Internet DRAFT - draft-tschofenig-oauth-hotk
draft-tschofenig-oauth-hotk
Network Working Group J. Bradley
Internet-Draft Ping Identity
Intended status: Standards Track P. Hunt
Expires: July 18, 2014 Oracle Corporation
T. Nadalin
Microsoft
H. Tschofenig
January 14, 2014
The OAuth 2.0 Authorization Framework: Holder-of-the-Key Token Usage
draft-tschofenig-oauth-hotk-03.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. The security model is based on
proof-of-possession: 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 for an increased level of security, particularly
to secure against token leakage. This document specifies an OAuth
security framework using the holder-of-the-key concept, which
requires the OAuth client when presenting an OAuth access token to
also demonstrate knowledge of keying material that is bound to the
token.
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 July 18, 2014.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Specification . . . . . . . . . . . . . . . . . . . 3
3.1. Binding a Key to an Access Token . . . . . . . . . . . . 4
3.1.1. Symmetric Keys . . . . . . . . . . . . . . . . . . . 4
3.1.2. Asymmetric Keys . . . . . . . . . . . . . . . . . . . 7
3.2. Accessing a Protected Resource . . . . . . . . . . . . . 9
3.2.1. Symmetric Keys . . . . . . . . . . . . . . . . . . . 9
3.2.2. Asymmetric Keys . . . . . . . . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
4.1. Security Threats . . . . . . . . . . . . . . . . . . . . 12
4.2. Threat Mitigation . . . . . . . . . . . . . . . . . . . . 12
4.3. Summary of Recommendations . . . . . . . . . . . . . . . 13
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
5.1. OAuth Parameters Registration . . . . . . . . . . . . . . 14
5.2. The 'hotk' JSON Web Token Claims . . . . . . . . . . . . 15
5.3. The 'hotk' OAuth Access Token Type . . . . . . . . . . . 15
5.4. Profile Registry . . . . . . . . . . . . . . . . . . . . 15
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Normative References . . . . . . . . . . . . . . . . . . 16
7.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
At the time of writing the OAuth 2.0 [3] and accompanying protocols
offer one main security mechanism to access protected resources,
namely the bearer token. In [12] a bearer token is defined as
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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.
The bearer token meets the security needs of number of use cases
OAuth had been designed for. There are, however, scenarios that
require stronger security properties and ask for active participation
of the OAuth client software in form of cryptographic computations
when presenting an access token to a resource server.
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 dynamic key distribution.
2. Data elements to bind emphemeral keying material to an access
token. For the access token we assume a JSON Web Token (JWT) [6]
in this specification to specify a complete solution. Future
specifications may make this functionality available to other
access token formats as well.
3. A mechanism to allow the OAuth client to demonstrate a proof of
possession.
The rest of the document describes how these different components
work together.
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 [1].
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)
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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 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
[3], 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 [3], is used
with every authorization grant except for the implicit grant type.
In the implicit grant type the access token is issued directly.
Two types of keying material can be bound to an access token, namely
symmetric keys and asymmetric keys, and we explain them in separate
sub-sections.
3.1.1. Symmetric Keys
In case a symmetric key shall be bound to an access token then the
following procedure is applicable. In the request message from the
OAuth client to the authorization server the following parameters
MUST be included:
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token_type: REQUIRED. For the symmetric holder-of-the-key variant
the value MUST be set to "hotk-sk".
profile: REQUIRED. The profile parameter provides information about
what mechanisms the client supports to provide proof of
possession of the key towards a resource server. The value
MUST be taken from the algorithm registry created in
Section 5.4. Algorithm names are case-sensitive. If the
client supports more than one profile then each individual
value MUST be separated by a comma.
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
&token_type=hotk-sk
&profile=jws,mac
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 [3].
The authorization server MUST include the following parameters in a
successful response, if it supports any of the profiles listed by the
client.
id: REQUIRED. An ephemeral and unique key identifier. The
authorization server MUST NOT select the same key identifier
twice within the lifetime of the access token, which is
indicated by the 'expires_in' parameter.
key: REQUIRED. A fresh and unique shared symmetric secret with
sufficient entrophy.
profile: REQUIRED. The profile parameter provides further
information about how the client has to provide proof of
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possession of the key with the resource server. The
authorization server chooses a value from the list of supported
mechanisms supported by the client.
For example:
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"access_token":"SlAV.....32hkKG",
"token_type":"hotk-sk",
"expires_in":3600,
"refresh_token":"8xLOxBtZp8",
"id":"client12345@example.com",
"key":"adijq39jdlaska9asud",
"profile":"jws"
}
The content of the 'access_token' MUST contain
the key identifier value in the 'hotk' element,
as shown in the example below.
{"typ":"JWT",
"alg":"HS256"
}
.
{"iss":"authorization-server-id",
"exp":1300819380,
"hotk":"client12345@example.com"
}
.
bbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZC
DISCUSSION: Should we put the encrypted key into the access token?
This would make the mechanism more similar to a Kerberos-based
scheme.
The key identifier, the key, and the profile name MUST NOT include
characters other than:
%x20-21 / %x23-5B / %x5D-7E
; Any printable ASCII character except for <"> and <\>
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3.1.2. Asymmetric Keys
In case an asymmetric key shall be bound to an access token then the
following procedure is applicable. In the request message from the
OAuth client to the authorization server the following parameters
MUST be included:
token_type: REQUIRED. For the asymmetric holder-of-the-key variant
the value MUST be set to "hotk-pk".
pk_info: REQUIRED. This field contains information about the public
key the client would like to bind to the access token in the
JSON Web Key format. The public key is "application/x-www-
form-urlencoded" encoded.
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
&token_type=hotk-pk
&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
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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 [3].
The authorization server also places information about the public key
used by the client into the access token to create the binding
between the two. The new token type, called 'hotk-pk', is placed
into the 'token_type' parameter.
An example of a successful response is shown below:
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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-pk",
"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
3.2. Accessing a Protected Resource
Accessing a protected resource depends on the chosen credential type.
3.2.1. Symmetric Keys
When a symmetric key was used as a holder-of-the-key then the client
has to demonstrate possession of the key that corresponds to the key
identifier found in the access token.
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This specification defines three ways for providing this proof of
possession, which are indicated as profiles in Section 3.1.1:
jws: When the 'jws' profile is chosen then the client MUST compute
the following string by concatenating together, in order, the
following HTTP request elements:
1. The HTTP request method in upper case. For example: "HEAD",
"GET", "POST", etc.
2. The HTTP request-URI as defined by Section 5.1.2 of [2].
3. The hostname included in the HTTP request using the "Host"
request header field in lower case.
4. The port as included in the HTTP request using the "Host"
request header field. If the header field does not include a
port, the default value for the scheme MUST be used (e.g., 80
for HTTP and 443 for HTTPS).
5. The value of the "ext" "Authorization" request header field
attribute if one was included in the request, otherwise, an
empty string.
Each element is followed by a new line character (%x0A) including
the last element and even when an element value is an empty
string. The resulting value MUST be put into the "request"
element of a JSON document that is then subject to JWS processing
[7]. The resulting JWS structure is put into the body of the HTTP
request. A receiving authorization server MUST use the value in
the 'kid' structure to identify the shared key and then use that
key to verify the keyed message digest. Additionally, the content
of the 'request' field needs to be verified against the HTTP
header information. If any of these verification steps fail then
the request to the protected resource MUST fail with a "401
Unauthorized" error message back to the OAuth client.
The following example shows and the corresponding encoding in a
JWS structure:
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1) HTTP Request
POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2+q HTTP/1.1
Host: example.com
2) JWS Document
{"typ":"HOTK-SK",
"alg":"HS256",
"kid":"client12345@example.com",
"timestamp":"2012-07-15T10:20:00.000-05:00" }
.
{"request":"POST/request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=
2+qexample.com80"}
.
dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk
JWS Example
mac: When the 'mac' profile is chosen then the client MUST follow
the description in [10].
3.2.2. Asymmetric Keys
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 [5].
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.
4. Security Considerations
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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 [14]. 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 and use 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
[8] 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) [9].
For the interaction between the client and the resource server this
specification requires a TLS extension for usage with out-of-band
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validation [5] to be used that allows clients to present raw public
keys for asymmetric holder-of-the-key usage.
With the usage of the holder-of-the-key concept it is not possible
for any party other than the legitimate client to use an access token
and to re-use it without knowing the corresponding asymmetric key
pair. This mechanism prevents against token disclosure.
With the usage of the asymmetric holder-of-the-key concept the
following deployment consideration needs to be taken into
consideration. 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.
Client implementations must be carefully implemented to avoid leaking
the ephemeral credentials (either the private key from the asymmetric
credential or the shared secret).
Token replay is also not possible since an eavesdropper will also
have to obtain the corresponding private key or shared secret that is
bound to the access token. Nevertheless, it is good practice to
limit the lifetime of the access token and therefore the lifetime of
associated key.
4.3. Summary of Recommendations
The following three items represent the main recommendations:
Safeguard the private key/shared secret: Client implementations MUST
ensure that the ephemeral private key / shared secret is not
leaked to third parties, since those will be able to use the
access token together with the keying material to gain access to
protected resources.
Switch keying material regularly: Clients can at any time create a
new ephemeral credential and associate it with an 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.
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5. IANA Considerations
This document requires IANA to take the following actions.
5.1. OAuth Parameters Registration
This specification registers the following parameters in the OAuth
Parameters Registry established by [3].
Parameter name: pk_info
Parameter usage location: token request
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
Parameter name: token_type
Parameter usage location: token request, token response,
authorization response
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
Parameter name: profile
Parameter usage location: token request, token response,
authorization response
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
Parameter name: id
Parameter usage location: token response, authorization response
Change controller: IETF
Specification document(s): [[ this document ]]
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Related information: None
Parameter name: key
Parameter usage location: token response, authorization response
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
5.2. The 'hotk' JSON Web Token Claims
[6] 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 [3] 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 ]]
5.4. Profile Registry
This document asks IANA to create a registry for profiles of
symmetric key-based holder-of-the-key mechanisms. The policy for
adding new entries to the registry is "Specification Required". IANA
is asked to populate the registry with the following values:
o Profile name: jws
o Change controller: IETF
o Specification document(s): [[ this document ]]
o Profile name: mac
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o Change controller: IETF
o Specification document(s): [[ this document ]]
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.
7. References
7.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[3] Hardt, D., "The OAuth 2.0 Authorization Framework", draft-
ietf-oauth-v2-31 (work in progress), August 2012.
[4] Jones, M., "JSON Web Key (JWK)", draft-ietf-jose-json-web-
key-19 (work in progress), December 2013.
[5] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Using Raw Public Keys in Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", draft-ietf-tls-oob-pubkey-10 (work in progress),
October 2013.
[6] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", draft-ietf-oauth-json-web-token-14 (work in
progress), December 2013.
[7] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", draft-ietf-jose-json-web-signature-19
(work in progress), December 2013.
[8] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[9] 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.
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Internet-Draft OAuth 2.0 HOTK Token Usage January 2014
[10] Richer, J., Mills, W., Tschofenig, H., and P. Hunt, "OAuth
2.0 Message Authentication Code (MAC) Tokens", draft-ietf-
oauth-v2-http-mac-04 (work in progress), July 2013.
7.2. Informative References
[11] Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0 Client Authentication
and Authorization Grants", draft-ietf-oauth-assertions-13
(work in progress), December 2013.
[12] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", draft-ietf-
oauth-v2-bearer-23 (work in progress), August 2012.
[13] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[14] Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S.,
and E. Nabbus, "NIST Special Publication 800-63-1,
INFORMATION SECURITY", December 2008.
Authors' Addresses
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
Phil Hunt
Oracle Corporation
Email: phil.hunt@yahoo.com
Tony Nadalin
Microsoft
Email: tonynad@microsoft.com
Hannes Tschofenig
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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