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RFC 7636

Proposed STD
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Proof Key for Code Exchange by OAuth Public Clients

 


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Internet Engineering Task Force (IETF)                  N. Sakimura, Ed.
Request for Comments: 7636                     Nomura Research Institute
Category: Standards Track                                     J. Bradley
ISSN: 2070-1721                                            Ping Identity
                                                              N. Agarwal
                                                                  Google
                                                          September 2015


          Proof Key for Code Exchange by OAuth Public Clients

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 is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   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/rfc7636.

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
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................3
      1.1. Protocol Flow ..............................................5
   2. Notational Conventions ..........................................6
   3. Terminology .....................................................7
      3.1. Abbreviations ..............................................7
   4. Protocol ........................................................8
      4.1. Client Creates a Code Verifier .............................8
      4.2. Client Creates the Code Challenge ..........................8
      4.3. Client Sends the Code Challenge with the
           Authorization Request ......................................9
      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 ............................10
      4.6. Server Verifies code_verifier before Returning the
           Tokens ....................................................10
   5. Compatibility ..................................................11
   6. IANA Considerations ............................................11
      6.1. OAuth Parameters Registry .................................11
      6.2. PKCE Code Challenge Method Registry .......................11
           6.2.1. Registration Template ..............................12
           6.2.2. Initial Registry Contents ..........................13
   7. Security Considerations ........................................13
      7.1. Entropy of the code_verifier ..............................13
      7.2. Protection against Eavesdroppers ..........................13
      7.3. Salting the code_challenge ................................14
      7.4. OAuth Security Considerations .............................14
      7.5. TLS Security Considerations ...............................15
   8. References .....................................................15
      8.1. Normative References ......................................15
      8.2. Informative References ....................................16
   Appendix A.  Notes on Implementing Base64url Encoding without
                Padding  .............................................17
   Appendix B.  Example for the S256 code_challenge_method ...........17
   Acknowledgements ..................................................19
   Authors' Addresses ................................................20

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1.  Introduction

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

   In this attack, the attacker intercepts the authorization code
   returned from the authorization endpoint within a communication path
   not protected by Transport Layer Security (TLS), such as inter-
   application communication within the client's operating system.

   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
   application running on the end device, such as a smartphone, 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 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).

   Note that it is possible for a malicious app to register itself as a
   handler for the custom scheme in addition to the legitimate OAuth 2.0
   app.  Once it does so, the malicious app 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., Smartphone)  |
    |                                |
    | +-------------+   +----------+ | (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 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 scheme 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.

   4. Either one of the following condition is met:

      4a. The attacker (via the installed application) is able to
          observe only the responses from the authorization endpoint.
          When "code_challenge_method" value is "plain", only this
          attack is mitigated.

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      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 was caused by leaking http
          log information in the OS.  To mitigate this,
          "code_challenge_method" value must be set either to "S256" or
          a value defined by a cryptographically secure
          "code_challenge_method" extension.

   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 of
   [RFC6819]) describes mitigation techniques, they are, unfortunately,
   not applicable since they rely on a per-client instance secret or a
   per-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

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   This specification adds additional parameters to the OAuth 2.0
   Authorization and Access Token Requests, shown in abstract form in
   Figure 2.

   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 code 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 natural language meanings.

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

   STRING denotes a sequence of zero or more ASCII [RFC20] characters.

   OCTETS denotes a sequence of zero or more octets.

   ASCII(STRING) denotes the octets of the ASCII [RFC20] representation
   of STRING where STRING is a sequence of zero or more ASCII
   characters.

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

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

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

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.

   code challenge method
      A method that was used to derive code challenge.

   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

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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 Section 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: The 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
   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.

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   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 is "S256" or "plain".

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 the "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.

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   If the server supporting PKCE does not support the requested
   transformation, 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".

4.6.  Server Verifies code_verifier before Returning the Tokens

   Upon receipt of the request at the token endpoint, the server
   verifies it by calculating the code challenge from the 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 4.3 was "S256", the
   received "code_verifier" is hashed by SHA-256, 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 4.3 was "plain", they are
   compared directly, i.e.:

   code_verifier == code_challenge.

   If the values are equal, the 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.

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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 the OAuth 2.0 [RFC6749]
   protocol without this extension.

   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 4 to
   all servers.

6.  IANA Considerations

   IANA has made the following registrations per this document.

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: token request
   o  Change controller: IESG
   o  Specification document(s): RFC 7636 (this document)

   o  Parameter name: code_challenge
   o  Parameter usage location: authorization request
   o  Change controller: IESG
   o  Specification document(s): RFC 7636 (this document)

   o  Parameter name: code_challenge_method
   o  Parameter usage location: authorization request
   o  Change controller: IESG
   o  Specification document(s): RFC 7636 (this document)

6.2.  PKCE Code Challenge Method Registry

   This specification establishes the "PKCE Code Challenge Methods"
   registry.  The new registry should be a sub-registry of the "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 (DEs).  The DEs will ensure that there

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   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,
      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) states
      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, and home page URI) may also be included.

   Specification Document(s):
      Reference to the document(s) that specifies 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.

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6.2.2.  Initial Registry Contents

   Per this document, IANA has registered 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 RFC 7636 (this document)

   o  Code Challenge Method Parameter Name: S256
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.2 of RFC 7636 (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 256 bits
   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.

7.2.  Protection against Eavesdroppers

   Clients MUST NOT downgrade to "plain" after trying the "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".  Because of this, an error when "S256" is presented
   can only mean that the server is faulty or that a MITM attacker is
   trying a downgrade attack.

   The "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, the "plain" method does not
   protect against the eavesdropping of the initial request.

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

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   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

   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
   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.

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7.5.  TLS Security Considerations

   Current security considerations can be found in "Recommendations for
   Secure Use of Transport Layer Security (TLS) and Datagram Transport
   Layer Security (DTLS)" [BCP195].  This supersedes the TLS version
   recommendations in OAuth 2.0 [RFC6749].

8.  References

8.1.  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,
              <http://www.rfc-editor.org/info/bcp195>.

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <http://www.rfc-editor.org/info/rfc20>.

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [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>.

   [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>.

   [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>.

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   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <http://www.rfc-editor.org/info/rfc6234>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://www.rfc-editor.org/info/rfc6749>.

8.2.  Informative References

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,
              <http://www.rfc-editor.org/info/rfc6819>.

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Appendix A.  Notes on Implementing Base64url Encoding without Padding

   This appendix describes how to implement a base64url-encoding
   function without padding, based upon the 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.

     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 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]

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

       E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM

   The authorization request includes:

       code_challenge=E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM
       &code_challenge_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
   parameter:

       code_verifier=dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

   The authorization server retrieves the information for the code
   grant.  Based on the recorded code_challenge_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.

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Acknowledgements

   The initial draft version 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
      Justin Richer, MIT Kerberos
      Josh Mandel, Boston Children's Hospital
      Lewis Adam, Motorola Solutions
      Madjid Nakhjiri, Samsung
      Michael B. Jones, Microsoft
      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

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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 Parkway
   Mountain View, CA  94043
   United States

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