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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: May 14, 2015                                      Ping Identity
                                                              N. Agarwal
                                                                  Google
                                                       November 12, 2014

  Symmetric Proof of Possession for the OAuth Authorization Code Grant
                        draft-ietf-oauth-spop-03

Abstract

   The OAuth 2.0 public client utilizing Authorization Code Grant (RFC
   6749 - 4.1) is susceptible to the code interception attack.  This
   specification describes a mechanism that acts as a control against
   this threat.

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

   This Internet-Draft will expire on May 14, 2015.

Copyright Notice

   Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . .  2

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     1.1.  Protocol Flow  . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Client creates a code verifier . . . . . . . . . . . . . .  4
     4.2.  Client creates the code challenge  . . . . . . . . . . . .  5
     4.3.  Client sends the code challenge with the authorization
           request  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.4.  Server returns the code  . . . . . . . . . . . . . . . . .  5
     4.5.  Client sends the code and the secret to the token endpoint  6
     4.6.  Server verifies code_verifier before returning the tokens   6
   5.  Compatibility  . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  OAuth Parameters Registry  . . . . . . . . . . . . . . . .  7
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
     7.1.  Entropy of the code verifier . . . . . . . . . . . . . . .  7
     7.2.  Protection against eavesdroppers . . . . . . . . . . . . .  7
     7.3.  Checking the Server support  . . . . . . . . . . . . . . .  7
     7.4.  OAuth security considerations  . . . . . . . . . . . . . .  8
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  Revision History . . . . . . . . . . . . . . . . . . . . . . .  8
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1.  Normative References  . . . . . . . . . . . . . . . . . .  9
     10.2.  Informative References  . . . . . . . . . . . . . . . . .  9
   Appendix A. Notes on implementing base64url encoding without paddi 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   Public clients in OAuth 2.0  [RFC6749] are susceptible to the
   authorization "code" interception attack.  A malicious client
   intercepts the authorization code returned from the authorization
   endpoint and uses it to obtain the access token.  This is possible on
   a public client as there is no client secret associated for it to be
   sent to the token endpoint.  This is especially true on Smartphone
   applications where the authorization code can be returned through
   custom URL Schemes where the same scheme can be registered by
   multiple applications.  Under this scenario, the mitigation strategy
   stated in section 4.4.1 of [RFC6819] does not work as they rely on
   per-client instance secret or per client instance redirect URI.

   To mitigate this attack, this extension utilizes a dynamically
   created cryptographically random key called 'code verifier'. The 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.

1.1.  Protocol Flow




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    +--------+                                  +---------------+
    |        |--(A)-- Authorization Request --->|               |
    |        |        + t(code_verifier), t     |   Resource    |
    |        |                                  |     Owner     |
    |        |<-(B)--- Authorization Grant -----|               |
    |        |                                  +---------------+
    | Client |
    |        |                                  +---------------+
    |        |--(C)--- Access Token Request --->|               |
    |        |          + code_verifier         | Authorization |
    |        |                                  |     Server    |
    |        |<-(D)------ Access Token ---------|               |
    +--------+                                  +---------------+

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

   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".

   B. The resource owner responds as usual, but records
      "t(code_verifier)" and the transformation method.

   C. The client then sends the code to 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].

   BASE64URL(OCTETS) denotes the base64url encoding of OCTETS, per




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   Section 3 producing a [US-ASCII] STRING.

   BASE64URL-DECODE(STRING) denotes the base64url decoding of STRING,
   per Section 3, producing a UTF-8 sequence of octets.

   SHA256(STRING) denotes a SHA2 256bit hash [RFC4634] of STRING.

   UTF8(STRING) denotes the octets of the UTF-8 [RFC3629] representation
   of STRING.

   ASCII(STRING) denotes the octets of the ASCII [US-ASCII]
   representation of STRING.

   The concatenation of two values A and B is denoted as A || B.

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 RFC 4648 [RFC4648], with all
      trailing '=' characters omitted (as permitted by Section 3.2) and
      without the inclusion of any line breaks, whitespace, or other
      additional characters.  (See Appendix Appendix A for notes on
      implementing base64url encoding without padding.)

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 [US-ASCII] sequence
   using the url and filename safe Alphabet [A-Z] / [a-z] / [0-9] / "-"
   / "_" from Sec 5 of RFC 4648 [RFC4648], with length less than 128
   characters.

   ABNF for "code_verifier" is as follows.

         code_verifier = 42*128unreserved
         unreserved    = [A-Z] / [a-z] / [0-9] / "-" / "_"






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   NOTE: code verifier SHOULD have enough entropy to make it impractical
   to guess the value.  It is RECOMMENDED that the output of a suitable
   random number generator be used to create a 32-octet sequence.  The
   Octet sequence is then BASE64URL encoded to produce a 42-octet URL
   safe string to use as the code verifier.

4.2.  Client creates the code challenge

   The client then creates a code challenge, "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(SHA256("code_verifier"))

   It is RECOMMENDED to use the S256 transformation when possible.

   ABNF for "code_challenge" is as follows.

         code_challenge = 42*128unreserved
         unreserved    = [A-Z] / [a-z] / [0-9] / "-" / "_"

4.3.  Client sends the code challenge with the authorization request

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

   code_challenge REQUIRED. Code challenge.

   code_challenge_method OPTIONAL, defaults to "plain".  Code verifier
      transformation method, "S256" or "plain".

4.4.  Server returns the code

   When the server issues the "code" in the Authorization Response, it
   MUST associate the "code_challenge" and "code_challenge_method"
   values with the "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







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   "code_challenge" with the issued "code" is out of scope for this
   specification.

4.5.  Client sends the code and the secret to the token endpoint

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

   code_verifier REQUIRED. Code verifier

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 3.2 was "S256", the received
   "code_verifier" is first hashed with SHA-256 then compared to the
   base64url decoded "code_challenge".  i.e.,

   SHA256("code_verifier" ) == BASE64URL-DECODE("code_challenge").

   If the "code_challenge_method" from 3.2 was "none", they are compared
   directly.  i.e.,

   "code_challenge" == "code_verifier".

   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 OAuth 2.0 [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 SHOULD 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:


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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

   o  Parameter usage location: Access 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

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.  It
   is RECOMMENDED that the output of a suitable random number generator
   be used to create a 32-octet sequence.

7.2.  Protection against eavesdroppers

   Unless there is a compelling reason, implementations SHOULD use
   "S256" method to protect against eavesdroppers intercepting the
   "code_challenge".  If the no transformation algorithm, which is the
   default algorithm, is used, the client SHOULD make sure that the
   authorization request is adequately protected from an eavesdropper.
   If "code_challenge" is to be returned inside authorization "code", it
   has to be encrypted in such a manner that only the server can decrypt
   and extract it.

7.3.  Checking the Server support

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   Before starting the authorization process, the client SHOULD check if
   the server supports this specification.  Confirmation of the server
   support may be obtained out-of-band or through some other mechanisms
   such as the discovery document in OpenID Connect Discovery
   [OpenID.Discovery].  The exact mechanism on how the client obtains
   this information, or the action it takes as a result is out of scope
   of this specification.

7.4.  OAuth security considerations

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

8.  Acknowledgements

   The initial draft of this specification was created by the OpenID AB/
   Connect Working Group of the OpenID Foundation, most notably by the
   following people:

   o  Naveen Agarwal, Google

   o  Dirk Balfanz, Google

   o  Sergey Beryozkin

   o  John Bradley, Ping Identity

   o  Brian Campbell, Ping Identity

   o  William Denniss, Google

   o  Eduardo Gueiros, Jive Communications

   o  Phil Hunt, Oracle

   o  Ryo Ito, mixi

   o  Michael B. Jones, Microsoft

   o  Torsten Lodderstedt, Deutsche Telekom

   o  Breno de Medeiros, Google

   o  Prateek Mishra, Oracle

   o  Anthony Nadalin, Microsoft

   o  Axel Nenker, Deutsche Telekom

   o  Nat Sakimura, Nomura Research Institute

9.  Revision History


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   -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

10.1.  Normative References

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

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

   [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

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

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

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

   [US-ASCII]
              American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

10.2.  Informative References

   [OpenID.Discovery]


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              Sakimura, N., Bradley, J., Jones, M.B. and E. Jay, "OpenID
              Connect Discovery 1.0", February 2014.

   [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 base64url encoding and
   decoding functions without padding based upon standard base64
   encoding and decoding functions that do use padding.

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

     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;
     }

     static byte [] base64urldecode(string arg)
     {
       string s = arg;
       s = s.Replace('-', '+'); // 62nd char of encoding
       s = s.Replace('_', '/'); // 63rd char of encoding
       switch (s.Length % 4) // Pad with trailing '='s
       {
         case 0: break; // No pad chars in this case
         case 2: s += "=="; break; // Two pad chars
         case 3: s += "="; break; // One pad char
         default: throw new System.Exception(
           "Illegal base64url string!");
       }
       return Convert.FromBase64String(s); // Standard base64 decoder
     }

   As per the example code above, the number of '=' padding characters
   that needs to be added to the end of a base64url encoded string
   without padding to turn it into one with padding is a deterministic
   function of the length of the encoded string.  Specifically, if the
   length mod 4 is 0, no padding is added; if the length mod 4 is 2, two
   '=' padding characters are added; if the length mod 4 is 3, one '='
   padding character is added; if the length mod 4 is 1, the input is
   malformed.

   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.


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   3 236 255 224 193

   A-z_4ME

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/


   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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