TURN Extension for Third Party Authorization
draft-reddy-tram-turn-third-party-authz-01

TRAM                                                            T. Reddy
Internet-Draft                                                  P. Patil
Intended status: Standards Track                         R. Ravindranath
Expires: October 25, 2014                                          Cisco
                                                               J. Uberti
                                                                  Google
                                                          April 23, 2014

              TURN Extension for Third Party Authorization
               draft-reddy-tram-turn-third-party-authz-01

Abstract

   This document proposes the use of OAuth to obtain and validate
   ephemeral tokens that can be used for TURN authentication.  The usage
   of ephemeral tokens ensure that access to a TURN server can be
   controlled even if the tokens are compromised, as is the case in
   WebRTC where TURN credentials must be specified in Javascript.

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
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   This Internet-Draft will expire on October 25, 2014.

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
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   to this document.  Code Components extracted from this document must

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Obtaining a Token Using OAuth . . . . . . . . . . . . . . . .   5
     4.1.  Key Establishment . . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  HTTP interactions . . . . . . . . . . . . . . . . . .   8
   5.  Forming a Request . . . . . . . . . . . . . . . . . . . . . .   9
   6.  STUN Attributes . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  THIRD-PARTY-AUTHORIZATION . . . . . . . . . . . . . . . .   9
     6.2.  ACCESS-TOKEN  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Receiving a request with ACCESS-TOKEN attribute . . . . . . .  11
   8.  Changes to Client . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     12.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   TURN [RFC5766] is a protocol that is often used to improve the
   connectivity of P2P applications.  By providing a cloud-based relay
   service, 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.

   TURN provides a mechanism to control access via "long-term" username/
   password credentials that are provided as part of the TURN protocol.
   It is expected that these credentials will be kept secret; if the
   credentials are discovered, the TURN server could be used by
   unauthorized users or applications.  However, in web applications,
   ensuring this secrecy is typically impossible.  To address this
   problem and the ones described in [I-D.ietf-tram-auth-problems], this
   document proposes the use of third party authorization using OAuth
   for TURN.

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   To achieve third party authorization, a resource owner e.g. WebRTC
   server, authorizes a TURN client to access resources on the TURN
   server.

   Using OAuth, a client obtains an ephemeral token from an
   authorization server e.g. WebRTC server, and the token is presented
   to the TURN server instead of the traditional mechanism of presenting
   username/password credentials.  The TURN server validates the
   authenticity of the token and provides required services.

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].

   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.
      Note that the lifetime of the session key is equal to the lifetime
      of 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.

3.  Solution Overview

   This specification uses the token type 'Assertion' (aka self-
   contained token) described in [RFC6819] where all the information
   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 TURN server and the authorization server for
   token validation, thus reducing latency.  The exact mechanism used by
   a client to obtain a token from the OAuth authorization server is
   outside the scope of this document.  For example, a client could make
   an HTTP request to an authorization server to obtain a token that can
   be used to avail TURN services.  The TURN token is returned in JSON,
   along with other OAuth Parameters like token type, mac_key, kid,
   token lifetime etc.  The client is oblivious to the content of the
   token.  The token is embedded within a TURN request sent to the TURN
   server.  Once the TURN server has determined the token is valid, TURN
   services are offered for a determined period of time.

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

                 Figure 1: TURN Third Party Authorization

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

   For example, the client 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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        POST /o/oauth2/token HTTP/1.1
        Host: server.example.com
        Content-Type: application/x-www-form-urlencoded
        kid=22BIjxU93h/IgwEb
        timestamp=1361471629
        grant_type=implicit

                             Figure 2: Request

   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":"mac",
          "expires_in":1800,
          "kid":"22BIjxU93h/IgwEb",
          "mac_key":"v51N62OM65kyMvfTI08O"
        }

                            Figure 3: Response

   Access token and other attributes issued by the authorization server
   are explained in Section 6.2.

4.  Obtaining a Token Using OAuth

   A TURN client should know the authentication capability of the TURN
   server before deciding to use third party authorization with it.  A
   TURN client initially makes a request without any authorization.  If
   the TURN server supports or mandates third party authorization, it
   will return an error message indicating support for third party
   authorization.  The TURN 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 TURN server's
   software.  The TURN servers also includes additional STUN attribute
   THIRD-PARTY-AUTHORIZATION signaling the TURN client that the TURN
   server supports third party authorization.

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   The following mapping of OAuth concepts to WebRTC is used :

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

         Figure 4: 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 (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 (in the mac_key parameter) and key id (kid).
   The TURN client conveys the access token and other OAuth parameters
   learnt from the authorization server to the resource server (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 5.

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

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

                          Figure 5: Interactions

   OAuth in [RFC6749] defines four grant types.  This specification uses
   the OAuth grant type "Implicit" explained in section 1.3.2 of
   [RFC6749] where the WebRTC client is issued an access token directly.
   The scope of the access token explained in section 3.3 of [RFC6749]
   MUST be TURN.

4.1.  Key Establishment

   The TURN and authorization servers establish symmetric key (AS-RS
   key) for confidentiality and authentication key (AUTH key) for
   integrity, using an out of band mechanism.  The Token is encrypted
   using Advanced Encryption Standard (AES) in Cipher Block Chaining
   (CBC) mode, using the AS-RS key for confidentiality.The default AS-RS
   key size is 128 bits, and all implementations MUST support this key
   size.  Implementations MAY also support key sizes of 192 bits and 256
   bits.  Symmetric keys MUST be chosen to ensure that the size of
   encrypted token is not large.  Usage of asymmetric keys will result
   in large encrypted tokens which may not fit into a single STUN
   message.  The integrity of the token is calculated with the AUTH key
   and using HMAC-SHA-256-128 algorithm.  AUTH key length MUST be
   256-bit (section 2.6 of [RFC4868]).  Authorization server MUST also

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   also signal a unique key identifier (kid) to the TURN server which
   will be used to select the appropriate keying material for
   decryption.

   The TURN and Authorization servers MUST establish the symmetric key
   over an authenticated secure channel.  For example, the two servers
   could choose to use Dynamic Symmetric Key Provisioning Protocol
   [RFC6063] or simply use HTTP for this exchange.  The following
   example describes HTTP interactions that could be used to obtain the
   symmetric key.

4.1.1.  HTTP interactions

   To retrieve a new set of symmetric keys, the TURN server makes an
   HTTP GET request to the Authorization server, specifying TURN as the
   service to allocate the symmetric keys for, and optionally specifying
   the domain name/IP Address of the TURN server.  The response is
   returned with content-type "application/json", and consists of a JSON
   object containing the symmetric key.

   Request
   -------

   service - specifies the desired service (turn)
   domain - an optional domain to be associated with the key

   example: GET /?service=turn&domain=turn.example.com

   Response
   --------

   ckey - symmetric key (AS-RS key)
   akey - authentication key (AUTH key)
   ttl - the duration for which the key is valid, in seconds.

   example:
       {
        "ckey" : "ESIzRFVmd4iZABEiM0RVZgKn6WjLaTC1",
        "akey" : "FXAghRMVTzkBGNaaN496523WIISKerLi",
        "ttl" : 86400,
        "kid" :"22BIjxU93h/IgwEb"
       }

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5.  Forming a Request

   When a TURN server responds that third party authorization is
   required, a TURN client re-attempts the request, this time including
   access token and kid values in ACCESS-TOKEN and USERNAME STUN
   attributes.  The TURN client includes a MESSAGE-INTEGRITY attribute
   as the last attribute in the message over the contents of the TURN
   message.  MESSAGE-INTEGRITY attribute is calculated using the long-
   term credentials mechanism specified in section 10.2 of [RFC5389],
   using the "kid" value from the returned JSON for its USERNAME
   attribute, and the mac_key for computing the message integrity.

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 TURN server to inform the client that
   it supports third party authorization.  This attribute is used by the
   TURN server to inform the client that it supports third party
   authorization.  This optional attribute value be a URL that the
   client should contact, to obtain a token for third party
   authorization.  The format for the URL will be as described in
   [RFC3986].  The THIRD-PARTY-AUTHORIZATION attribute is a
   comprehension-optional attribute (see Section 15 from [RFC5389]).

   The TURN server may have tie-up with multiple authorization servers
   so the client MUST provide the TURN server domain name or IP address
   to the authorization server so that it can select the appropriate
   keying material to generate the self-contained token.

6.2.  ACCESS-TOKEN

   The access token is issued by the authorization server.  OAuth 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]), 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 it
   MUST NOT examine the ticket.  The ACCESS-TOKEN attribute is a
   comprehension-optional attribute (see Section 15 from [RFC5389]).

   The token is structured as follows:

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         struct {
             ushort key_length;
             opaque mac_key[key_length];
             opaque timestamp[8];
             long   lifetime;
             opaque mac[16];
         } token;

                   Figure 6: Self-contained token format

   The fields are described below:

   key_length:  Length of the session key. 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
      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
      SHOULD be equal to the "expires_in" parameter defined in section
      4.2.2 of [RFC6749].

   mac:  The Message Authentication Code (MAC) is calculated with AUTH
      key and using HMAC-SHA-256-128 algorithm over key, timestamp and
      lifetime fields to verify the integrity of the access token.

   The token MUST be encoded as defined in Section 4 of [RFC4648] and
   then encrypted using the symmetric long-term key established between
   the resource server and the authorization server, as shown in
   Figure 5 as AS-RS key.  Since the access token is valid for a
   specific period of time the resource server MUST cache it so that it
   need not to be provided in every request within an existing
   allocation.  The access token can be re-used for multiple Allocate
   requests to the same TURN server.  The TURN client MUST only include
   the ACCESS-TOKEN attribute in Allocate and Refresh requests.

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7.  Receiving a request with ACCESS-TOKEN attribute

   The TURN 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  When the TURN server receives a message with a new access token,
      it 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 TURN server calculates HMAC-SHA-256-128 over key, timestamp
      and lifetime fields in the self-contained token and if the
      resulting value does not match the mac field in self-contained
      token then it rejects the request with an error response 401
      (Unauthorized).

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

   o

      *  The received timestamp (timestamp field in self-contained
         token) is checked and the access token is accepted if the
         timestamp is recent enough to the reception time of the TURN
         request (RDnew) using the following formula: Lifetime + Delta >
         abs(RDnew - TSnew).  The RECOMMENDED value for the allowed
         Delta is 5 seconds.  If the timestamp is NOT within the
         boundaries then the TURN server discards the request with error
         response 401 (Unauthorized).

   o  The TURN server uses the mac_key to compute the value for the
      message integrity 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 TURN server continues to process the
      request.  Any response generated by the server MUST include the
      MESSAGE-INTEGRITY attribute, computed using the mac_key.

   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.

8.  Changes to Client

   o  A TURN response is discarded by the client if the value computed
      for message integrity using mac_key does not match the contents of
      the MESSAGE-INTEGRITY attribute.

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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 also use the new token to refresh
      existing allocations.  This way client has to maintain only one
      token per TURN server.

9.  Security Considerations

   When OAuth is used the interaction between the client and the
   authorization server requires Transport Layer Security (TLS) with a
   ciphersuite offering confidentiality protection.  The session key
   MUST NOT be transmitted in clear since this would completely destroy
   the security benefits of the proposed scheme.

   Security considerations discussed in [I-D.ietf-oauth-v2-http-mac] are
   to be taken into account.

10.  IANA Considerations

   IANA is requested to add the following attributes to the STUN
   attribute registry [iana-stun],

   o  THIRD-PARTY-AUTHORIZATION

   o  ACCESS-TOKEN

11.  Acknowledgements

   Authors would like to thank Dan Wing, Pal Martinsen and Oleg
   Moskalenko for comments and review.  The authors would like to give
   special thanks to Brandon Williams for his help.

12.  References

12.1.  Normative References

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

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

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

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

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   [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>.

12.2.  Informative References

   [I-D.ietf-oauth-v2-http-mac]
              Richer, J., Mills, W., Tschofenig, H., and P. Hunt, "OAuth
              2.0 Message Authentication Code (MAC) Tokens", draft-ietf-
              oauth-v2-http-mac-05 (work in progress), January 2014.

   [I-D.ietf-rtcweb-overview]
              Alvestrand, H., "Overview: Real Time Protocols for Brower-
              based Applications", draft-ietf-rtcweb-overview-09 (work
              in progress), February 2014.

   [I-D.ietf-tram-auth-problems]
              Reddy, T., R, R., Perumal, M., and A. Yegin, "Problems
              with STUN Authentication for TURN", draft-ietf-tram-auth-
              problems-00 (work in progress), March 2014.

   [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.

   [RFC6063]  Doherty, A., Pei, M., Machani, S., and M. Nystrom,
              "Dynamic Symmetric Key Provisioning Protocol (DSKPP)", RFC
              6063, December 2010.

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

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

Reddy, et al.           Expires October 25, 2014               [Page 14]