Internet Engineering Task Force (IETF) T. Reddy

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.

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RFC 7635 STUN for Third-Party Authorization August 2015

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

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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

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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.

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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 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.

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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 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.

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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,
which signals 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 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

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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
&[email protected]

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]).

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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]).

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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
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.

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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),

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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)

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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.

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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

Reddy, et al. Standards Track [Page 15]

RFC 7635 STUN for Third-Party Authorization August 2015

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>.

Reddy, et al. Standards Track [Page 16]

RFC 7635 STUN for Third-Party Authorization August 2015

[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>.

Reddy, et al. Standards Track [Page 17]

RFC 7635 STUN for Third-Party Authorization August 2015

[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>.

Reddy, et al. Standards Track [Page 18]

RFC 7635 STUN for Third-Party Authorization August 2015

[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.

Reddy, et al. Standards Track [Page 19]

RFC 7635 STUN for Third-Party Authorization August 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

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RFC 7635 STUN for Third-Party Authorization August 2015

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

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RFC 7635 STUN for Third-Party Authorization August 2015

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 "[email protected]" 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):

Reddy, et al. Standards Track [Page 22]

RFC 7635 STUN for Third-Party Authorization August 2015

HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
[email protected]
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

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RFC 7635 STUN for Third-Party Authorization August 2015

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: [email protected]

Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
Email: [email protected]

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

Justin Uberti
Google
747 6th Ave S.
Kirkland, WA 98033
United States
Email: [email protected]

Reddy, et al. Standards Track [Page 24]