| TRAM | T. Reddy |
| Internet-Draft | P. Patil |
| Intended status: Standards Track | R. Ravindranath |
| Expires: August 18, 2014 | Cisco |
| J. Uberti | |
| February 14, 2014 |
TURN Extension for Third Party Authorization
draft-reddy-tram-turn-third-party-authz-00
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. It also addresses the need for stronger authentication described in [I-D.reddy-behave-turn-auth].
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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.reddy-behave-turn-auth], this document proposes the use of third party authorization using OAuth for TURN.
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.
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 specification uses token type 'Handle' (or artifact) described in [RFC6819]. A handle token is a reference to some internal data structure within the OAuth authorization server; the internal data structure contains the attributes of the token such as mac_key, lifetime of the access token etc. 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.
+-------------------+ +--------+ +---------+
| ......... 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 HTTP response from Authorization server:
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"access_token":
"eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJBMTI4Q0JDK0hTMjU2In0.
kwx9txo_sKRasjlXc8RYP-evLCmT1XRXKjtY5l44Gnh0A84hGvVfMxMfCWXh38hi",
"token_type":"mac",
"expires_in":1800,
"refresh_token":"8xLOxBtZp8",
"kid":"22BIjxU93h/IgwEb4zCRu5WF37s=",
"mac_key":"adijq39jdlaska9asud"
}
Figure 2: Example
Handle token type is selected for the following reasons:
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.
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 3: 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 obtains the session key via 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 4.
+---------------+ Token metadata
| + (4)
+------------->| Authorization |------------+
| | Server | |
| +----------|(WebRTC Server)|<------+ |
| | | | | |
(1) | | +--------------+ | |
Access | | (2) | |
Token | | Access Token | |
Request | | + Get Token | |
| | Session Key (3) | |
| | | |
| V | V
+-------+---+ +-+----=-----+
| | (5) | |
| | TURN Request + Access | |
| WebRTC | Token | TURN |
| Client |---------------------->| Server |
| (Alice) | | |
| | | |
+-----------+ +------------+
User : Alice
Figure 4: 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.
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" value for the password input to the MESSAGE-INTEGRITY hash.
The TURN server, on receiving a request, performs checks listed in section 10.2.2 of [RFC5389] in addition to the following steps to verify that the access token is valid:
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.
The following new STUN attributes are introduced by this specification to accomplish 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 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 access token is issued by the authorization server. OAuth does not impose any limitation on the length of the access token but since STUN messages cannot exceed 548 bytes (Section 7.1 of [RFC5389]), access token length needs to be restricted to fit within the maximum STUN message size. The value of ACCESS-TOKEN is a variable-length value. Its length MUST be less than 256 bytes and SHOULD be less than 64 bytes.
Since the access token is valid for a period of time the resource server MUST cache it so that it does not need to be provided in every request from the client. The ACCESS-TOKEN MUST only be included in the first request from the client to the server but MUST NOT be included in a subsequent request/response.
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. The TURN server can also maintain a cache of used kid as an effective countermeasure against replay attacks.
IANA is requested to add the following attributes to the STUN attribute registry [iana-stun],
Authors would like to thank Dan Wing, Pal Martinsen for comments and review.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [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. |
| [RFC3986] | Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. |
| [iana-stun] | IANA, , "IANA: STUN Attributes", April 2011. |
| [I-D.ietf-rtcweb-overview] | Alvestrand, H., "Overview: Real Time Protocols for Brower-based Applications", Internet-Draft draft-ietf-rtcweb-overview-08, September 2013. |
| [I-D.reddy-behave-turn-auth] | Reddy, T., R, R., Perumal, M. and A. Yegin, "Problems with STUN Authentication for TURN", Internet-Draft draft-reddy-behave-turn-auth-04, September 2013. |
| [I-D.richer-oauth-introspection] | Richer, J., "OAuth Token Introspection", Internet-Draft draft-richer-oauth-introspection-04, May 2013. |
| [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. |
| [RFC6819] | Lodderstedt, T., McGloin, M. and P. Hunt, "OAuth 2.0 Threat Model and Security Considerations", RFC 6819, January 2013. |