Tunneled HTTP Authentication For SASLYahoo! Inc.wmills@yahoo-inc.com Yahoo! Inc.timshow@yahoo-inc.comNokia Siemens NetworksLinnoitustie 6Espoo02600Finland+358 (50) 4871445Hannes.Tschofenig@gmx.nethttp://www.tschofenig.priv.atKITTENSimple 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 framework for delegated HTTP 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 HTTP authentication over SASL, and additionally defines
authorization and token issuing endpoint discovery. Thereby, it enables
schemes defined within the OAuth framework for non-HTTP-based application protocols.
A significant benefit of OAuth for usage in clients that usually
store passwords is storing tokens instead of passwords. This is much lower risk
since tokens can be more limited in scope of access and can be managed and
revoked separately from the user credential (password).
OAuth offers a standard mechanism for delegating
authentication typically used for the purpose of control access to resources. The core OAuth
specification defines a framework for authentication and token usage in an HTTP-based environment.
The HTTP authorization schemes and tokens in this model are defined separately,
some are defined within the OAuth 2 framework such as OAuth 2.0 Protocol: Bearer Tokens , and some are free standing with OAuth 2
framework bindings such as MAC Authentication tokens. This mechanism takes
advantage of the OAuth protocol and infrastructure to provide a way to use
SASL for access to resources for non-HTTP-based protocols
such as IMAP , which is what this memo uses in the examples. The general authentication flow is that the application will first obtain an access
token from an OAuth token service for the resource. Once the client has obtained
an OAuth access token it then connects and authenticated using this SASL mechanism.
shows the relationship between SASL and OAuth graphically. Item
(1) denotes the part of the OAuth exchange that remains unchanged from , 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.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 trade off between a generic, externally available
defined discovery mechanisms (such as Webfinger using host-meta ) and configuration information exchanged in band between
the protocol endpoints. 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
.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.Note that the IMAP SASL specification requires base64 encoding message, not this memo.SASL is used as a generalized authentication method in a variety of protocols. This
document defines the "OAUTH" mechanism to allow HTTP Authorization schemes in the OAuth
framework to be used within the SASL framework. In this
model a client authenticates to an OAuth-capable authorization server over HTTPS. 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. This mechanism further provides compatibility with OAuth 1.0a and the "OAuth" authentication scheme defined there.[[TODO: make this -PLUS not -SSL since if you support CB you have to support all types.]]Channel binding in this mechanism is defined in order to allow
satisfying the security requirements of the authorization schemes used. This document defines the
"OAUTH-PLUS" mechanism to provide channel binding for the OAUTH mechanism.
If the specification for the underlying
authorization scheme requires a security layer such as TLS the server
SHOULD only provide that scheme in a mechanism with channel binding enabled.
The channel binding data is computed by the client based on it's choice of
preferred channel binding type. As specified in the
channel binding information must start with the channel binding unique prefix followed
by a colon (ASCII 0x3A), this is followed by base64 encoded channel binding
payload. The channel binding payload is the raw data from the channel binding
type if the raw channel binding data is less than 500 bytes, if 500 bytes or
larger the channel binding payload is a SHA-1 hash of
the raw channel binding data. If the client is using tls-unique for channel binding then the raw channel
binding data is the first TLS finished message. This is under the 500 byte
limit, so the channel binding payload sent to the server would be the base64
encoded first TLS finished message.
In the case where the client has chosen tls-endpoint, the raw channel binding
data is the certificate of the server the client connected to. This will
frequently be 500 bytes or more, and if it is then the channel binding payload
is the base64 encoded SHA-1 hash of the server certificate.
The client response is formatted as an HTTP request. The HTTP request is limited in
that the path MUST be "/". In the OAUTH mechanism no query string is allowed. The following header lines are
defined in the client response:
Contains the user identifier being authenticated, and is
provided to allow correct discovery information to be returned.Contains the host name to which the client connected.An HTTP Authorization header.. The user name is provided by the client to allow the discovery information to be
customized for the user, a given server could allow multiple authenticators and it needs to
return the correct one. 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. In the OAUTH-PLUS mechanism the channel binding information is carried in
the query string. OAUTH-PLUS defines following query parameter(s):
Contains the
base64 encoded channel binding data, properly
escaped as an HTML query parameter value.The server validates the response per the specification for the authorization
scheme used. If the authorization scheme used includes signing of the request
parameters the client must provide a complete HTTP style request that satisfies
the data requirements for the scheme in use.
In the OAUTH-PLUS mechanism the server examines the channel binding data,
extracts the channel binding unique prefix, and extracts the raw channel biding
data based on the channel binding type used. It then computes it's own copy of
the channel binding payload and compares that to the payload sent by the client in
the query parameters of the tunneled HTTP request. Those two must be equal for
channel binding to succeed.
The server responds to a successful OAuth authentication by completing the SASL
negotiation. The authentication scheme MUST carry the user ID to be used as the
authorization identity (identity to act as). The server
MUST use that ID as the user being authorized, that is the user assertion
we accept and not other information such as from the URL or
"User:" header. The server responds to failed authentication by sending discovery information
in an HTTP style response with the HTTP status code set to 401, and then
failing the authentication.
If channel binding is in use and the channel
binding fails the server responds with a minimal HTTP response without discovery
information and the HTTP status code set to 412 to indicate that the channel
binding precondition failed. If the authentication scheme in use does not include
signing the server SHOULD revoke the presented credential and the client SHOULD
discard that credential.
Some OAuth mechanisms can provide both an authorization identity and an
authentication identity. An example of this is OAuth 1.0a where
the consumer key (oauth_consumer_key) identifies the entity using to token which equates to the
SASL authentication identity, and is authenticated using the shared secret. The
authorization identity in the OAuth 1.0a case is carried in the token (per the requirement
above), which SHOULD validated independently. The server MAY use a consumer key or
other comparable identity in the OAuth authorization scheme as the SASL authentication identity.
If an appropriate authentication identity is not available the server MUST use the
identity asserted in the token.The server MUST send discovery information in response to a failed authentication
exchange or a request with an empty Authorization header. If discovery information is
returned it MUST include an authentication
endpoint appropriate for the user. If the "User" header is present the discovery
information MUST be for that user. Discovery information is provided by the server to the
client to allow a client to discover the appropriate OAuth authentication and token
endpoints. The client then uses that information to obtain the access token needed for
OAuth authentication. The client SHOULD cache and re-use the user specific discovery
information for service endpoints. Discovery information makes use of both the WWW-Authenticate header as defined
in HTTP Authentication: Basic and Digest Access Authentication and Link headers as
defined in . The following elements are defined for discovery information:
A WWW-Authenticate header for each authentication scheme
supported by the server. Authentication scheme names are case
insensitive. The following authentication
parameters are defined:
REQUIRED -- (as defined by RFC2617)OPTIONAL -- A quoted string. This provides the
client an OAuth 2 scope known to be valid for the resource. An Link header specifying
the authentication endpoint. This link has an
OPTIONAL link-extension "scheme", if included this link applies ONLY to the
specified scheme.
An Link header specifying
the token endpoint. This link has an
OPTIONAL link-extension "scheme", if included this link applies ONLY to the
specified scheme.
(Optional) An Link header specifying
the Oauth 1.0a initiation endpoint. The server MUST send this if
"OAuth" is included in the supported list of HTTP authentication schemes for the
server.
(Optional) An Link header specifying
the Oauth 1.0a authentication endpoint. The server MUST send this if
"OAuth" is included in the supported list of HTTP authentication schemes for the
server.
(Optional) An Link header specifying
the Oauth 1.0a token endpoint. The server MUST send this if
"OAuth" is included in the supported list of HTTP authentication schemes for the
server. This link type has one link-extension "grant-types" which is a space
separated list of the the OAuth 2.0 grant types that can be used at the token
endpoint to obtain a token.
Usage of the URLs provided in the discovery information is defined in the relevant
specifications. 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 supports multiple types of authentication schemes and the server
MUST specify at least one supported authentication scheme in the discovery information. The server
MAY support multiple schemes and MAY support schemes not listed in the discovery
information.If the resource server provides a scope the client SHOULD always request scoped
tokens from the token endpoint. The client MAY use a scope other than the one
provided by the resource server. Scopes other than those advertised by the
resource server must be defined by the resource owner and provided in service
documentation (which is beyond the scope of this memo).
This mechanism supports authorization using signatures, which requires that both client and
server construct the string to be signed. OAuth 2 is designed for
authentication/authorization to access specific URIs. 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 "/". 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 folder or FTP access
limited to a specific directory. Using the example in the MAC specification
as a starting point, on an IMAP server running on port 143 and given
the MAC style authorization request (with long lines wrapped for readability) below:
The normalized request string would be constructed per the MAC specifcation . In this example the normalized
request string with the new line separator character is represented by
"\n" for display purposes only would be:
Tokens typically have a restricted lifetime. In addition a previously obtained
token MAY be revoked or rendered invalid at any time. The client MAY request a new access token for each
connection to a resource server, but it SHOULD cache and re-use access credentials that appear
to be valid. Credential lifetime and how that is communicated to the client is defined in
the authentication scheme specifications. Clients MAY implement any of the OAuth 2
profiles since they are largely outside the scope of this specification, and the mentioned
profiles in this document are just examples.These example illustrate exchanges between an IMAP client and an IMAP server.This example shows a successful OAuth 2.0 bearer token exchange with an initial client
response. Note that line breaks are inserted for readability.As required by IMAP , the payloads are base64-encoded. The
decoded initial client response is:
The line containing just a "+" and a space is an empty response from the server. This response contains
discovery information, and in the success case no discovery information is necessary so the
response is empty. Like other messages, and in accordance with the IMAP SASL binding, the
empty response is base64-encoded. This example shows a channel binding failure. The example sends the same
request as above, but in the context of an OAUTH-PLUS exchange the channel binding
information is missing. Note that line breaks are inserted for
readability.As required by IMAP , the payloads are
base64-encoded. The
decoded initial client response is:
The line conaining just a "+" and a space is an empty response from the server. This response contains
discovery information, and in the success case no discovery information is necessary so the
response is empty. Like other messages, and in accordance with the IMAP SASL binding, the
empty response is base64-encoded.
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. The decoded initial client response is:
The decoded server discovery response is:
This example shows a channel binding failure in a discovery request.
The channel binding information is empty. Note that line breaks are inserted for
readability. The decoded initial client response is:
The decoded server response is:
This mechanism does not provide a security layer, but does provide a provision for
channel binding. The OAuth 2 specification allows for a variety of usages, and the security properties
of these profiles vary. 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 or an equivalent secure channel MUST be implemented and its
usage is RECOMMENDED.
Channel binding in this mechanism has different properties based on the authentication
scheme used. Bearer tokens have the same properties as cookies, and the bearer token
authentication scheme has no signature or message integrity. Channel binding to TLS with a
bearer token provides only a binding to the TLS layer. Authentication schemes like MAC
tokens have a signature over the channel binding information. These provide additional
protection against a man in the middle, and the MAC authorization header is bound to the
channel and only valid in that context.
It is possible that SASL will be authenticating a connection and the life of that
connection may 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. Servers MAY unilaterally disconnect
clients in accordance with the application protocol.
An OAuth credential is not equivalent to the password or primary account
credential. There are protocols like XMPP that allow actions like change
password. The server SHOULD ensure that actions taken in the authenticated
channel are appropriate to the strength of the presented credential.
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. The IANA is requested to register the following SASL profile: SASL mechanism profile: OAUTHSecurity Considerations: See this documentPublished Specification: See this documentFor further information: Contact the authors of this document.Owner/Change controller: the IETFNote: None The IANA is requested to register the following SASL profile: SASL mechanism profile: OAUTH-PLUSSecurity Considerations: See this documentPublished Specification: See this documentFor further information: Contact the authors of this document.Owner/Change controller: the IETFNote: None Pursuant to The following link type registrations
[[will be]] registered by mail to link-relations@ietf.org.
Relation Name: oauth2-authenticator
Description: An OAuth 2.0 authentication endpoint.
Reference:
Notes: This link type indicates an OAuth 2.0 authentication endpoint
that can be used for user authentication/authorization for the endpoint
providing the link.
Application Data: [optional]
Relation Name: oauth2-token
Description: The OAuth token endpoint used to get tokens for access.
Reference:
Notes: The OAuth 2.0 token endpoint to be used for obtaining tokens
to access the endpoint providing the link.
Application Data: This link type has one link-extension "grant-types"
which is the OAuth 2.0 grant types that can be used at the token endpoint
to obtain a token. This is not an exclusive list, it provides a hint
to the application of what SHOULD be valid. A token endpoint MAY support
additional grant types not advertised by a resource endpoint.
Relation Name: oauth-initiate
Description: The OAuth 1.0a request initiation endpoint used to get tokens for access.
Reference:
Notes: The OAuth 1.0a endpoint used to initiate the sequence, this temporary
request is what the user approves to grant access to the resource.
Application Data: Relation Name: oauth-authorize
Description: The OAuth 1.0a authorization endpoint used to approve an access request.
Reference:
Notes:
Application Data: Relation Name: oauth-token
Description: The OAuth 1.0a token endpoint used to get tokens for access.
Reference:
Notes:
Application Data:
[[ to be removed by RFC editor before publication as an RFC ]]
-03
Fixing channel binding, not tls-unique specific. Also defining how the CB data is
properly generated.
Various small editorial changes and embarassing spelling fixes.
-02
Filling out Channel Binding
Added text clarifying how to bind to the 2 kinds of SASL identities.
-01
Bringing this into line with draft 12 of the core spec, the
bearer token spec, and references the MAC token spec
Changing discovery over to using the Link header construct from RFC5988.
Added the seeds of channel binding.
-00
Initial revision