STUN Third-Party Auth: Session Traversal Utilities for NAT (STUN) Extension for Third-Party Authorization

Internet Engineering Task Force (IETF)                          T. Reddy
Request for Comments: 7635                                      P. Patil
Category: Standards Track                                R. Ravindranath
ISSN: 2070-1721                                                    Cisco
                                                               J. Uberti
                                                                  Google
                                                             August 2015


          Session Traversal Utilities for NAT (STUN) Extension
                     for Third-Party Authorization

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 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/rfc7635.

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.

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 Behavior  . . . . . . . . . . . . . . . . . . . .  13
   8.  STUN Client Behavior  . . . . . . . . . . . . . . . . . . . .  14
   9.  TURN Client and Server Behavior . . . . . . . . . . . . . . .  14
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  15
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Well-Known 'stun-key' URI  . . . . . . . . . . . . . . .  16
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     13.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Sample Tickets . . . . . . . . . . . . . . . . . . .  20
   Appendix B.  Interaction between the Client and Authorization
                Server . . . . . . . . . . . . . . . . . . . . . . .  22
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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 [WEBRTC] where JavaScript uses the browser functionality
   for making real-time audio and/or video calls, web conferencing, and
   direct data transfer, ensuring this secrecy is typically not
   possible.

   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., a 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 [WEBRTC].

   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
      is 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.

   o  RS: Resource server.

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

3.  Solution Overview

   The 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' (a.k.a.  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 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.  The 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'
   as 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 peer-to-peer (P2P) applications.  TURN ensures that a
   connection can be established even when one or both sides are
   incapable of a direct P2P connection.  However, as a relay service,
   it imposes a non-trivial 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., a
   WebRTC server, authorizes a TURN client to access resources on the
   TURN server.

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

           +----------------------+----------------------------+
           |     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 the
   access token, lifetime, session key, and kid.  The TURN client
   conveys the access token and other OAuth 2.0 parameters learned 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.

                           +---------------+
                           |               +<******+
            +------------->| 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 Symmetric 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 the STUN
   attribute THIRD-PARTY-AUTHORIZATION.  The WebRTC client obtains an
   access token from the WebRTC server, provides the access token to the
   TURN client, and it tries again, this time including the 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 (among other things) the relayed transport
   address assigned to the allocation.

   +-------------------+                         +--------+  +---------+
   | .........  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,
   which signals the STUN client that the STUN server supports third-
   party authorization.

   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 beforehand.  For example, once a client has
   learned 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.  An example of this
   approach can be found with the OAuth 2.0 Proof-of-Possession (PoP)
   Security Architecture [POP-ARCH].  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 the 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 long-term
   symmetric key (K) and a 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
   [ENCRYPT]) 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 the Authenticated Encryption with Associated Data
   (AEAD) algorithm defined in [RFC5116] is used, then there is no need
   to generate the AUTH key, and the AS-RS key will have the same value
   as K.

   The procedure for establishment of the long-term symmetric key is
   outside the scope of this specification, and this specification does
   not mandate support of any given mechanism.  Sections 4.1.1 and 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 Representational State
   Transfer (REST) API over HTTPS to establish a long-term symmetric
   key.  HTTPS MUST be used for data confidentiality, and TLS based on a
   client certificate MUST be used for mutual authentication.  To
   retrieve a new long-term symmetric key, the STUN server makes an HTTP
   GET request to the authorization server, specifying STUN as the
   service to allocate the long-term 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 long-term symmetric key.

   Request
   -------

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

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


   Response
   --------

   k   - long-term symmetric key
   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
   [RFC7518], 'enc' is defined in Section 4.1.2 of [RFC7516], 'kid' is
   defined in Section 4.1.4 of [RFC7515], and 'exp' is defined in
   Section 4.1.4 of [RFC7519].  A256GCM and other authenticated
   encryption algorithms are defined in Section 5.1 of [RFC7518].  A
   STUN server and authorization server implementation MUST support
   A256GCM as the authenticated encryption algorithm.

   If A256CBC-HS512 as defined in [RFC7518] is used, then the AS-RS and
   AUTH keys are derived from K using the mechanism explained in
   Section 5.2.2.1 of [RFC7518].  In this case, the AS-RS key length
   must be 256 bits and the AUTH key length must be 256 bits
   (Section 2.6 of [RFC4868]).

4.1.2.  Manual Provisioning

   The STUN and AS servers could be manually configured with a long-term
   symmetric key, an authenticated encryption algorithm, and kid.

   Note: The mechanism specified in this section requires configuration
   to change the long-term symmetric key and/or authenticated encryption
   algorithm.  Hence, a STUN server and authorization server
   implementation SHOULD support REST as 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 the 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 an 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 (as
   explained in Section 10 of [RFC5389]).

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
      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.  The key length of
      160 bits MUST be supported (i.e., only the 160-bit key is used by
      HMAC-SHA-1 for message integrity of STUN messages).  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 1/64000 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 the 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 long-term symmetric 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 of
   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 long-term symmetric key with an authorization server.

   If authenticated encryption with AES-CBC and HMAC-SHA (explained in
   Section 2.1 of [ENCRYPT]) is used, then the encryption process is as
   illustrated below.  The ciphertext consists of the string S, with the
   string T appended to it.  Here, C and A denote ciphertext and the
   STUN server name, respectively.  The octet string AL (Section 2.1 of
   [ENCRYPT]) 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),

      *  The 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,

   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 Behavior

   The STUN server, on receiving a request with the 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  The STUN server selects the keying material based on kid signaled
      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].  The 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 the 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 STUN server name,
      the encrypted portion in the self-contained token, and the AL
      using the 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  The 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 shortly before the reception time of
         the STUN request (RDnew).  The following formula is used:

            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).

   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 Behavior

   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 Behavior

   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.

   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, the 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 long-
      term symmetric keys, the necessary information, which includes
      long-term secret (K) and the authenticated encryption algorithm,
      has 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 and access token depending on 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 the message with the ACCESS-TOKEN attribute
   can be mitigated by frequent changes of the nonce value as discussed
   in Section 10.2 of [RFC5389].  The client may know some (but not all)
   of the token fields encrypted with an unknown secret key, and the
   token can be subjected to known-plaintext attacks, 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 TLS
   [RFC5246] or Datagram Transport Layer Security (DTLS) [RFC6347] as a
   transport protocol for STUN, as defined in [RFC5389]and [RFC7350].

   Threat mitigation discussed in Section 5 of [POP-ARCH] and security
   considerations in [RFC5389] are to be taken into account.

12.  IANA Considerations

   This document defines the THIRD-PARTY-AUTHORIZATION STUN attribute,
   described in Section 6.  IANA has allocated the comprehension-
   optional codepoint 0x802E for this attribute.

   This document defines the ACCESS-TOKEN STUN attribute, described in
   Section 6.  IANA has allocated the comprehension-required codepoint
   0x001B 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.  References

13.1.  Normative References

   [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>.

   [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>.

   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
              HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868,
              DOI 10.17487/RFC4868, May 2007,
              <http://www.rfc-editor.org/info/rfc4868>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <http://www.rfc-editor.org/info/rfc5389>.

   [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>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <http://www.rfc-editor.org/info/rfc7518>.

13.2.  Informative References

   [ENCRYPT]  McGrew, D., Foley, J., and K. Paterson, "Authenticated
              Encryption with AES-CBC and HMAC-SHA", Work in Progress,
              draft-mcgrew-aead-aes-cbc-hmac-sha2-05, July 2014.

   [POP-ARCH] Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
              Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
              Architecture", Work in Progress,
              draft-ietf-oauth-pop-architecture-02, July 2015.

   [POP-KEY-DIST]
              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", Work in Progress,
              draft-ietf-oauth-pop-key-distribution-01, March 2015.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [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,
              DOI 10.17487/RFC5766, April 2010,
              <http://www.rfc-editor.org/info/rfc5766>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [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>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7350]  Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as Transport for Session Traversal
              Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
              August 2014, <http://www.rfc-editor.org/info/rfc7350>.

   [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, DOI 10.17487/RFC7376, September
              2014, <http://www.rfc-editor.org/info/rfc7376>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <http://www.rfc-editor.org/info/rfc7516>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://www.rfc-editor.org/info/rfc7519>.

   [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, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.

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

   [WEBRTC]   Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", Work in Progress, draft-ietf-
              rtcweb-overview-14, June 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
      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 the Client and Authorization Server

   The 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 as
   defined in [POP-KEY-DIST].

   +-------------------+                         +--------+  +---------+
   | .........  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

   [POP-KEY-DIST] describes the interaction between the client and the
   authorization server.  For example, the client learns the STUN server
   name "stun1@example.com" from the THIRD-PARTY-AUTHORIZATION attribute
   value and makes the following HTTP request for the access token using
   TLS (with extra line breaks for display purposes only):

        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

   [STUN] supports hash agility and accomplishes 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
   [POP-KEY-DIST].  The authorization server determines the 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 a 256-bit
   key from the authorization server, and calculates a 160-bit key for
   HMAC-SHA-1 using SHA1 and taking the 256-bit key as input.

   If the client is authorized, then the authorization server issues an
   access token.  An example of a 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

Acknowledgements

   The 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.

   Thanks to Oleg Moskalenko for providing token samples in Appendix A.

Authors' Addresses

   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
   United States
   Email: justin@uberti.name