[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]
Versions: 00 01 02 03
Network Working Group H. Tschofenig
Internet-Draft Nokia Siemens Networks
Intended status: Standards Track July 9, 2012
Expires: January 10, 2013
The OAuth 2.0 Authorization Framework: Holder-of-the-Key Token Usage
draft-tschofenig-oauth-hotk-00.txt
Abstract
OAuth 2.0 deployments currently rely on bearer tokens for securing
access to protected resources. Bearer tokens require Transport Layer
Security to be used between an OAuth client and the resource server
when presenting the access token in order to get access. The
security model is based on proof-of-possession of the access token:
access token storage and transfer has to be done with care to prevent
leakage.
There are, however, use cases that require a more active involvement
of the OAuth client to offer increased security, particularly against
token leakage. This document specifies an OAuth security framework
using ephemeral asymmetric credentials that are bound to the access
token. A client can create these key pairs dynamically and use them,
after they are bound to an access token by the authorization server,
in communication interactions with resource servers.
This document is discussed at
https://www.ietf.org/mailman/listinfo/oauth. This initial version of
the specification shall serve as a discussion starter.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 10, 2013.
Tschofenig Expires January 10, 2013 [Page 1]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Specification . . . . . . . . . . . . . . . . . . . . 5
3.1. Binding a Public Key to an Access Token . . . . . . . . . 5
3.2. Accessing a Protected Resource . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4.1. Security Threats . . . . . . . . . . . . . . . . . . . . . 9
4.2. Threat Mitigation . . . . . . . . . . . . . . . . . . . . 9
4.3. Summary of Recommendations . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5.1. OAuth Parameters Registration . . . . . . . . . . . . . . 11
5.2. The 'hotk' JSON Web Token Claims . . . . . . . . . . . . . 11
5.3. The 'hotk' OAuth Access Token Type . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Tschofenig Expires January 10, 2013 [Page 2]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
1. Introduction
At the time of writing the OAuth 2.0 [1] and accompanying protocols
offer one main security mechanism to access protected resources,
namely the bearer token. In [8] a bearer token is defined as
A security token with the property that any party in possession of
the token (a "bearer") can use the token in any way that any other
party in possession of it can. Using a bearer token does not
require a bearer to prove possession of cryptographic key material
(proof-of-possession).
The bearer token provides sufficient security properties for a number
of use cases OAuth had been designed for, if certain conditions are
met (which are documented in [8]). Some usage scenarios, however,
require stronger security guarantees and ask for active participation
of the client software in form of cryptographic computations when
presenting an access token.
In addition to the bearer token a MAC token has been specified, see
[9]. The design of the MAC token was inspired by features in the
OAuth 1.0 [10]. Unfortunately, the MAC token has not received a lot
of deployment attention.
This specification defines a new security mechanism for usage with
OAuth that combines various existing specifications to offer enhanced
security properties for OAuth. The incredients for this security
solution are:
1. A mechanism for on-the-fly provisioning of ephemeral asymmetric
credentials using the JSON Web Key (JWK) format [2].
2. The ability to access a protected resource using this ephemeral
asymmetric credentials for client authentication using a
transport layer extension that allows out-of-band key validation
[3].
3. A data structure to bind the emphemeral asymmetric credential to
an access token. The structure uses the JSON Web Token (JWT)
[4].
The rest of the document describes how these different components
work together.
Tschofenig Expires January 10, 2013 [Page 3]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
2. Terminology
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
specification are to be interpreted as described in [5].
Tschofenig Expires January 10, 2013 [Page 4]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
3. Protocol Specification
To describe the architecture of the proposed security mechanism it is
best to start by looking at the main OAuth 2.0 protocol exchange
sequence. Figure 1 shows the abstract OAuth 2.0 protocol exchanges
graphically. The exchange in this document will focus on two
interactions, namely
1. to allow the client to obtain the ephemeral asymmetric
credentails in step (D)
2. to use the obtained asymmetric credentials for the interaction
with the resource server in step (E)
+--------+ +---------------+
| |--(A)- Authorization Request ->| Resource |
| | | Owner |
| |<-(B)-- Authorization Grant ---| |
| | +---------------+
| |
| | +---------------+
| |--(C)-- Authorization Grant -->| Authorization |
| Client | | Server |
| |<-(D)----- Access Token -------| |
| | +---------------+
| |
| | +---------------+
| |--(E)----- Access Token ------>| Resource |
| | | Server |
| |<-(F)--- Protected Resource ---| |
+--------+ +---------------+
Figure 1: Abstract OAuth 2.0 Protocol Flow
3.1. Binding a Public Key to an Access Token
OAuth 2.0 offers different ways to obtain an access token, namely
using authorization grants and using a refresh token. The core OAuth
specification defines four authorization grants, see Section 1.3 of
[1], and [11] adds an assertion-based authorization grant to that
list.
This document extends the communication with the token endpoint. The
token endpoint, which is described in Section 3.2 of [1], is used
with every authorization grant except for the implicit grant type
since an access token is issued directly. The request contains
information about the public key the client would like to bind to the
Tschofenig Expires January 10, 2013 [Page 5]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
access token in the JSON Web Key format. This parameter also
provides an indication to the authorization servers about the support
by the client for this specification. Since the client makes a
request to the token endpoint by adding a new of parameters using the
"application/x-www-form-urlencoded" format in the HTTP request
entity-body the public key information must be encoded into a new
parameter, called 'pk-info'. A new token type, called 'hotk', is
also defined by this specification.
For example, the client makes the following HTTP request using TLS
(extra line breaks are for display purposes only):
POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded;charset=UTF-8
grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
&pk-info=eZQQYbYS6WxS...lxlOB
whereby the content of the pk-info field represents the following
structure:
{"keys":
[
{"alg":"RSA",
"mod": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2
QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw",
"exp":"AQAB",
"kid":"2011-04-29"}
]
}
Example Request to the Authorization Server
If the access token request is valid and authorized, the
authorization server issues an access token and optionally a refresh
token. If the request client authentication failed or is invalid,
the authorization server returns an error response as described in
Section 5.2 of [1].
The authorization server also places information about the public key
Tschofenig Expires January 10, 2013 [Page 6]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
used by the client into the access token to create the binding
between the two.
An example successful response:
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"2YotnFZFE....jr1zCsicMWpAA",
"token_type":"hotk",
"expires_in":3600,
"refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA"
}
whereby the content of the 'access_token' field, for example,
contains an encoded JWT with the following raw structure:
{"typ":"JWT",
"alg":"HS256"}
.
{"iss":"authorization-server-id",
"exp":1300819380,
"hotk": {"keys":
[
{"alg":"RSA",
"mod": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2
QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw",
"exp":"AQAB",
"kid":"2011-04-29"}
]
}
}
.
bbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZC
Example Response from the Authorization Server
Tschofenig Expires January 10, 2013 [Page 7]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
3.2. Accessing a Protected Resource
The client accesses protected resources by presenting the access
token to the resource server. It does so via a Transport Layer
Security (TLS) secured channel. Since the client had previously
bound a public key to an access token it selects this key for usage
with TLS as described in [3].
The resource server validates the access token and ensure it has not
expired and that its scope covers the requested resource.
Additionally, the resource server verifies that the public key
presented during the TLS handshake corresponds to the public key that
is contained in the access token.
Note that this step confirms that the client is in possession of the
private key corresponding to the public key previously bound to the
access token. Information about the client authentication may be
contained in the token in case the authorization server added this
information when it authenticated the client.
Tschofenig Expires January 10, 2013 [Page 8]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
4. Security Considerations
4.1. Security Threats
The following list presents several common threats against protocols
utilizing some form of tokens. This list of threats is based on NIST
Special Publication 800-63 [12]. We exclude a discussion of threats
related to any form of registration and authentication.
Token manufacture/modification: An attacker may craft a fake token
or modify the token content (such as the authentication or
attribute statements), causing a resource server to grant
inappropriate access to the attacker. For example, an attacker
may modify the token to extend the validity period or the scope to
have extended access to information.
Token disclosure: Tokens may contain authentication and attribute
statements that include sensitive information.
Token redirect: An attacker uses a token generated for consumption
by one resource server to gain access to a different resource
server that mistakenly believes the token to be for it.
Token reuse: An attacker attempts to use a token that has already
been used with that resource server in the past.
4.2. Threat Mitigation
A large range of threats can be mitigated by protecting the contents
of the access token by using a digital signature or a Message
Authentication Code (MAC). Consequently, the token integrity
protection MUST be sufficient to prevent the token from being
modified.
To deal with token redirect, it is important for the authorization
server to include the identity of the intended recipients (the
audience), typically a single resource server (or a list of resource
servers), in the token. Restricting the use of the token to a
specific scope is also RECOMMENDED.
The authorization server MUST implement TLS. Which version(s) ought
to be implemented will vary over time, and depend on the widespread
deployment and known security vulnerabilities at the time of
implementation. At the time of this writing, TLS version 1.2 [6] is
the most recent version. The client MUST validate the TLS
certificate chain when making requests to protected resources,
including checking the Certificate Revocation List (CRL) [7]. In
addition, this specificatio requires a TLS extension for usage with
Tschofenig Expires January 10, 2013 [Page 9]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
out-of-band validation [3] to be used that allows clients to present
raw public keys.
With the usage of the holder-of-the-key concept it is not possible
for any party other than the legitimate client to use a token and to
re-use it without knowing the corresponding asymmetric key pair.
This mechanism prevents against token disclosure. In some
deployments, including those utilizing load balancers, the TLS
connection to the resource server terminates prior to the actual
server that provides the resource. This could leave the token
unprotected between the front end server where the TLS connection
terminates and the back end server that provides the resource. Even
in such deployments, token leakage is not a problem.
Client implementations must be carefully implemented to avoid leaking
the ephemeral asymmetric key pair.
Token replay is also not possible since an eavesdropper will also
have to obtain the corresponding asymmetric key pair that corresponds
to the access token. Nevertheless, it is good practice to limit the
lifetime of the and therefore the lifetime of the ephemeral
asymmetric key associated with it.
4.3. Summary of Recommendations
The following three items represent the main recommendations:
Safeguard the private key: Client implementations MUST ensure that
the ephemeral private key is not leaked to third parties, since
those will be able to use the access together with the key pair to
gain access to protected resources.
Switch keying material regularly: Clients can at any time create a
new ephemeral key pair and request the public key to be associated
with the access token. For example, a client presents a new
public key when requesting an access token with the help of a
refresh token. Nevertheless, the lifetime of these access token
may be longer than the lifetime of bearer tokens.
Issue scoped bearer tokens: Token servers SHOULD issue bearer tokens
that contain an audience restriction, scoping their use to the
intended relying party or set of relying parties.
Tschofenig Expires January 10, 2013 [Page 10]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
5. IANA Considerations
This document requires IANA to take the following actions.
5.1. OAuth Parameters Registration
This specification registers the 'pk-info' parameter in the OAuth
Parameters Registry established by [1].
Parameter name: pk-info
Parameter usage location: token request
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
5.2. The 'hotk' JSON Web Token Claims
[4] established the IANA JSON Web Token Claims registry for reserved
JWT Claim Names and this document adds the 'hotk' name to that
registry.
5.3. The 'hotk' OAuth Access Token Type
Section 11.1 of [1] defines the OAuth Access Token Type Registry and
this document adds another token type to this registry.
Type name: hotk
Additional Token Endpoint Response Parameters: (none)
HTTP Authentication Scheme(s): Holder of the key confirmation using
TLS
Change controller: IETF
Specification document(s): [[ this document ]]
Tschofenig Expires January 10, 2013 [Page 11]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
6. Acknowledgements
The author would like to thank the OAuth working group and
participants of the Internet Identity Workshop for their discussion
input that lead to this document.
Tschofenig Expires January 10, 2013 [Page 12]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
7. References
7.1. Normative References
[1] Hammer-Lahav, E., Recordon, D., and D. Hardt, "The OAuth 2.0
Authorization Framework", draft-ietf-oauth-v2-28 (work in
progress), June 2012.
[2] Jones, M., "JSON Web Key (JWK)",
draft-ietf-jose-json-web-key-03 (work in progress), July 2012.
[3] Wouters, P., Gilmore, J., Weiler, S., Kivinen, T., and H.
Tschofenig, "TLS Out-of-Band Public Key Validation",
draft-ietf-tls-oob-pubkey-03 (work in progress), April 2012.
[4] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", draft-ietf-oauth-json-web-token-01 (work in progress),
July 2012.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.
[7] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley,
R., and W. Polk, "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile",
RFC 5280, May 2008.
7.2. Informative References
[8] Jones, M., Hardt, D., and D. Recordon, "The OAuth 2.0
Authorization Framework: Bearer Token Usage",
draft-ietf-oauth-v2-bearer-21 (work in progress), June 2012.
[9] Hammer-Lahav, E., "HTTP Authentication: MAC Access
Authentication", draft-ietf-oauth-v2-http-mac-01 (work in
progress), February 2012.
[10] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[11] Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0",
draft-ietf-oauth-assertions-04 (work in progress), July 2012.
[12] Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S., and E.
Tschofenig Expires January 10, 2013 [Page 13]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
Nabbus, "NIST Special Publication 800-63-1, INFORMATION
SECURITY", December 2008.
Tschofenig Expires January 10, 2013 [Page 14]
Internet-Draft OAuth 2.0 HOTK Token Usage July 2012
Author's Address
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Tschofenig Expires January 10, 2013 [Page 15]
Html markup produced by rfcmarkup 1.129d, available from
https://tools.ietf.org/tools/rfcmarkup/