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Versions: (draft-reddy-tram-turn-third-party-authz) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 7635

TRAM                                                            T. Reddy
Internet-Draft                                                  P. Patil
Intended status: Standards Track                         R. Ravindranath
Expires: November 14, 2015                                         Cisco
                                                               J. Uberti
                                                                  Google
                                                            May 13, 2015


  Session Traversal Utilities for NAT (STUN) Extension for Third Party
                             Authorization
               draft-ietf-tram-turn-third-party-authz-16

Abstract

   This document proposes the use of OAuth 2.0 to obtain and validate
   ephemeral tokens that can be used for Session Traversal Utilities for
   NAT (STUN) authentication.  The usage of ephemeral tokens ensures
   that access to a STUN server can be controlled even if the tokens are
   compromised.

Status of This Memo

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

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

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

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

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



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   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Usage with TURN . . . . . . . . . . . . . . . . . . . . .   4
   4.  Obtaining a Token Using OAuth . . . . . . . . . . . . . . . .   7
     4.1.  Key Establishment . . . . . . . . . . . . . . . . . . . .   8
       4.1.1.  HTTP interactions . . . . . . . . . . . . . . . . . .   8
       4.1.2.  Manual provisioning . . . . . . . . . . . . . . . . .  10
   5.  Forming a Request . . . . . . . . . . . . . . . . . . . . . .  10
   6.  STUN Attributes . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  THIRD-PARTY-AUTHORIZATION . . . . . . . . . . . . . . . .  10
     6.2.  ACCESS-TOKEN  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  STUN server behaviour . . . . . . . . . . . . . . . . . . . .  13
   8.  STUN client behaviour . . . . . . . . . . . . . . . . . . . .  14
   9.  TURN client and server behaviour  . . . . . . . . . . . . . .  14
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  15
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Well-Known 'stun-key' URI  . . . . . . . . . . . . . . .  16
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     14.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Sample tickets . . . . . . . . . . . . . . . . . . .  19
   Appendix B.  Interaction between client and authorization server   20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Session Traversal Utilities for NAT (STUN) [RFC5389] provides a
   mechanism to control access via "long-term" username/ password
   credentials that are provided as part of the STUN protocol.  It is
   expected that these credentials will be kept secret; if the
   credentials are discovered, the STUN server could be used by
   unauthorized users or applications.  However, in web applications
   like WebRTC [I-D.ietf-rtcweb-overview] where JavaScript uses the
   browser functionality to make real-time audio and/or video calls, Web
   conferencing, and direct data transfer, ensuring this secrecy is
   typically not possible.





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   To address this problem and the ones described in [RFC7376], this
   document proposes the use of third party authorization using OAuth
   2.0 [RFC6749] for STUN.  Using OAuth 2.0, a client obtains an
   ephemeral token from an authorization server e.g.  WebRTC server, and
   the token is presented to the STUN server instead of the traditional
   mechanism of presenting username/password credentials.  The STUN
   server validates the authenticity of the token and provides required
   services.  Third party authorization using OAuth 2.0 for STUN
   explained in this specification can also be used with Traversal Using
   Relays around NAT (TURN) [RFC5766].

2.  Terminology

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

   This document uses the following abbreviations:

   o  WebRTC Server: A web server that supports WebRTC
      [I-D.ietf-rtcweb-overview].

   o  Access Token: OAuth 2.0 access token.

   o  mac_key: The session key generated by the authorization server.
      This session key has a lifetime that corresponds to the lifetime
      of the access token, is generated by the authorization server and
      bound to the access token.

   o  kid: An ephemeral and unique key identifier.  The kid also allows
      the resource server to select the appropriate keying material for
      decryption.

   o  AS: Authorization server

   Some sections in this specification show WebRTC server as the
   authorization server and client as the WebRTC client, however WebRTC
   is intended to be used for illustrative purpose only.

3.  Solution Overview

   STUN client knows that it can use OAuth 2.0 with the target STUN
   server either through configuration or when it receives the new STUN
   attribute THIRD-PARTY-AUTHORIZATION in the error response with an
   error code of 401(Unauthorized).

   This specification uses the token type 'Assertion' (aka self-
   contained token) described in [RFC6819] where all the information



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   necessary to authenticate the validity of the token is contained
   within the token itself.  This approach has the benefit of avoiding a
   protocol between the STUN server and the authorization server for
   token validation, thus reducing latency.  The content of the token is
   opaque to the client.  The client embeds the token within a STUN
   request sent to the STUN server.  Once the STUN server has determined
   the token is valid, its services are offered for a determined period
   of time.  Access token issued by the authorization server is
   explained in Section 6.2.  OAuth 2.0 in [RFC6749] defines four grant
   types.  This specification uses the OAuth 2.0 grant type "Implicit"
   explained in section 1.3.2 of [RFC6749] where the client is issued an
   access token directly.  The string 'stun' is defined by this
   specification for use as the OAuth scope parameter (see section 3.3
   of [RFC6749]) for the OAuth token.

   The exact mechanism used by a client to obtain a token and other
   OAuth 2.0 parameters like token type, mac_key, token lifetime and kid
   is outside the scope of this document.  Appendix B provides an
   example deployment scenario of interaction between the client and
   authorization server to obtain a token and other OAuth 2.0
   parameters.

   Section 3.1 illustrates the use of OAuth 2.0 to achieve third party
   authorization for TURN.

3.1.  Usage with TURN

   TURN, an extension to the STUN protocol, is often used to improve the
   connectivity of P2P applications.  TURN ensures that a connection can
   be established even when one or both sides is incapable of a direct
   P2P connection.  However, as a relay service, it imposes a nontrivial
   cost on the service provider.  Therefore, access to a TURN service is
   almost always access-controlled.  In order to achieve third party
   authorization, a resource owner e.g.  WebRTC server, authorizes a
   TURN client to access resources on the TURN server.

   In this example, a resource owner i.e., WebRTC server, authorizes a
   TURN client to access resources on a TURN server.













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                 +----------------------+----------------------------+
                 |     OAuth 2.0        |            WebRTC          |
                 +======================+============================+
                 | Client               | WebRTC client              |
                 +----------------------+----------------------------+
                 | Resource owner       | WebRTC server              |
                 +----------------------+----------------------------+
                 | Authorization server | Authorization server       |
                 +----------------------+----------------------------+
                 | Resource server      | TURN Server                |
                 +----------------------+----------------------------+

         Figure 1: OAuth terminology mapped to WebRTC terminology

   Using the OAuth 2.0 authorization framework, a WebRTC client (third-
   party application) obtains limited access to a TURN server (resource
   server) on behalf of the WebRTC server (resource owner or
   authorization server).  The WebRTC client requests access to
   resources controlled by the resource owner (WebRTC server) and hosted
   by the resource server (TURN server).  The WebRTC client obtains
   access token, lifetime, session key and kid.  The TURN client conveys
   the access token and other OAuth 2.0 parameters learnt from the
   authorization server to the TURN server.  The TURN server obtains the
   session key from the access token.  The TURN server validates the
   token, computes the message integrity of the request and takes
   appropriate action i.e, permits the TURN client to create
   allocations.  This is shown in an abstract way in Figure 2.
























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                           +---------------+
                           |               +<******+
            +------------->| Authorization |       *
            |              | Server        |       *
            |   +----------|(WebRTC Server)|       *  AS-RS,
            |   |          |               |       *  AUTH keys
   (1)      |   |           +---------------+      *   (0)
   Access   |   |  (2)                             *
   Token    |   | Access Token                     *
   Request  |   |    +                             *
            |   | Session Key                      *
            |   |                                  *
            |   V                                  V
        +-------+---+                       +-+----=-----+
        |           |         (3)           |            |
        |           | TURN Request + Access |            |
        | WebRTC    | Token                 | TURN       |
        | Client    |---------------------->| Server     |
        | (Alice)   | Allocate Response (4) |            |
        |           |<----------------------|            |
        +-----------+                       +------------+

   User : Alice
   ****: Out-of-Band Long-Term Key Establishment

                          Figure 2: Interactions

   In the below figure, the TURN client sends an Allocate request to the
   TURN server without credentials.  Since the TURN server requires that
   all requests be authenticated using OAuth 2.0, the TURN server
   rejects the request with a 401 (Unauthorized) error code and STUN
   attribute THIRD-PARTY-AUTHORIZATION.  The WebRTC client obtains
   access token from the WebRTC server, provides the access token to the
   TURN client and it tries again, this time including access token in
   the allocate request.  This time, the TURN server validates the
   token, accepts the Allocate request and returns an Allocate success
   response containing (amongst other things) the relayed transport
   address assigned to the allocation.













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   +-------------------+                         +--------+  +---------+
   | .........  TURN   |                         |  TURN  |  |  WebRTC |
   | .WebRTC .  Client |                         |        |  |         |
   | .Client .         |                         | Server |  |  Server |
   | .........         |                         |        |  |         |
   +-------------------+                         +--------+  +---------+
     |       |           Allocate request                |         |
     |       |------------------------------------------>|         |
     |       |                                           |         |
     |       |         Allocate error response           |         |
     |       |         (401 Unauthorized)                |         |
     |       |<------------------------------------------|         |
     |       |         THIRD-PARTY-AUTHORIZATION         |         |
     |       |                                           |         |
     |       |                                           |         |
     |       |      HTTP Request for token               |         |
     |------------------------------------------------------------>|
     |       |      HTTP Response with token parameters  |         |
     |<------------------------------------------------------------|
     |OAuth 2.0                                          |         |
      Attributes                                         |         |
     |------>|                                           |         |
     |       |    Allocate request ACCESS-TOKEN          |         |
     |       |------------------------------------------>|         |
     |       |                                           |         |
     |       |         Allocate success response         |         |
     |       |<------------------------------------------|         |
     |       |             TURN Messages                 |         |
     |       |      ////// integrity protected //////    |         |
     |       |      ////// integrity protected //////    |         |
     |       |      ////// integrity protected //////    |         |

                 Figure 3: TURN Third Party Authorization

4.  Obtaining a Token Using OAuth

   A STUN client needs to know the authentication capability of the STUN
   server before deciding to use third party authorization.  A STUN
   client initially makes a request without any authorization.  If the
   STUN server supports third party authorization, it will return an
   error message indicating that the client can authorize to the STUN
   server using an OAuth 2.0 access token.  The STUN server includes an
   ERROR-CODE attribute with a value of 401 (Unauthorized), a nonce
   value in a NONCE attribute and a SOFTWARE attribute that gives
   information about the STUN server's software.  The STUN server also
   includes the additional STUN attribute THIRD-PARTY-AUTHORIZATION
   signaling the STUN client that the STUN server supports third party
   authorization.



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   Note: An implementation may choose to contact the authorization
   server to obtain a token even before it makes a STUN request, if it
   knows the server details before hand.  For example, once a client has
   learnt that a STUN server supports third party authorization from a
   authorization server, the client can obtain the token before making
   subsequent STUN requests.

4.1.  Key Establishment

   In this model the STUN server would not authenticate the client
   itself but would rather verify whether the client knows the session
   key associated with a specific access token.  Example of this
   approach can be found with the OAuth 2.0 Proof-of-Possession (PoP)
   Security Architecture [I-D.ietf-oauth-pop-architecture].  The
   authorization server shares a long-term secret (K) with the STUN
   server.  When the client requests an access token the authorization
   server creates a fresh and unique session key (mac_key) and places it
   into the token encrypted with the long term secret.  Symmetric
   cryptography MUST be chosen to ensure that the size of encrypted
   token is not large because usage of asymmetric cryptography will
   result in large encrypted tokens which may not fit into a single STUN
   message.

   The STUN server and authorization server can establish a symmetric
   key (K) and certain authenticated encryption algorithm, using an out
   of band mechanism.  The STUN and authorization servers MUST establish
   K over an authenticated secure channel.  If Authenticated Encryption
   with AES-CBC and HMAC-SHA (defined in
   [I-D.mcgrew-aead-aes-cbc-hmac-sha2]) is used then the AS-RS and AUTH
   keys will be derived from K.  The AS-RS key is used for encrypting
   the self-contained token and the message integrity of the encrypted
   token is calculated using the AUTH key.  If Authenticated Encryption
   with Associated Data (AEAD) algorithm defined in [RFC5116] is used
   then there is no need to generate the AUTH key and AS-RS key will
   have the same value as K.

   The procedure for establishment of the symmetric key is outside the
   scope of this specification, and this specification does not mandate
   support of any given mechanism.  Section 4.1.1 and Section 4.1.2 show
   examples of mechanisms that can be used.

4.1.1.  HTTP interactions

   The STUN and AS servers could choose to use REST API over HTTPS to
   establish a symmetric key.  HTTPS MUST be used for data
   confidentiality and TLS based on client certificate MUST be used for
   mutual authentication.  To retrieve a new symmetric key, the STUN
   server makes an HTTP GET request to the authorization server,



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   specifying STUN as the service to allocate the symmetric keys for,
   and specifying the name of the STUN server.  The response is returned
   with content-type "application/json", and consists of a JavaScript
   Object Notation (JSON) [RFC7159] object containing the symmetric key.

   Request
   -------

   service - specifies the desired service (turn)
   name    -  STUN server name be associated with the key

   example:
   GET https://www.example.com/.well-known/stun-key?service=stun
   &name=turn1@example.com


   Response
   --------

   k - Long-term key (K)
   exp - identifies the time after which the key expires.


   example:
   {
      "k" :
   "ESIzRFVmd4iZABEiM0RVZgKn6WjLaTC1FXAghRMVTzkBGNaaN496523WIISKerLi",
      "exp" : 1300819380,
      "kid" :"22BIjxU93h/IgwEb"
      "enc" : A256GCM
     }

   The authorization server must also signal kid to the STUN server
   which will be used to select the appropriate keying material for
   decryption.  The parameter "k" is defined in Section 6.4.1 of
   [I-D.ietf-jose-json-web-algorithms], "enc" is defined in
   Section 4.1.2 of [I-D.ietf-jose-json-web-encryption], "kid" is
   defined in Section 4.1.4 of [I-D.ietf-jose-json-web-signature] and
   "exp" is defined in Section 4.1.4 of [I-D.ietf-oauth-json-web-token].
   A256GCM and other authenticated encryption algorithms are defined in
   section 5.1 of [I-D.ietf-jose-json-web-algorithms].  A STUN server
   and authorization server implementation MUST support A256GCM as the
   authenticated encryption algorithm.

   If A256CBC-HS512 defined in [I-D.ietf-jose-json-web-algorithms] is
   used then the AS-RS and AUTH keys are derived from K using the
   mechanism explained in section 5.2.2.1 of
   [I-D.ietf-jose-json-web-algorithms].  In this case AS-RS key length



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   must be 256-bit, AUTH key length must be 256-bit (section 2.6 of
   [RFC4868]).

4.1.2.  Manual provisioning

   The STUN and AS servers could be manually configured with a symmetric
   key (K), authenticated encryption algorithm and kid.

   Note : The mechanism specified in this section requires configuration
   to change the symmetric key (K) and/or authenticated encryption
   algorithm.  Hence a STUN server and authorization server
   implementation SHOULD support REST explained in Section 4.1.1.

5.  Forming a Request

   When a STUN server responds that third party authorization is
   required, a STUN client re-attempts the request, this time including
   access token and kid values in ACCESS-TOKEN and USERNAME STUN
   attributes.  The STUN client includes a MESSAGE-INTEGRITY attribute
   as the last attribute in the message over the contents of the STUN
   message.  The HMAC for the MESSAGE-INTEGRITY attribute is computed as
   described in section 15.4 of [RFC5389] where the mac_key is used as
   the input key for the HMAC computation.  The STUN client and server
   will use the mac_key to compute the message integrity and do not
   perform MD5 hash on the credentials.

6.  STUN Attributes

   The following new STUN attributes are introduced by this
   specification to accomplish third party authorization.

6.1.  THIRD-PARTY-AUTHORIZATION

   This attribute is used by the STUN server to inform the client that
   it supports third party authorization.  This attribute value contains
   the STUN server name.  The authorization server may have tie-ups with
   multiple STUN servers and vice versa, so the client MUST provide the
   STUN server name to the authorization server so that it can select
   the appropriate keying material to generate the self-contained token.
   If the authorization server does not have tie-up with the STUN server
   then it returns error to the client.  If the client does not support
   or is not capable of doing third party authorization then it defaults
   to first party authentication.  The THIRD-PARTY-AUTHORIZATION
   attribute is a comprehension-optional attribute (see Section 15 from
   [RFC5389]).  If the client is able to comprehend THIRD-PARTY-
   AUTHORIZATION it MUST ensure that third party authorization takes
   precedence over first party authentication (explained in section 10
   of [RFC5389]).



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6.2.  ACCESS-TOKEN

   The access token is issued by the authorization server.  OAuth 2.0
   does not impose any limitation on the length of the access token but
   if path MTU is unknown then STUN messages over IPv4 would need to be
   less than 548 bytes (Section 7.1 of [RFC5389]).  The access token
   length needs to be restricted to fit within the maximum STUN message
   size.  Note that the self-contained token is opaque to the client and
   the client MUST NOT examine the token.  The ACCESS-TOKEN attribute is
   a comprehension-required attribute (see Section 15 from [RFC5389]).

   The token is structured as follows:

         struct {
             uint16_t nonce_length;
             opaque nonce[nonce_length];
             opaque {
                 uint16_t key_length;
                 opaque mac_key[key_length];
                 uint64_t timestamp;
                 uint32_t lifetime;
             } encrypted_block;
         } token;

                   Figure 4: Self-contained token format

   Note: uintN_t means an unsigned integer of exactly N bits.  Single-
   byte entities containing uninterpreted data are of type opaque.  All
   values in the token are stored in network byte order.

   The fields are described below:

   nonce_length:  Length of the nonce field.  The length of nonce for
      authenticated encryption with additional data (AEAD) algorithms is
      explained in [RFC5116].

   Nonce:  Nonce (N) formation is explained in section 3.2 of [RFC5116].

   key_length:  Length of the session key in octets.  Key length of
      160-bits MUST be supported (i.e., only 160-bit key is used by
      HMAC-SHA-1 for message integrity of STUN message).  The key length
      facilitates the hash agility plan discussed in section 16.3 of
      [RFC5389].

   mac_key:  The session key generated by the authorization server.

   timestamp:  64-bit unsigned integer field containing a timestamp.
      The value indicates the time since January 1, 1970, 00:00 UTC, by



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      using a fixed point format.  In this format, the integer number of
      seconds is contained in the first 48 bits of the field, and the
      remaining 16 bits indicate the number of 1/64K fractions of a
      second (Native format - Unix).

   lifetime:  The lifetime of the access token, in seconds.  For
      example, the value 3600 indicates one hour.  The lifetime value
      MUST be greater than or equal to the "expires_in" parameter
      defined in section 4.2.2 of [RFC6749], otherwise resource server
      could revoke the token but the client would assume that the token
      has not expired and would not refresh the token.

   encrypted_block:  The encrypted_block (P) is encrypted and
      authenticated using the symmetric long-term key established
      between the STUN server and the authorization server.

   The AEAD encryption operation has four inputs: K , N, A, and P, as
   defined in section 2.1 of [RFC5116] and there is a single output a
   ciphertext C or an indication that the requested encryption operation
   could not be performed.

   The associated data (A) MUST be the STUN server name.  This ensures
   that the client does not use the same token to gain illegal access to
   other STUN servers provided by the same administrative domain i.e.,
   when multiple STUN servers in a single administrative domain share
   the same symmetric key with an authorization server.

   If AES_CBC_HMAC_SHA2 (explained in section 2.1 of
   [I-D.mcgrew-aead-aes-cbc-hmac-sha2])) is used then the encryption
   process is illustrated below.  The ciphertext consists of the string
   S, with the string T appended to it.  Here C and A denote Ciphertext
   and STUN server name respectively.  The octet string AL (section 2.1
   of [I-D.mcgrew-aead-aes-cbc-hmac-sha2]) is equal to the number of
   bits in A expressed as a 64-bit unsigned big endian integer.

   o  AUTH = initial authentication key length octets of K,

   o  AS-RS = final encryption key length octets of K,

   o  S = CBC-PKCS7-ENC(AS-RS, encrypted_block),

      *  Initialization vector is set to zero because the
         encrypted_block in each access token will not be identical and
         hence will not result in generation of identical ciphertext.

   o  mac = MAC(AUTH, A || S || AL),

   o  T = initial T_LEN octets of mac,



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   o  C = S || T.

   The entire token i.e., the 'encrypted_block' is base64 encoded (see
   section 4 of [RFC4648]) and the resulting access token is signaled to
   the client.

7.  STUN server behaviour

   The STUN server, on receiving a request with ACCESS-TOKEN attribute,
   performs checks listed in section 10.2.2 of [RFC5389] in addition to
   the following steps to verify that the access token is valid:

   o  STUN server selects the keying material based on kid signalled in
      the USERNAME attribute.

   o  The AEAD decryption operation has four inputs: K, N, A, and C, as
      defined in section 2.2 of [RFC5116].  AEAD decryption algorithm
      has only a single output, either a plaintext or a special symbol
      FAIL that indicates that the inputs are not authentic.  If
      authenticated decrypt operation returns FAIL then the STUN server
      rejects the request with an error response 401 (Unauthorized).

   o  If AES_CBC_HMAC_SHA2 is used then the final T_LEN octets are
      stripped from C.  It performs the verification of the token
      message integrity by calculating HMAC over the the STUN server
      name, the encrypted portion in the self-contained token and the AL
      using AUTH key and if the resulting value does not match the mac
      field in the self-contained token then it rejects the request with
      an error response 401 (Unauthorized).

   o  STUN server obtains the mac_key by retrieving the content of the
      access token (which requires decryption of the self-contained
      token using the AS-RS key).

   o  The STUN server verifies that no replay took place by performing
      the following check:

      *  The access token is accepted if the timestamp field (TS) in the
         self-contained token is recent enough to the reception time of
         the STUN request (RDnew) using the following formula:

        lifetime + Delta > abs(RDnew - TS)

         The RECOMMENDED value for the allowed Delta is 5 seconds.  If
         the timestamp is NOT within the boundaries then the STUN server
         discards the request with error response 401 (Unauthorized).





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   o  The STUN server uses the mac_key to compute the message integrity
      over the request and if the resulting value does not match the
      contents of the MESSAGE-INTEGRITY attribute then it rejects the
      request with an error response 401 (Unauthorized).

   o  If all the checks pass, the STUN server continues to process the
      request.

   o  Any response generated by the server MUST include the MESSAGE-
      INTEGRITY attribute, computed using the mac_key.

   If a STUN server receives an ACCESS-TOKEN attribute unexpectedly
   (because it had not previously sent out a THIRD-PARTY-AUTHORIZATION),
   it will respond with an error code of 420 (Unknown Attribute) as
   specified in Section 7.3.1 of [RFC5389].

8.  STUN client behaviour

   o  The client looks for the MESSAGE-INTEGRITY attribute in the
      response.  If MESSAGE-INTEGRITY is absent or the value computed
      for message integrity using mac_key does not match the contents of
      the MESSAGE-INTEGRITY attribute then the response MUST be
      discarded.

   o  If the access token expires then the client MUST obtain a new
      token from the authorization server and use it for new STUN
      requests.

9.  TURN client and server behaviour

   Changes specific to TURN are listed below:

   o  The access token can be reused for multiple Allocate requests to
      the same TURN server.  The TURN client MUST include the ACCESS-
      TOKEN attribute only in Allocate and Refresh requests.  Since the
      access token is valid for a specific period of time, the TURN
      server can cache it so that it can check if the access token in a
      new allocation request matches one of the cached tokens and avoids
      the need to decrypt the token.

   o  The lifetime provided by the TURN server in the Allocate and
      Refresh responses MUST be less than or equal to the lifetime of
      the token.  It is RECOMMENDED that the TURN server calculate the
      maximum allowed lifetime value using the formula:

        lifetime + Delta - abs(RDnew - TS)

      The RECOMMENDED value for the allowed Delta is 5 seconds.



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   o  If the access token expires then the client MUST obtain a new
      token from the authorization server and use it for new
      allocations.  The client MUST use the new token to refresh
      existing allocations.  This way client has to maintain only one
      token per TURN server.

10.  Operational Considerations

   The following operational considerations should be taken into
   account:

   o  Each authorization server should maintain the list of STUN servers
      for which it will grant tokens, and the long-term secret shared
      with each of those STUN servers.

   o  If manual configuration (Section 4.1.2) is used to establish
      symmetric keys, the necessary information which includes long-term
      secret (K) and authenticated encryption algorithm have to be
      configured on each authorization server and STUN server for each
      kid.  The client obtains the session key and HMAC algorithm from
      the authorization server in company with the token.

   o  When a STUN client sends a request to get access to a particular
      STUN server (S) the authorization server must ensure that it
      selects the appropriate kid, access-token depending on the server
      S.

11.  Security Considerations

   When OAuth 2.0 is used, the interaction between the client and the
   authorization server requires Transport Layer Security (TLS) with a
   ciphersuite offering confidentiality protection and the guidance
   given in [RFC7525] must be followed to avoid attacks on TLS.  The
   session key MUST NOT be transmitted in clear since this would
   completely destroy the security benefits of the proposed scheme.  An
   attacker trying to replay message with ACCESS-TOKEN attribute can be
   mitigated by frequent changes of nonce value as discussed in section
   10.2 of [RFC5389].  The client may know some (but not all) of the
   token fields encrypted with a unknown secret key and the token can be
   subjected to known-plaintext attack, but AES is secure against this
   attack.

   An attacker may remove the THIRD-PARTY-AUTHORIZATION STUN attribute
   from the error message forcing the client to pick first party
   authentication, this attack may be mitigated by opting for Transport
   Layer Security (TLS) [RFC5246] or Datagram Transport Layer Security
   (DTLS) [RFC6347] as a transport protocol for Session Traversal
   Utilities for NAT (STUN), as defined in [RFC5389]and [RFC7350].



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   Threat mitigation discussed in section 5 of
   [I-D.ietf-oauth-pop-architecture] and security considerations in
   [RFC5389] are to be taken into account.

12.  IANA Considerations

   [Paragraphs below in braces should be removed by the RFC Editor upon
   publication]

   [IANA is requested to add the following attributes to the STUN
   attribute registry [iana-stun], The THIRD-PARTY-AUTHORIZATION
   attribute requires that IANA allocate a value in the "STUN attributes
   Registry" from the comprehension-optional range (0x8000-0xBFFF)]

   This document defines the THIRD-PARTY-AUTHORIZATION STUN attribute,
   described in Section 6.  IANA has allocated the comprehension-
   optional codepoint TBD for this attribute.

   [The ACCESS-TOKEN attribute requires that IANA allocate a value in
   the "STUN attributes Registry" from the comprehension-required range
   (0x0000-0x3FFF)]

   This document defines the ACCESS-TOKEN STUN attribute, described in
   Section 6.  IANA has allocated the comprehension-required codepoint
   TBD for this attribute.

12.1.  Well-Known 'stun-key' URI

   This memo registers the 'stun-key' well-known URI in the Well-Known
   URIs registry as defined by [RFC5785].

   URI suffix: stun-key

   Change controller: IETF

   Specification document(s): This RFC

   Related information: None

13.  Acknowledgements

   Authors would like to thank Dan Wing, Pal Martinsen, Oleg Moskalenko,
   Charles Eckel, Spencer Dawkins, Hannes Tschofenig, Yaron Sheffer, Tom
   Taylor, Christer Holmberg, Pete Resnick, Kathleen Moriarty, Richard
   Barnes, Stephen Farrell, Alissa Cooper and Rich Salz for comments and
   review.  The authors would like to give special thanks to Brandon
   Williams for his help.




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   Thanks to Oleg Moskalenko for providing token samples in the
   Appendix section.

14.  References

14.1.  Normative References

   [I-D.ietf-jose-json-web-algorithms]
              Jones, M., "JSON Web Algorithms (JWA)", draft-ietf-jose-
              json-web-algorithms-40 (work in progress), January 2015.

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

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

   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
              384, and HMAC-SHA-512 with IPsec", RFC 4868, May 2007.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

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

   [iana-stun]
              IANA, , "IANA: STUN Attributes", April 2011,
              <http://www.iana.org/assignments/stun-parameters/stun-pa
              rameters.xml>.

14.2.  Informative References

   [I-D.ietf-jose-json-web-encryption]
              Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              draft-ietf-jose-json-web-encryption-40 (work in progress),
              January 2015.

   [I-D.ietf-jose-json-web-signature]
              Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", draft-ietf-jose-json-web-signature-41
              (work in progress), January 2015.





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   [I-D.ietf-oauth-json-web-token]
              Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", draft-ietf-oauth-json-web-token-32 (work in
              progress), December 2014.

   [I-D.ietf-oauth-pop-architecture]
              Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
              Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
              Architecture", draft-ietf-oauth-pop-architecture-01 (work
              in progress), March 2015.

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", draft-ietf-oauth-pop-key-
              distribution-01 (work in progress), March 2015.

   [I-D.ietf-rtcweb-overview]
              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", draft-ietf-rtcweb-overview-13
              (work in progress), November 2014.

   [I-D.ietf-tram-stunbis]
              Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
              D., Mahy, R., and P. Matthews, "Session Traversal
              Utilities for NAT (STUN)", draft-ietf-tram-stunbis-04
              (work in progress), March 2015.

   [I-D.mcgrew-aead-aes-cbc-hmac-sha2]
              McGrew, D., Foley, J., and K. Paterson, "Authenticated
              Encryption with AES-CBC and HMAC-SHA", draft-mcgrew-aead-
              aes-cbc-hmac-sha2-05 (work in progress), July 2014.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785, April
              2010.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.





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

   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

   [RFC7350]  Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as Transport for Session Traversal
              Utilities for NAT (STUN)", RFC 7350, August 2014.

   [RFC7376]  Reddy, T., Ravindranath, R., Perumal, M., and A. Yegin,
              "Problems with Session Traversal Utilities for NAT (STUN)
              Long-Term Authentication for Traversal Using Relays around
              NAT (TURN)", RFC 7376, September 2014.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, May 2015.

Appendix A.  Sample tickets

   Input data (same for all samples below):

      //STUN SERVER NAME
      server_name = "blackdow.carleon.gov";

      //Shared key between AS and RS

      long_term_key = \x48\x47\x6b\x6a\x33\x32\x4b\x4a\x47\x69\x75\x79
                      \x30\x39\x38\x73\x64\x66\x61\x71\x62\x4e\x6a\x4f
                      \x69\x61\x7a\x37\x31\x39\x32\x33

      //MAC key of the session (included in the token)
      mac_key = \x5a\x6b\x73\x6a\x70\x77\x65\x6f\x69\x78\x58\x6d\x76\x6e
                \x36\x37\x35\x33\x34\x6d;

      //length of the MAC key
      mac_key_length  =  20;

      //The timestamp field in the token
      token_timestamp = 92470300704768;

      //The lifetime of the token
      token_lifetime = 3600;

      //nonce for AEAD



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      aead_nonce = \x68\x34\x6a\x33\x6b\x32\x6c\x32\x6e\x34\x62\x35;

      Samples:

        1) token encryption algorithm = AEAD_AES_256_GCM

        Encrypted token (64 bytes = 2 + 12 +34 + 16) =

       \x00\x0c\x68\x34\x6a\x33\x6b\x32\x6c\x32\x6e\x34\x62
       \x35\x61\x7e\xf1\x34\xa3\xd5\xe4\x4e\x9a\x19\xcc\x7d
       \xc1\x04\xb0\xc0\x3d\x03\xb2\xa5\x51\xd8\xfd\xf5\xcd
       \x3b\x6d\xca\x6f\x10\xcf\xb7\x7e\x5b\x2d\xde\xc8\x4d
       \x29\x3a\x5c\x50\x49\x93\x59\xf0\xc2\xe2\x6f\x76

     2) token encryption algorithm = AEAD_AES_128_GCM

        Encrypted token (64 bytes = 2 + 12 +34 + 16) =

      \x00\x0c\x68\x34\x6a\x33\x6b\x32\x6c\x32\x6e\x34\x62
      \x35\x7f\xb9\xe9\x9f\x08\x27\xbe\x3d\xf1\xe1\xbd\x65
      \x14\x93\xd3\x03\x1d\x36\xdf\x57\x07\x97\x84\xae\xe5
      \xea\xcb\x65\xfa\xd4\xf2\x7f\xab\x1a\x3f\x97\x97\x4b
      \x69\xf8\x51\xb2\x4b\xf5\xaf\x09\xed\xa3\x57\xe0


   Note:
   [1]
   After EVP_EncryptFinal_ex encrypts the final data
   EVP_CIPHER_CTX_ctrl must be called to append
   the authentication tag to the ciphertext.
   //EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_GET_TAG, taglen, tag);
   [2]
   EVP_CIPHER_CTX_ctrl must be invoked to set the
   authentication tag before calling EVP_DecryptFinal.
   //EVP_CIPHER_CTX_ctrl (&ctx, EVP_CTRL_GCM_SET_TAG, taglen, tag);

                         Figure 5: Sample tickets

Appendix B.  Interaction between client and authorization server

   Client makes an HTTP request to an authorization server to obtain a
   token that can be used to avail itself of STUN services.  The STUN
   token is returned in JSON syntax [RFC7159], along with other OAuth
   2.0 parameters like token type, key, token lifetime and kid defined
   in [I-D.ietf-oauth-pop-key-distribution].






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   +-------------------+                         +--------+  +---------+
   | .........  STUN   |                         |  STUN  |  |  WebRTC |
   | .WebRTC .  Client |                         |        |  |         |
   | .Client .         |                         | Server |  |  Server |
   | .........         |                         |        |  |         |
   +-------------------+                         +--------+  +---------+
     |       |           STUN request                    |         |
     |       |------------------------------------------>|         |
     |       |                                           |         |
     |       |         STUN error response               |         |
     |       |         (401 Unauthorized)                |         |
     |       |<------------------------------------------|         |
     |       |         THIRD-PARTY-AUTHORIZATION         |         |
     |       |                                           |         |
     |       |                                           |         |
     |       |      HTTP Request for token               |         |
     |------------------------------------------------------------>|
     |       |      HTTP Response with token parameters  |         |
     |<------------------------------------------------------------|
     |OAuth 2.0                                          |         |
      Attributes                                         |         |
     |------>|                                           |         |
     |       |    STUN request with ACCESS-TOKEN         |         |
     |       |------------------------------------------>|         |
     |       |                                           |         |
     |       |         STUN success response             |         |
     |       |<------------------------------------------|         |
     |       |             STUN Messages                 |         |
     |       |      ////// integrity protected //////    |         |
     |       |      ////// integrity protected //////    |         |
     |       |      ////// integrity protected //////    |         |

                 Figure 6: STUN Third Party Authorization

   [I-D.ietf-oauth-pop-key-distribution] describes the interaction
   between the client and the authorization server.  For example, the
   client learns the STUN server name "stun1@example.com" from THIRD-
   PARTY-AUTHORIZATION attribute value and makes the following HTTP
   request for the access token using transport-layer security (with
   extra line breaks for display purposes only):











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        HTTP/1.1
        Host: server.example.com
        Content-Type: application/x-www-form-urlencoded
        aud=stun1@example.com
        timestamp=1361471629
        grant_type=implicit
        token_type=pop
        alg=HMAC-SHA-256-128

                             Figure 7: Request

   [I-D.ietf-tram-stunbis] supports hash agility and accomplish this
   agility by computing message integrity using both HMAC-SHA-1 and
   HMAC-SHA-256-128.  The client signals the algorithm supported by it
   to the authorization server in the 'alg' parameter defined in
   [I-D.ietf-oauth-pop-key-distribution].  The authorization server
   determines length of the mac_key based on the HMAC algorithm conveyed
   by the client.  If the client supports both HMAC-SHA-1 and HMAC-SHA-
   256-128 then it signals HMAC-SHA-256-128 to the authorization server,
   gets 256-bit key from the authorization server and calculates 160-bit
   key for HMAC-SHA-1 using SHA1 taking the 256-bit key as input.

   If the client is authorized then the authorization server issues an
   access token.  An example of successful response:

        HTTP/1.1 200 OK
        Content-Type: application/json
        Cache-Control: no-store

        {
          "access_token":
   "U2FsdGVkX18qJK/kkWmRcnfHglrVTJSpS6yU32kmHmOrfGyI3m1gQj1jRPsr0uBb
   HctuycAgsfRX7nJW2BdukGyKMXSiNGNnBzigkAofP6+Z3vkJ1Q5pWbfSRroOkWBn",
          "token_type":"pop",
          "expires_in":1800,
          "kid":"22BIjxU93h/IgwEb",
          "key":"v51N62OM65kyMvfTI08O"
          "alg":HMAC-SHA-256-128
        }

                            Figure 8: Response

Authors' Addresses








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   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: tireddy@cisco.com


   Prashanth Patil
   Cisco Systems, Inc.
   Bangalore
   India

   Email: praspati@cisco.com


   Ram Mohan Ravindranath
   Cisco Systems, Inc.
   Cessna Business Park,
   Kadabeesanahalli Village, Varthur Hobli,
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: rmohanr@cisco.com


   Justin Uberti
   Google
   747 6th Ave S
   Kirkland, WA
   98033
   USA

   Email: justin@uberti.name














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