing '='s
       s = s.Replace('+', '-'); // 62nd char of encoding
       s = s.Replace('/', '_'); // 63rd char of encoding
       return s;
     }


   An example correspondence between unencoded and encoded values
   follows.  The octet sequence below encodes into the string below,
   which when decoded, reproduces the octet sequence.

   3 236 255 224 193

   A-z_4ME

Appendix B.  Example for the S256 code_challenge_method

   The client uses output of a suitable random number generator to
   create a 32-octet sequence.  The octets representing the value in
   this example (using JSON array notation) are:"

      [116, 24, 223, 180, 151, 153, 224, 37, 79, 250, 96, 125, 216, 173,
      187, 186, 22, 212, 37, 77, 105, 214, 191, 240, 91, 88, 5, 88, 83,
      132, 141, 121]

   Encoding this octet sequence as a Base64url provides the value of the
   code_verifier:

       dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

   The code_verifier is then hashed via the SHA256 hash function to
   produce:

     [19, 211, 30, 150, 26, 26, 216, 236, 47, 22, 177, 12, 76, 152, 46,
      8, 118, 168, 120, 173, 109, 241, 68, 86, 110, 225, 137, 74, 203,
      112, 249, 195]

   Encoding this octet sequence as a base64url provides the value of the
   code_challenge:

       E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM

   The authorization request includes:





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       code_challenge=E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM
       &code_challange_method=S256

   The Authorization server then records the code_challenge and
   code_challenge_method along with the code that is granted to the
   client.

   in the request to the token_endpoint the client includes the code
   received in the authorization response as well as the additional
   paramater:

       code_verifier=dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

   The Authorization server retrieves the information for the code
   grant.  Based on the recorded code_challange_method being S256, it
   then hashes and base64url encodes the value of code_verifier.
   BASE64URL-ENCODE(SHA256(ASCII("code_verifier" )))

   The calculated value is then compared with the value of
   "code_challenge":

   BASE64URL-ENCODE(SHA256(ASCII("code_verifier" ))) == code_challenge

   If the two values are equal then the Authorization server can provide
   the tokens as long as there are no other errors in the request.  If
   the values are not equal then the request must be rejected, and an
   error returned.

Authors' Addresses

   Nat Sakimura (editor)
   Nomura Research Institute
   1-6-5 Marunouchi, Marunouchi Kitaguchi Bldg.
   Chiyoda-ku, Tokyo  100-0005
   Japan

   Phone: +81-3-5533-2111
   Email: n-sakimura@nri.co.jp
   URI:   http://nat.sakimura.org/












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   John Bradley
   Ping Identity
   Casilla 177, Sucursal Talagante
   Talagante, RM
   Chile

   Phone: +44 20 8133 3718
   Email: ve7jtb@ve7jtb.com
   URI:   http://www.thread-safe.com/


   Naveen Agarwal
   Google
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043
   USA

   Phone: +1 650-253-0000
   Email: naa@google.com
   URI:   http://google.com/































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