TRAM T. Reddy
Internet-Draft P. Patil
Intended status: Standards Track R. Ravindranath
Expires: August 10, 2015 Cisco
J. Uberti
Google
February 6, 2015
Session Traversal Utilities for NAT (STUN) Extension for Third Party
Authorization
draft-ietf-tram-turn-third-party-authz-10
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
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This Internet-Draft will expire on August 10, 2015.
Copyright Notice
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document authors. All rights reserved.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 3
4. Obtaining a Token Using OAuth . . . . . . . . . . . . . . . . 4
4.1. Key Establishment . . . . . . . . . . . . . . . . . . . . 4
4.1.1. DSKPP . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1.2. HTTP interactions . . . . . . . . . . . . . . . . . . 5
4.1.3. Manual provisioning . . . . . . . . . . . . . . . . . 6
5. Forming a Request . . . . . . . . . . . . . . . . . . . . . . 7
6. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. THIRD-PARTY-AUTHORIZATION . . . . . . . . . . . . . . . . 7
6.2. ACCESS-TOKEN . . . . . . . . . . . . . . . . . . . . . . 7
7. STUN Server Behaviour . . . . . . . . . . . . . . . . . . . . 9
8. STUN Client Behaviour . . . . . . . . . . . . . . . . . . . . 10
9. Usage with TURN . . . . . . . . . . . . . . . . . . . . . . . 11
10. Operational Considerations . . . . . . . . . . . . . . . . . 14
11. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
14.1. Normative References . . . . . . . . . . . . . . . . . . 15
14.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Sample tickets . . . . . . . . . . . . . . . . . . . 17
Appendix B. Interaction between client and authorization server 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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,
ensuring this secrecy is typically impossible.
To address this problem and the ones described in [RFC7376], this
document proposes the use of third party authorization using OAuth
2.0 [RFC6749] for STUN. Using OAuth 2.0, a client obtains an
ephemeral token from an authorization server e.g. WebRTC server, and
the token is presented to the STUN server instead of the traditional
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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].
o WebRTC Server: A web server that supports WebRTC
[I-D.ietf-rtcweb-overview].
o Access Token: OAuth 2.0 access token.
o mac_key: The session key generated by the authorization server.
This session key has a lifetime that corresponds to the lifetime
of the access token, is generated by the authorization server and
bound to the access token.
o kid: An ephemeral and unique key identifier. The kid also allows
the resource server to select the appropriate keying material for
decryption.
Some sections in this specification show WebRTC server as the
authorization server and client as the WebRTC client, however WebRTC
is intended to be used for illustrative purpose only.
3. Solution Overview
STUN client knows that it can use OAuth 2.0 with the target STUN
server either through configuration or when it receives the new STUN
attribute THIRD-PARTY-AUTHORIZATION in the error response with an
error code of 401(Unauthorized).
This specification uses the token type 'Assertion' (aka self-
contained token) described in [RFC6819] where all the information
necessary to authenticate the validity of the token is contained
within the token itself. This approach has the benefit of avoiding a
protocol between the STUN server and the authorization server for
token validation, thus reducing latency. The content of the token is
opaque to the client. The client embeds the token within a STUN
request sent to the STUN server. Once the STUN server has determined
the token is valid, its services are offered for a determined period
of time. Access token issued by the authorization server is
explained in Section 6.2. OAuth 2.0 in [RFC6749] defines four grant
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types. This specification uses the OAuth 2.0 grant type "Implicit"
explained in section 1.3.2 of [RFC6749] where the client is issued an
access token directly. The value of the scope parameter explained in
section 3.3 of [RFC6749] MUST be string 'stun'.
The exact mechanism used by a client to obtain a token from the OAuth
2.0 authorization server 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.
4. Obtaining a Token Using OAuth
A STUN client MUST know the authentication capability of the STUN
server before deciding to use third party authorization. A STUN
client initially makes a request without any authorization. If the
STUN server supports third party authorization, it will return an
error message indicating that the client can authorize to the STUN
server using an OAuth 2.0 access token. The STUN server includes an
ERROR-CODE attribute with a value of 401 (Unauthorized), a nonce
value in a NONCE attribute and a SOFTWARE attribute that gives
information about the STUN server's software. The STUN server also
includes the additional STUN attribute THIRD-PARTY-AUTHORIZATION
signaling the STUN client that the STUN server supports third party
authorization.
Note: An implementation may choose to contact the authorization
server to obtain a token even before it makes a STUN request, if it
knows the server details before hand. For example, once a client has
learnt that a STUN server supports third party authorization from a
authorization server, the client can obtain the token before making
subsequent STUN requests.
4.1. Key Establishment
The authorization server shares a long-term secret (like asymmetric
credentials) with the STUN server for mutual authentication.
Symmetric-key algorithm with Hash based Message Authentication Codes
(HMACs) MUST be chosen to ensure that the size of encrypted token is
not large because usage of asymmetric keys will result in large
encrypted tokens which may not fit into a single STUN message.
The STUN server and authorization server can establish a symmetric
key (K), using an out of band mechanism. If symmetric key 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.
The STUN and authorization servers MUST establish the symmetric key
over an authenticated secure channel. The procedure for
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establishment of the symmetric key is outside the scope of this
specification. For example, implementations could use one of the
following mechanisms to establish a symmetric key.
4.1.1. DSKPP
The two servers could choose to use Dynamic Symmetric Key
Provisioning Protocol (DSKPP) [RFC6063] to establish a symmetric key
(K). The encryption and MAC algorithms will be negotiated using the
KeyProvClientHello, KeyProvServerHello messages. A unique key
identifier (referred to as KeyID) for the symmetric key is generated
by the DSKPP server (i.e., Authorization server) and signalled to the
DSKPP client (i.e., STUN server) which is equivalent to the kid
defined in this specification. The AS-RS, AUTH keys would be derived
from the symmetric key using (HMAC)-based key derivation function
(HKDF) [RFC5869] and the default hash function MUST be SHA-256. For
example if the input symmetric key (K) is 32 octets length,
encryption algorithm is AES_256_CBC and HMAC algorithm is HMAC-SHA1
[RFC2104] then the secondary keys AS-RS, AUTH are generated from the
input key K as follows
1. HKDF-Extract(zero, K) -> PRK
2. HKDF-Expand(PRK, "AUTH key", 20) -> AUTH key
3. HKDF-Expand(PRK, "AS-RS key", 32) -> AS-RS key
If Authenticated Encryption with Associated Data (AEAD) algorithm
defined in [RFC5116] is used then there is no need to generate the
AUTH key.
4.1.2. HTTP interactions
The two servers could choose to use REST API over HTTPS to establish
a symmetric key. HTTPS MUST be used for mutual authentication and
confidentiality. To retrieve a new symmetric key, the STUN server
makes an HTTP GET request to the authorization server, specifying
STUN as the service to allocate the symmetric keys for, and
specifying the name of the STUN server. The response is returned
with content-type "application/json", and consists of a JSON object
containing the symmetric key.
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Request
-------
service - specifies the desired service (turn)
name - STUN server name be associated with the key
example: GET /?service=stun&name=turn1@example.com
Response
--------
key - Long-term key (K)
ttl - the duration for which the key is valid, in seconds.
example:
{
"key" :
"ESIzRFVmd4iZABEiM0RVZgKn6WjLaTC1FXAghRMVTzkBGNaaN496523WIISKerLi",
"ttl" : 86400,
"kid" :"22BIjxU93h/IgwEb"
"enc" : A256CBC-HS512
}
The AS-RS, AUTH keys are derived from K using HKDF as discussed in
Section 4.1.1. The authorization server must also signal kid to the
STUN server which will be used to select the appropriate keying
material for decryption. A256CBC-HS512 and other encryption
algorithms are defined in [I-D.ietf-jose-json-web-algorithms]. In
this case AS-RS key length must be 256-bit, AUTH key length must be
256-bit (section 2.6 of [RFC4868]).
4.1.3. Manual provisioning
The STUN and authorization servers could be manually configured with
a symmetric key (K) and kid. If manual provisioning is supported,
support MUST also be provided for AES_256_CBC_HMAC_SHA_512 (explained
in [I-D.ietf-jose-json-web-algorithms]) as the authenticated
encryption algorithm .
Note : The mechanism specified in Section 4.1.3 is easy to implement
and deploy compared to DSKPP, REST but lacks encryption and HMAC
algorithm agility.
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5. Forming a Request
When a STUN server responds that third party authorization is
required, a STUN client re-attempts the request, this time including
access token and kid values in ACCESS-TOKEN and USERNAME STUN
attributes. The STUN client includes a MESSAGE-INTEGRITY attribute
as the last attribute in the message over the contents of the STUN
message. The HMAC for the MESSAGE-INTEGRITY attribute is computed as
described in section 15.4 of [RFC5389] where the mac_key is used as
the input key for the HMAC computation. The STUN client and server
will use the mac_key to compute the message integrity and do not
perform MD5 hash on the credentials.
6. STUN Attributes
The following new STUN attributes are introduced by this
specification to accomplish third party authorization.
6.1. THIRD-PARTY-AUTHORIZATION
This attribute is used by the STUN server to inform the client that
it supports third party authorization. This attribute value contains
the STUN server name. The STUN server may have tie-ups with multiple
authorization 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.
The THIRD-PARTY-AUTHORIZATION attribute is a comprehension-optional
attribute (see Section 15 from [RFC5389]). If the client is able to
comprehend THIRD-PARTY-AUTHORIZATION it MUST ensure that third party
authorization takes precedence over first party authentication
(explained in section 10 of [RFC5389]). If the client does not
support or is not capable of doing third party authorization then it
defaults to first party authentication.
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 ticket. The ACCESS-TOKEN attribute
is a comprehension-required attribute (see Section 15 from
[RFC5389]).
The token is structured as follows:
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struct {
opaque {
uint16_t key_length;
opaque mac_key[key_length];
uint64_t timestamp;
uint32_t lifetime;
uint8_t padding_length;
uint8_t padding[padding_length];
} encrypted_block;
opaque mac[mac_length];
uint8_t mac_length;
} token;
Figure 1: 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:
key_length: Length of the session key in octets. Key length of
160-bits MUST be supported (i.e., only 160-bit key is used by
HMAC-SHA-1 for message integrity of STUN message). The key length
facilitates the hash agility plan discussed in section 16.3 of
[RFC5389].
mac_key: The session key generated by the authorization server.
timestamp: 64-bit unsigned integer field containing a timestamp.
The value indicates the time since January 1, 1970, 00:00 UTC, by
using a fixed point format. In this format, the integer number of
seconds is contained in the first 48 bits of the field, and the
remaining 16 bits indicate the number of 1/64K fractions of a
second (Native format - Unix).
lifetime: The lifetime of the access token, in seconds. For
example, the value 3600 indicates one hour. The lifetime value
MUST be greater than or equal to the "expires_in" parameter
defined in section 4.2.2 of [RFC6749], otherwise resource server
could revoke the token but the client would assume that the token
has not expired and would not refresh the token.
padding_length: The padding length MUST be such that the total size
of the encrypted_block structure is a multiple of the cipher's
block length.
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padding: Padding that is added to force the length of the plaintext
to be an integral multiple of the block cipher's block length.
encrypted_block: The encrypted_block is encrypted using the
symmetric long-term key established between the STUN server and
the authorization server. Shown in Figure 3 as AS-RS key.
mac: The Hashed Message Authentication Code (HMAC) is calculated
with the AUTH key over the 'encrypted_block' and the STUN server
name (N) conveyed in the THIRD-PARTY-AUTHORIZATION response. This
ensures that the client does not use the same token to gain
illegal access to other STUN servers provided by the same
administrative domain i.e., when multiple STUN servers in a single
administrative domain share the same symmetric key with an
authorization server.
mac_length: Length of the mac field.
An example encryption process is illustrated below. Here C, N denote
Ciphertext and STUN server name respectively.
o C = AES_256_CBC(AS-RS, encrypted_block)
* Initialization vector can be 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 = HMAC-SHA-256-128(AUTH, C | | N)
Encryption is applied before message authentication on the sender
side and conversely on the receiver side. The entire token i.e., the
'encrypted_block' and 'mac' is base64 encoded (see section 4 of
[RFC4648]) and the resulting access token is signaled to the client.
If AEAD algorithm is used then there is no need to explicitly compute
HMAC, the associated data MUST be the STUN server name (N) and the
mac field MUST carry the nonce. The length of nonce for AEAD
algorithms is explained in [RFC5116].
7. STUN Server Behaviour
The STUN server, on receiving a request with ACCESS-TOKEN attribute,
performs checks listed in section 10.2.2 of [RFC5389] in addition to
the following steps to verify that the access token is valid:
o STUN server selects the keying material based on kid signalled in
the USERNAME attribute.
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o It performs the verification of the token message integrity by
calculating HMAC over the encrypted portion in the self-contained
token and STUN server name using AUTH key and if the resulting
value does not match the mac field in the self-contained token
then it rejects the request with an error response 401
(Unauthorized). If AEAD algorithm is used then it has only a
single output, either a plaintext or a special symbol FAIL that
indicates that the inputs are not authentic.
o STUN server obtains the mac_key by retrieving the content of the
access token (which requires decryption of the self-contained
token using the AS-RS key).
o The STUN server verifies that no replay took place by performing
the following check:
* The access token is accepted if the timestamp field (TS) in the
self-contained token is recent enough to the reception time of
the STUN request (RDnew) using the following formula: Lifetime
+ Delta > abs(RDnew - TS). The RECOMMENDED value for the
allowed Delta is 5 seconds. If the timestamp is NOT within the
boundaries then the STUN server discards the request with error
response 401 (Unauthorized).
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. Any response generated by the server MUST include the
MESSAGE-INTEGRITY attribute, computed using the mac_key.
If a STUN server receives an ACCESS-TOKEN attribute unexpectedly
(because it had not previously sent out a THIRD-PARTY-AUTHORIZATION),
it will respond with an error code of 420 (Unknown Attribute) as
specified in Section 7.3.1 of [RFC5389].
8. STUN Client Behaviour
o The client looks for the MESSAGE-INTEGRITY attribute in the
response. If MESSAGE-INTEGRITY is absent or the value computed
for message integrity using mac_key does not match the contents of
the MESSAGE-INTEGRITY attribute then the response MUST be
discarded.
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o If the access token expires then the client MUST obtain a new
token from the authorization server and use it for new STUN
requests.
9. Usage with TURN
Traversal Using Relay NAT (TURN) [RFC5766] an extension to the STUN
protocol is often used to improve the connectivity of P2P
applications. TURN ensures that a connection can be established even
when one or both sides is incapable of a direct P2P connection.
However, as a relay service, it imposes a nontrivial cost on the
service provider. Therefore, access to a TURN service is almost
always access-controlled. In order to achieve third party
authorization, a resource owner e.g. WebRTC server, authorizes a
TURN client to access resources on the TURN server.
Consider the following example that illustrates the use of OAuth 2.0
to achieve third party authorization for TURN. In this example, a
resource owner i.e., 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 2: 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 and kid. The TURN client conveys the access
token and other OAuth 2.0 parameters learnt from the authorization
server to the TURN server. The TURN server obtains the session key
from the access token. The TURN server validates the token, computes
the message integrity of the request and takes appropriate action
i.e, permits the TURN client to create allocations. This is shown in
an abstract way in Figure 3.
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+---------------+
| +<******+
+------------->| Authorization | *
| | Server | *
| +----------|(WebRTC Server)| * AS-RS,
| | | | * AUTH keys
(2) | | +---------------+ * (1)
Access | | (3) *
Token | | Access Token *
Request | | + *
| | Session Key *
| | *
| V V
+-------+---+ +-+----=-----+
| | (4) | |
| | TURN Request + Access | |
| WebRTC | Token | TURN |
| Client |---------------------->| Server |
| (Alice) | Allocate Response (5) | |
| |<----------------------| |
+-----------+ +------------+
User : Alice
****: Out-of-Band Long-Term Key Establishment
Figure 3: Interactions
In the below figure, the client sends an Allocate request to the
server without credentials. Since the server requires that all
requests be authenticated using OAuth 2.0, the server rejects the
request with a 401 (Unauthorized) error code and STUN attribute
THIRD-PARTY-AUTHORIZATION. The WebRTC client obtains access token
from the WebRTC server and then tries again, this time including
access token. This time, the server validates the token, accepts the
Allocate request and returns an Allocate success response containing
(amongst other things) the relayed transport address assigned to the
allocation.
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+-------------------+ +--------+ +---------+
| ......... TURN | | TURN | | WebRTC |
| .WebRTC . Client | | | | |
| .Client . | | Server | | Server |
| ......... | | | | |
+-------------------+ +--------+ +---------+
| | Allocate request | |
| |------------------------------------------>| |
| | | |
| | Allocate error response | |
| | (401 Unauthorized) | |
| |<------------------------------------------| |
| | THIRD-PARTY-AUTHORIZATION | |
| | | |
| | | |
| | HTTP Request for token | |
|------------------------------------------------------------>|
| | HTTP Response with token parameters | |
|<------------------------------------------------------------|
|OAuth 2.0 | |
Attributes | |
|------>| | |
| | Allocate request ACCESS-TOKEN | |
| |------------------------------------------>| |
| | | |
| | Allocate success response | |
| |<------------------------------------------| |
| | TURN Messages | |
| | ////// integrity protected ////// | |
| | ////// integrity protected ////// | |
| | ////// integrity protected ////// | |
Figure 4: TURN Third Party Authorization
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:
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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 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 4.1.3) is used to establish
symmetric keys, the necessary information which includes long-term
key (K), encryption and HMAC algorithms have to be configured on
each authorization server and STUN server for each kid. The
client obtains the session key and HMAC algorithm from the
authorization server in company with the token.
o When a STUN client sends a request to get access to a particular
STUN server (S) the authorization server must ensure that it
selects the appropriate kid, access-token depending on the server
S.
11. Security Considerations
When OAuth 2.0 is used the interaction between the client and the
authorization server requires Transport Layer Security (TLS) with a
ciphersuite offering confidentiality protection. The session key
MUST NOT be transmitted in clear since this would completely destroy
the security benefits of the proposed scheme. An attacker trying to
replay message with ACCESS-TOKEN attribute can be mitigated by
frequent changes of nonce value as discussed in section 10.2 of
[RFC5389]. The client may know some (but not all) of the token
fields encrypted with a unknown secret key and the token can be
subjected to known-plaintext attack, but AES is secure against this
attack.
An attacker may remove the THIRD-PARTY-AUTHORIZATION STUN attribute
from the error message forcing the client to pick first party
authentication, this attack may be mitigated by opting for Transport
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Layer Security (TLS) [RFC5246] or Datagram Transport Layer Security
(DTLS) [RFC6347] as a transport protocol for Session Traversal
Utilities for NAT (STUN), as defined in [RFC5389]and [RFC7350].
Threat mitigation discussed in section 5 of
[I-D.ietf-oauth-pop-architecture] and security considerations in
[RFC5389] are to be taken into account.
12. IANA Considerations
[Paragraphs below in braces should be removed by the RFC Editor upon
publication]
[IANA is requested to add the following attributes to the STUN
attribute registry [iana-stun], The THIRD-PARTY-AUTHORIZATION
attribute requires that IANA allocate a value in the "STUN attributes
Registry" from the comprehension-optional range (0x8000-0xBFFF)]
This document defines the THIRD-PARTY-AUTHORIZATION STUN attribute,
described in Section 6. IANA has allocated the comprehension-
optional codepoint TBD for this attribute.
[The ACCESS-TOKEN attribute requires that IANA allocate a value in
the "STUN attributes Registry" from the comprehension-required range
(0x0000-0x3FFF)]
This document defines the ACCESS-TOKEN STUN attribute, described in
Section 6. IANA has allocated the comprehension-required codepoint
TBD for this attribute.
13. Acknowledgements
Authors would like to thank Dan Wing, Pal Martinsen, Oleg Moskalenko,
Charles Eckel, Spencer Dawkins, Hannes Tschofenig, Yaron Sheffer and
Tom Taylor for comments and review. The authors would like to give
special thanks to Brandon Williams for his help.
Thanks to Oleg Moskalenko for providing ticket samples in the
Appendix section.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4868] Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
384, and HMAC-SHA-512 with IPsec", RFC 4868, May 2007.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[iana-stun]
IANA, , "IANA: STUN Attributes", April 2011,
<http://www.iana.org/assignments/stun-parameters/stun-pa
rameters.xml>.
14.2. Informative References
[I-D.ietf-jose-json-web-algorithms]
Jones, M., "JSON Web Algorithms (JWA)", draft-ietf-jose-
json-web-algorithms-40 (work in progress), January 2015.
[I-D.ietf-oauth-pop-architecture]
Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
Architecture", draft-ietf-oauth-pop-architecture-00 (work
in progress), July 2014.
[I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
"OAuth 2.0 Proof-of-Possession: Authorization Server to
Client Key Distribution", draft-ietf-oauth-pop-key-
distribution-00 (work in progress), July 2014.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-13
(work in progress), November 2014.
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[I-D.ietf-tram-stunbis]
Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", draft-ietf-tram-stunbis-00
(work in progress), November 2014.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, May 2010.
[RFC6063] Doherty, A., Pei, M., Machani, S., and M. Nystrom,
"Dynamic Symmetric Key Provisioning Protocol (DSKPP)", RFC
6063, December 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6819] Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
January 2013.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
Layer Security (DTLS) as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, August 2014.
[RFC7376] Reddy, T., Ravindranath, R., Perumal, M., and A. Yegin,
"Problems with Session Traversal Utilities for NAT (STUN)
Long-Term Authentication for Traversal Using Relays around
NAT (TURN)", RFC 7376, September 2014.
Appendix A. Sample tickets
Input data (same for all samples below):
//STUN SERVER NAME
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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 when AEAD is used
aead_nonce = \x68\x34\x6a\x03\x6b\x32\x6c\x32\6E\0x4\0x62\0x05;
Samples:
1)
hkdf hash function = SHA-256,
token encryption algorithm = AES-256-CBC
token auth algorithm = HMAC-SHA-256
Result:
AS_RS key (32 bytes) = \xd\x7e\x54\x5b\x7e\x15\xc9\x81\x8c\x81\x4b\x83
\xdc\x4e\xce\x24\x55\xde\x73\xe\xab\x8\x8a\x94
\xc4\x29\xab\x45\xfd\x61\xa\xb5
AUTH key (32 bytes) = \xd\x7e\x54\x5b\x7e\x15\xc9\x81\x8c\x81\x4b\x83
\xdc\x4e\xce\x24\x55\xde\x73\xe\xab\x8\x8a\x94
\xc4\x29\xab\x45\xfd\x61\xa\xb5
Encrypted token (80 bytes = 48+32) =
\x1b\xb6\x4b\x4f\xbf\x99\x6d\x60\x55\xda\xf3\x9f\xa1\xed\x3\x73\x4e
\x1c\x95\x64\x84\xc1\xeb\xc3\x63\x9b\x70\xe6\xb8\x21\x45\xe6\x45\xa0
\x23\xaf\xc1\xee\x87\x91\x7b\xea\xb8\x4a\x7f\x80\xb2\x0\xa5\xad\x14
\x97\x17\xf9\xbc\xfa\xa1\xc6\x2f\x4d\xfc\xaf\xc1\xc5\x11\xc5\x55\x7d
\xb0\x35\x58\xcf\xc6\xce\x6e\x10\x7\xd1\x98\xbd
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2)
hkdf hash function = SHA-256,
token encryption algorithm = AEAD_AES_256_GCM
token auth algorithm = N/A
Result:
AS_RS key (32 bytes) = \xd\x7e\x54\x5b\x7e\x15\xc9\x81\x8c\x81\x4b\x83
\xdc\x4e\xce\x24\x55\xde\x73\xe\xab\x8\x8a\x94
\xc4\x29\xab\x45\xfd\x61\xa\xb5
AUTH key = N/A
Encrypted token (62 bytes = 34 + 16 + 12) =
\xa8\x52\x90\x64\xc7\xd9\x3b\x6c\xe\x9\xe\xcf\x9e\x7d\x0\x70\x47\xe2
\x99\x8d\xe3\x31\xe1\x39\x20\xed\x88\x90\x4\xd8\xcf\x82\x93\x3f\xc6\
x4\xd1\xaa\xe6\xf5\x62\xea\x3c\x94\x45\x8\x3d\xfa\xe9\x5f\x68\x34\x6a
\x33\x6b\x32\x6c\x32\x6e\x34\x62\x35
3)
hkdf hash function = SHA-1,
token encryption algorithm = AES-128-CBC
token auth algorithm = HMAC-SHA-256-128
Result:
AS_RS key (16 bytes) = \x8c\x48\x5f\x1e\x1\x3a\xc6\x50\x36\x70\x84\x37
\xa5\x4e\xd7\x70
AUTH key (32 bytes) = \x8c\x48\x5f\x1e\x1\x3a\xc6\x50\x36\x70\x84\x37
\xa5\x4e\xd7\x70\x17\xcc\xcd\xa1\x7c\xd7\x8\x39
\xfa\xc8\xee\x14\xf9\x77\xb4\xcf
Encrypted token (64 bytes = 48+16) =
\x13\xcd\x17\x4a\xde\x54\xe1\xe6\x65\xe6\xbb\x3a\xb9\x4d\x1c\xf7\x3b
\x60\x31\x8b\xc4\x7\x4b\x3b\x5f\x1c\xda\xf4\x60\x4\x7\x88\x8e\xc9\xc7
\xd3\xf4\x71\x94\x87\x85\xd9\xad\xf7\x6a\xda\x77\x4e\x11\x13\x8d\x8e
\xe8\x93\x9\x76\xa3\x85\x96\x1f\x5e\xd3\xc4\x55
Figure 5: Sample tickets
Appendix B. Interaction between client and authorization server
Client could make 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
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OAuth 2.0 parameters like token type, key, token lifetime and kid
defined in [I-D.ietf-oauth-pop-key-distribution].
+-------------------+ +--------+ +---------+
| ......... STUN | | STUN | | WebRTC |
| .WebRTC . Client | | | | |
| .Client . | | Server | | Server |
| ......... | | | | |
+-------------------+ +--------+ +---------+
| | STUN request | |
| |------------------------------------------>| |
| | | |
| | STUN error response | |
| | (401 Unauthorized) | |
| |<------------------------------------------| |
| | THIRD-PARTY-AUTHORIZATION | |
| | | |
| | | |
| | HTTP Request for token | |
|------------------------------------------------------------>|
| | HTTP Response with token parameters | |
|<------------------------------------------------------------|
|OAuth 2.0 | |
Attributes | |
|------>| | |
| | STUN request with ACCESS-TOKEN | |
| |------------------------------------------>| |
| | | |
| | STUN success response | |
| |<------------------------------------------| |
| | STUN Messages | |
| | ////// integrity protected ////// | |
| | ////// integrity protected ////// | |
| | ////// integrity protected ////// | |
Figure 6: STUN Third Party Authorization
[I-D.ietf-oauth-pop-key-distribution] describes the interaction
between the client and the authorization server. For example, the
client learns the STUN server name "stun1@example.com" from THIRD-
PARTY-AUTHORIZATION attribute value and makes the following HTTP
request for the access token using transport-layer security (with
extra line breaks for display purposes only):
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HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
aud=stun1@example.com
timestamp=1361471629
grant_type=implicit
token_type=pop
alg=HMAC-SHA-1 HMAC-SHA-256-128
Figure 7: Request
[I-D.ietf-tram-stunbis] will support hash agility and accomplish this
agility by conveying the HMAC algorithms supported by the STUN server
along with a STUN error message to the client. The client then
signals the intersection-set of algorithms supported by it and the
STUN server to the authorization server in the 'alg' parameter
defined in [I-D.ietf-oauth-pop-key-distribution]. The authorization
server selects an HMAC algorithm from the list of algorithms the
client provided and determines length of the mac_key based on the
selected HMAC algorithm. Note that until STUN supports hash agility
HMAC-SHA1 is the only valid hash algorithm that the client can signal
to the authorization server and vice-versa.
If the client is authorized then the authorization server issues an
access token. An example of successful response:
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"access_token":
"U2FsdGVkX18qJK/kkWmRcnfHglrVTJSpS6yU32kmHmOrfGyI3m1gQj1jRPsr0uBb
HctuycAgsfRX7nJW2BdukGyKMXSiNGNnBzigkAofP6+Z3vkJ1Q5pWbfSRroOkWBn",
"token_type":"pop",
"expires_in":1800,
"kid":"22BIjxU93h/IgwEb",
"key":"v51N62OM65kyMvfTI08O"
"alg":HMAC-SHA-256-128
}
Figure 8: Response
Authors' Addresses
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Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
Email: praspati@cisco.com
Ram Mohan Ravindranath
Cisco Systems, Inc.
Cessna Business Park,
Kadabeesanahalli Village, Varthur Hobli,
Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: rmohanr@cisco.com
Justin Uberti
Google
747 6th Ave S
Kirkland, WA
98033
USA
Email: justin@uberti.name
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