KITTEN W. Mills
Internet-Draft T. Showalter
Intended status: Standards Track Yahoo Inc.
Expires: January 27, 2011 H. Tschofenig
Nokia Siemens Networks
July 26, 2010
A SASL Mechanism for OAuth
draft-mills-kitten-sasl-oauth-00.txt
Abstract
Simple Authentication and Security Layer (SASL) is a framework for
providing authentication and data security services in connection-
oriented protocols via replaceable mechanisms. OAuth is a protocol
for delegated authentication and thereby provides a method for
clients to access a protected resource on behalf of a resource owner.
This document defines the use of OAuth over SASL. Thereby, it
enables OAuth usage for non-HTTP-based application protocols. A
future version of this document will describe the integration into
the Generic Security Services Application Program Interface (GSS-
APIO).
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 27, 2011.
Copyright Notice
Copyright (c) 2010 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The OAuth SASL Mechanism . . . . . . . . . . . . . . . . . . . 6
3.1. Initial Client Response . . . . . . . . . . . . . . . . . 6
3.2. Server's Response . . . . . . . . . . . . . . . . . . . . 6
3.3. Discovery Information . . . . . . . . . . . . . . . . . . 7
3.4. Use of Signature Type Authorization . . . . . . . . . . . 7
3.5. Formal Syntax of Messages . . . . . . . . . . . . . . . . 8
4. Implementation Requirements . . . . . . . . . . . . . . . . . 9
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Successful Bearer Token Exchange . . . . . . . . . . . . . 10
5.2. Failed Exchange . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
[I-D.ietf-oauth-v2] offers a standard mechanism for delegating
authentication typically used for the purpose of control access to
resources. The core OAuth specification defines a number of profiles
but focuses on an HTTP-based environment. This mechanism takes
advantage of the OAuth protocol and infrastructure to provide a way
to use SASL [RFC4422] for access to resources for non-HTTP-based
protocols. One example for such a protocol is the Internet Message
Access Protocol (IMAP) [RFC3501], which is what we use in our
examples.
The general authentication flow is that the application will first
obtain an OAuth access token from an OAuth service for the resource.
Once the client has obtained an OAuth access token it then connects
and authenticated using this SASL mechanism.
Figure 1 shows the relationship between SASL and OAuth graphically.
Item (1) denotes the part of the OAuth exchange that remains
unchanged from [I-D.ietf-oauth-v2], i.e. where the client obtains and
refreshes Access Tokens. This document focuses on item (2) where the
Access Token is presented to the resource server over SASL.
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----+
+--------+ +---------------+ |
| |--(C)-- Authorization Request --->| Resource | |
| | | Owner | |Plain
| |<-(D)------ Access Grant ---------| | |OAuth
| | +---------------+ |2.0
| | |(1)
| | Client Credentials & +---------------+ |
| |--(E)------ Access Grant -------->| Authorization | |
| Client | | Server | |
| |<-(F)------ Access Token ---------| | |
| | (w/ Optional Refresh Token) +---------------+ |
| | ----+
| |
| | ----+
| | (Optional discovery) +---------------+ |
| |--(A)------- User Name --------->| | |
| Client | | | |
| |<-(B)------ Authentication -------| | |
| | endpoint information | Resource | |OAuth
| | | Server | |over
| |--(G)------ Access Token -------->| | |SASL
| | | | |
| |<-(H)---- Protected Resource -----| | |(2)
+--------+ +---------------+ |
----+
Figure 1: Interworking Architecture
Note: The discovery procedure in OAuth is still work in progress.
Hence, the discovery components described in this document should
be considered incomplete and a tentative proposal. In general,
there is a tradeoff between a generic, externally available
defined discovery mechanisms (such as Webfinger using host-meta
[I-D.hammer-hostmeta]) and configuration information exchanged
inband between the protocol endpoints.
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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 RFC 2119 [RFC2119].
The reader is assumed to be familiar with the terms used in the OAuth
2.0 specification.
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. Line breaks have been inserted for readability.
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3. The OAuth SASL Mechanism
SASL is used as a generalized authentication method in a variety of
protocols. This document defines a mechanism to allow OAuth to be
used within the SASL framework. In this model a client authenticates
to an OAuth-capable authorization server over HTTP. This server then
issues tokens after successfully authenticating the resource owner.
Subsequently, the obtained token may be presented in an OAuth-
authenticated request to the resource server.
3.1. Initial Client Response
The client response is formatted in the style of an HTTP request, a
GET line is included for the purposes of extensibility. The
following key-value header lines are defined in the client response:
User (OPTIONAL): Contains the user identifier being authenticated,
and is provided to allow correct discovery information to be
returned.
Host (REQUIRED): Contains the host name to which the client
connected.
Authorization (REQUIRED): Contains the authenticator as specified in
OAuth.
The user name is provided before discovery information because a
given server could allow multiple authenticators. For instance, a
large ISP could provide mail service for several domains who manage
their own user information. For instance, users at foo-example.com
could be authenticated by an OAuth service at
https://oauth.foo-example.com/, and users at bar-example.com could be
authenticated by https://oauth.bar-example.com, but both could be
served by a hypothetical IMAP server running at a third domain,
imap.example.net.
3.2. Server's Response
The server validates the response as per the OAuth specification. If
the protected resource requires a signed request (using one of the
available signature method), the URL for the resource being
authenticated is reconstructed per the OAuth specification from the
HTTP style request passed by the client.
The server responds to a successful OAuth authentication by
completing the SASL negotiation. The OAuth token MUST carry the user
id to be authenticated and the server MUST use the user in the OAuth
credential as the user being authenticated, the assertion we accept
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is that of the OAuth token and not other information such as from the
URL or "User:" header.
The server responds to failed authentication by sending discovery
information and then failing the authentication.
3.3. Discovery Information
The server MUST send discovery information in response to a failed
OAuth authentication exchange or a request with an empty Authenticate
header. If discovery information is returned the server MUST return
discovery information containing an authentication endpoint
appropriate for the user. If the "User" header is present the
discovery information MUST be for that user. In the absence of the
"User" header the server SHOULD send discovery information for the
user from the OAuth token. Discovery information is provided by the
server to the client to allow a client to discover the appropriate
OAuth authenticator service. The client then uses that information
to obtain the refresh token and the access token needed for OAuth
authentication. The client SHOULD cache and re-use the user specific
discovery information for service endpoints. The following key-value
pairs are defined for discovery information:
WWW-Authenticate: As specified in [I-D.ietf-oauth-v2].
Usage of the URL provided in the discovery information is defined in
the OAuth specification. If the server supports multiple
authenticators the discovery information returned for unknown users
MUST be consistent with the discovery information for known users to
prevent user enumeration. The OAuth 2.0 specification
[I-D.ietf-oauth-v2] has multiple types of authentication flows and
the server MUST specify the supported authorization flows in the
discovery information. The server MUST support at least one
authorziation flow, and MAY support multiple flows.
3.4. Use of Signature Type Authorization
OAuth supports authorization using signatures, which requires that
both client and server construct the string to be signed. OAuth is
designed for authentication/authorization to use a resource. SASL is
designed for user authentication, and has no facility for being more
specific. In this mechanism we require an HTTP style format
specifically to support signature type authentication, but this is
extremely limited. The HTTP style request is limited to a path of
"/", because this mechanism is authenticating the user to the server.
This mechanism is in the SASL model, but is designed so that no
changes are needed if there is a revision of SASL which supports more
specific resource authorization, e.g. IMAP access to a specific
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folder or FTP access limited to a specific directory.
So for example, given that OAuth has a port number component for the
signature, on an IMAP server running on port 143 and given the the
OAuth style authorization request (with long lines wrapped for
readability) below:
GET / HTTP/1.1
Host: server.example.com
Authorization: Token token="vF9dft4qmT", nonce="s8djwd",
timestamp="137131200", algorithm="hmac-sha256",
signature="wOJIO9A2W5mFwDgiDvZbTSMK/PY="
The normalized request string would be constructed per
[I-D.ietf-oauth-v2]. In this example the normalized request string
would be:
137131200,s8djwd,hmac-sha256,GET,server.example.com:143,
http://example.com/
3.5. Formal Syntax of Messages
;; CRLF, ... defined in RFC 5234
client_response = header user* host authorization
header = "GET / HTTP/1.1" CRLF
user = 'User:' SPACE field-value CRLF
host = 'Host:' SPACE field-value CRLF
authorization = 'Authorization:' SPACE field-value CRLF
field-name = *(%x20-%x39 / %x3b-%xff) ;; no ":", ascii printable
field-value = *(%x01-%x09 / %x0b / %x0c / %x0e-%xff) CRLF
;; no CR or LF
server_discovery_info = [ "WWW-Authenticate:" SPACE
field_value extended* CRLF ]
extended = field-name ":" SPACE field-value
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4. Implementation Requirements
Tokens typically have a restricted lifetime. In addition, the policy
of a client MAY revoke a previously obtained token at any time. The
client MAY request a new access token for each connection to a
resource server be made. In use cases like IMAP where clients
frequently make multiple connections at the same time the client it
is RECOMMENDED to re-use the same access token, if permitted by the
resource server. Clients MAY implement any of the OAuth profiles
since they are largely outside the scope of this specification, and
the mentioned profiles in this document are just examples.
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5. Examples
These example illustrate exchanges between an IMAP client and an IMAP
server.
5.1. Successful Bearer Token Exchange
This example shows a successful OAuth 2.0 bearer token exchange with
an initial client response. Note that line breaks are inserted for
readability.
S: * IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTH
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTH R0VUIC8gSFRUUC8xLjENClVzZXI6IHNjb290ZXJAYWx0
YXZpc3RhLmNvbQ0KSG9zdDogaW1hcC55YWhvby5jb20NCkF1dGhvcml6YXRpb24
6IFRva2VuIHRva2VuPSJ2RjlkZnQ0cW1UYzJOdmIzUmxja0JoYkhSaGRtbHpkR0
V1WTI5dENnPT0iDQoNCg==
S: +
S: t1 OK SASL authentication succeeded
As required by IMAP [RFC3501], the payloads are base64-encoded. The
initial client response is:
GET / HTTP/1.1
Host: imap.example.com
Authorization: Token
token="vF9dft4qmTc2Nvb3RlckBhbHRhdmlzdGEuY29tCg=="
The "blank" line here is an empty response from the server. This
response contains discovery information, in the success case no
discovery information is necessary to the response is empty. Like
other messages, and in accordance with the IMAP SASL binding, the
empty response is base64-encoded.
5.2. Failed Exchange
This example shows a failed exchange because of the empty
Authorization header, which is how a client can query for discovery
information. Note that line breaks are inserted for readability.
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S: * IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTH SASL-IR
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTH R0VUIC8gSFRUUC8xLjENClVzZXI6IHNjb290ZXJAYW
x0YXZpc3RhLmNvbQ0KSG9zdDogaW1hcC55YWhvby5jb20NCkF1dGhlbnRpY2F0ZT
ogDQoNCg==
S: + V1dXLUF1dGhlbnRpY2F0ZTogcmVhbG09ImltYXAueWFob28uY29tIiwgYXV0aH
otdXJpPSJodHRwczovL2xvZ2luLnlhaG9vLmNvbS9vYXV0aCIsIHRva2VuLXVyaT
0iaHR0cHM6Ly9sb2dpbi55YWhvby5jb20vb2F1dGgiLCBhbGdvcml0aG09ImhtYW
Mtc2hhMjU2Ig0KDQo=
S: t1 NO SASL authentication failed
The initial client response is:
GET / HTTP/1.1
User: alice@example.com
Host: imap.example.com
Authorization:
The server discovery response is:
WWW-Authenticate: Token realm="mail",
authz-uri="https://login.example.com/oauth",
token-uri="https://login.example.com/oauth",
algorithm="hmac-sha256"
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6. Security Considerations
This mechanism does not provide a security layer. The OAuth
specification [I-D.ietf-oauth-v2] allows for a variety of usages, and
the security properties of these profiles varies. The usage of
bearer tokens, for example, provide security features similar to
cookies. Applications using this mechanism SHOULD exercise the same
level of care using this mechanism as they would in using the SASL
PLAIN mechanism. In particular, TLS 1.2 MUST be implemented and its
usage is RECOMMENDED unless tokens expire quickly.
A significant benefit of OAuth for usage in IMAP, POP, SMTP, or other
clients that usually store passwords, is that the password is not
stored in the client, a token is. This means that the password is
not exposed, what we risk is a token that can be more limited or can
be easily revoked.
It is possible that SASL will be authenticating a connection, indeed
our examples are IMAP, and the life of that connection my outlast the
life of the token used to authenticate it. This is a common problem
in application protocols where connections are long-lived, and not a
problem with this mechanism per se.
It is possible for an application server running on Evil.example.com
to tell a client to request a token from Good.example.org. A client
following these instructions will pass a token from Good to Evil.
This is by design, since it is possible that Good and Evil are merely
names, not descriptive, and that this is an innocuous activity
between cooperating two servers in different domains. For instance,
a site might operate their authentication service in-house, but
outsource their mail systems to an external entity.
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7. IANA Considerations
The IANA is requested to register the following SASL profile:
SASL mechanism profile: OAUTH
Security Considerations: See this document
Published Specification: See this document
For further information: Contact the authors of this document.
Owner/Change controller: the IETF
Note: None
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8. References
8.1. Normative References
[I-D.ietf-oauth-v2]
Hammer-Lahav, E., Recordon, D., and D. Hardt, "The OAuth
2.0 Protocol", draft-ietf-oauth-v2-10 (work in progress),
July 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
8.2. Informative References
[I-D.hammer-hostmeta]
Hammer-Lahav, E., "Web Host Metadata",
draft-hammer-hostmeta-13 (work in progress), June 2010.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
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Authors' Addresses
William Mills
Yahoo Inc.
Phone:
Email: wmills@yahoo-inc.com
Tim Showalter
Yahoo Inc.
Phone:
Email: timshow@yahoo-inc.com
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
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