INTERNET-DRAFT Magnus Nystrom
February, 2003 RSA Security
Expires: August, 2003 Alexey Melnikov
Intended category: Standards track ACI WorldWide/MessagingDirect
Robert Zuccherato
Entrust, Inc.
SASL in HTTP/1.1
draft-nystrom-http-sasl-06.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
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The list of current Internet-Drafts can be accessed at
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo suggest the use of SASL [RFC2222] as a framework to enable
the use of strong authentication mechanisms in HTTP/1.1 [RFC2616],
and defines one approach to accomplish this.
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].
Please send comments on this document to the relevant mailing lists,
e.g. ietf-sasl@imc.org.
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Table of contents
1 Introduction .............................................. 3
2 Document conventions and examples ......................... 4
2.1 Conventions used in this memo ............................ 4
3 Relationship with the HTTP/1.1 specification .............. 4
4 SASL framework ............................................ 4
4.1 The HTTP/1.1 challenge-response framework ................ 4
4.2 SASL authentication scheme ............................... 5
4.2.1 Recognition of the scheme .............................. 5
4.2.2 SASL authentication response header .................... 5
4.2.3 SASL authorization request header ...................... 6
4.2.4 New Expect header value used for authentication ........ 7
4.3 Usage model .............................................. 7
4.3.1 SASL handshake initiation .............................. 7
4.3.2 Client response ........................................ 8
4.3.3 Server behavior upon receiving a "SASL" <auth-scheme>
token .................................................. 9
4.3.4 Final part of handshake ............................... 11
4.3.5 Subsequent requests ................................... 11
4.3.6 Example sequence diagram .............................. 12
4.3.7 Pipelining considerations ............................. 12
4.3.8 Caching considerations................................. 13
4.3.9 "Web farm" considerations ............................. 13
4.3.10 Other considerations ................................. 13
4.4 Request/response encoding ............................... 14
4.4.1 SASL challenge/response encoding ...................... 14
4.4.2 Security layer......................................... 14
4.4.3 Interaction with TLS....................................14
4.5 Error handling .......................................... 15
4.5.1 Client errors ......................................... 15
4.5.2 Server errors ......................................... 16
4.6 Authorization identity .................................. 16
4.7 Examples ................................................ 17
4.7.1 Example 1 - Server requires authentication ............ 17
4.7.2 Example 2 - Initial response .......................... 18
4.7.3 Example 3 - One mechanism only ........................ 19
4.7.4 Example 4 - Server sends additional data .............. 20
4.7.5 Example 5 - Abort ..................................... 21
4.7.6 Example 6 - Client requires authentication ............ 22
4.7.7 Example 7 - Client uses POST request .................. 24
4.8 Interoperability with existing HTTP/1.1 clients and
servers ................................................. 25
4.9 Preferences ............................................. 26
5 IANA considerations ...................................... 26
5.1 GSSAPI/SASL service name ................................ 26
6 Security considerations .................................. 26
6.1 Introduction ............................................ 26
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6.2 Active attacks .......................................... 26
6.2.1 Man-in-the-middle ..................................... 26
6.2.2 Denial of service ..................................... 27
6.2.3 Replay ................................................ 27
6.3 Passive attacks ......................................... 27
6.4 Other considerations .................................... 27
7 Acknowledgements ......................................... 28
8 Copyright ................................................ 28
9 References ............................................... 28
9.1 Normative references .................................... 28
9.2 Informative references .................................. 29
10 Authors' addresses ...................................... 29
1 Introduction
"The Hypertext Transfer Protocol, HTTP/1.1 [RFC2616], supports only
two authentication schemes, namely the "Basic Access Authentication
Scheme" and the "Digest Access Authentication Scheme [RFC2617]."
Neither of these can be considered to be strong authentication
schemes. The former is extremely insecure unless used in conjunction
with a lower-level protocol offering security services, since it
sends cleartext passwords. The latter is an improvement, but is still
vulnerable to man-in-the-middle attacks.
The Simple Authentication and Security Layer Protocol (SASL
[RFC2222]) provides a method for adding authentication and security
services to connection-oriented protocols in a flexible manner,
enabling a variety of authentication and security mechanisms (e.g.
those based on one-time-passwords, public key technology or password-
based public-key cryptography) to be used with any protocol
supporting SASL. One major benefit of using SASL with HTTP is that
since the security technology is not built in to HTTP it is possible
to easily remove support for mechanisms based on technology that has
been proven to be vulnerable, and to easily add mechanisms that
support the latest and greatest security technology.
This memo suggests a method to use SASL in HTTP/1.1 and solicit
comments on the suggested approach.
<<Editorial comments are in angle brackets, like this. See also
appendix B for the list of major Open Issues>>.
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2 Document conventions and examples
2.1 Conventions used in this memo
In examples, "C:" and "S:" indicate lines sent by a client and a
server respectively.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [RFC2119].
3 Relationship with the HTTP/1.1 specification
This memo relies on the HTTP/1.1 [RFC2616] specification. As with RFC
2616, it uses the ABNF [RFC2234] grammar of that document and relies
on both non-terminals and other aspects of it.
Further, this memo requires persistent connections.
4 SASL framework
4.1 The HTTP/1.1 challenge-response framework
HTTP/1.1 provides a simple challenge-response mechanism that can be
used by a server or proxy to challenge a client request and by a
client to provide authentication information. The reader is referred
to [RFC2616] and [RFC2617] for a more detailed description of this
mechanism. The relevant ABNF productions are:
challenge = auth-scheme 1*SP 1#auth-param
auth-scheme = token
auth-param = token "=" (token | quoted-string)
The challenge will be found in a WWW-Authenticate or a Proxy-
Authenticate header field.
The client response, containing the client's credentials is defined
as follows:
credentials = auth-scheme 1*SP 1#auth-param
The response will be found in an Authorization or a Proxy-
Authorization header field.
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4.2 SASL authentication scheme
4.2.1 Recognition of the scheme
A server MUST use the auth-scheme token "SASL" if it supports SASL
and is willing to perform authentication using a SASL-based
mechanism.
4.2.2 SASL authentication response header
For the "SASL" <auth-scheme>, the authentication response header is
as follows:
challenge = SASL 1*SP sasl-response-parameters
sasl-response-parameters
= [sasl-mechanisms WSAC] [realm WSAC] sasl-sid
[WSAC (sasl-challenge | sasl-credentials)]
sasl-mechanisms = "mechanisms" "=" <"> 1#sasl-mech-name <">
realm = "realm" "=" quoted-string
; See RFC 2617
sasl-sid = "id" "=" quoted-string
sasl-challenge = "challenge" "=" base64-string
sasl-credentials = "credentials" "=" base64-string
; Only sent if mutual authentication
sasl-mech-name = 1*20 SASLCHAR
; Name must be from IANA set of registered SASL mechanisms,
; e.g. "SECURID"
base64-string = *BASE64 *2"="
; Encoding must be in accordance with [RFC2045], except not
; limited to 76 chars/line
SASLCHAR = UPALPHA | DIGIT | "-" | "_"
; Characters allowed in SASL mechanism name
BASE64 = DIGIT | ALPHA | "+" | "/"
WSAC = *LWS "," *LWS
Note: All directives ("mechanism", "id", "realm", "challenge", etc.)
are case-insensitive. All directive values are case-sensitive.
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The meanings of the values of the directives used above are as
follows:
sasl-mechanisms
A list of registered SASL mechanisms acceptable to the
server. MUST be sent by the server unless a mechanism already has
been agreed upon (see example 2 in section 4.7.2). Servers MUST
list supported SASL mechanisms in their preferred order.
realm
As defined in [RFC2617]. The directive MUST be present in initial
challenges and when the realm otherwise would not be known by the
client.
sasl-sid
A session identifier identifying a particular SASL context (see
below). MUST always be present. Sasl-sids are chosen by the server
and MUST be unique for each connection. The state associated with
a particular sasl-sid MUST be deleted once the connection goes away
or otherwise becomes invalid.
sasl-challenge
A Base64-encoded challenge in accordance with a selected SASL
mechanism. MUST NOT be sent unless there is exactly one SASL
mechanism in the <sasl-mechanisms> directive.
sasl-credentials
Base64-encoded credentials in accordance with a selected SASL
mechanism. This is only sent when the server is authenticating to
the client, see Section 4.3.3 below.
4.2.3 SASL authorization request header
For the SASL scheme, the authorization request header is as follows:
credentials = SASL [1*SP sasl-request-parameters]
sasl-request-parameters
= [sasl-mechanism WSAC] [sasl-sid WSAC]
[realm WSAC] [sasl-credentials]
sasl-mechanism = "mechanism" "=" <"> sasl-mech-name <">
The meanings of the values of the directives used above are as
follows:
sasl-mechanism
A SASL mechanism acceptable to the client, chosen from the list
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provided by the server or set by some configuration. MUST be sent
by the client unless a mechanism already has been agreed upon.
sasl-sid
A session identifier identifying a particular SASL context (see
below), previously set by a server. MUST always be sent by the
client except for the case of "initial responses," see Section
4.3.1 below.
realm
As defined in [RFC2617]. MUST always be sent by the client unless
the realm is possible to determine by other means (e.g. server
provided only one realm in its "SASL" <auth-scheme> token).
sasl-credentials
Base64-encoded credentials in accordance with a selected SASL
mechanism. MUST be sent if a <sasl-challenge> directive has been
received by the client.
4.2.4 New Expect header value used for authentication
The new value "102-continue" of the Expect header is introduced by
this document and is used solely for SASL authentication. It allows
the client to perform authentication exchange without the need to
send the body of the message with each request.
expectation-extension =/ expect-sasl-exchange
expect-sasl-exchange = "102-continue"
;; this conforms to the syntax
;; of expectation-extension
4.3 Usage model
4.3.1 SASL handshake initiation
When a client makes a request for a resource on a server that
requires SASL-based authentication, the server MUST respond with a
401 - Unauthorized (407 - Proxy Authentication Required) response
including a WWW-Authenticate (or Proxy-Authenticate) header field
that contains "SASL" <auth-scheme>.
The server MUST list all supported and acceptable SASL mechanisms in
the <sasl-mechanisms> directive. If the server only supports one SASL
mechanism, it MAY include a <sasl-challenge> directive in order to
reduce the number of roundtrips (see the example in Section 4.7.3).
The server MUST include a <sasl-sid> directive to identify the secure
session being negotiated. This value MUST be the same for all
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messages associated with that session. Multiple <sasl-sid> directives
MUST NOT be mixed on the same connection. However, there is one
exception to this rule. If the client starts authentication exchange
with the target server and than a proxy server asks the client to
authenticate, than the client must perform authentication exchange to
the proxy first and than resume authentication to the end server.
Further, the server MUST include a <realm> directive in accordance
with [RFC2617], however if a particular SASL mechanism defines its
own "realm" as a part of an authentication exchange, the mechanism
specific version of "realm" must be used by the mechanism.
A client, which is about to issue a request to a server, and knows
that the server requires a certain SASL mechanism, MAY include a a
"SASL" <auth-scheme> token in an Authorization (or Proxy-
Authorization) header field in its request. If the client chooses to
do so, it MUST include a <sasl-mechanism> directive identifying the
used SASL mechanism, but MUST NOT include a <sasl-sid> directive, as
session identifiers are chosen by the server. If the chosen SASL
mechanism requires that the client sends data first, the client MUST
also include a <sasl-credentials> directive, c.f. the "initial
response" in [RFC2222] (see the example in Section 4.7.2). This
minimizes the number of roundtrips, otherwise the server would be
required to send an empty challenge.
If the client requires authentication, but doesn't know which
mechanisms are supported by the server, the client MUST issue an
OPTION request that includes a Request-URI header for the desired
resource and Authorization (or Proxy-Authorization) header field
containing "SASL" <auth-scheme> token that is not followed by any
sasl-request-parameters. This provides a way for the client to query
the server about supported SASL mechanisms for the requested
resource. After the request the session continues as if the server
initiated an authentication exchange.
4.3.2 Client response
A client, which receives a "SASL" <auth-scheme> authentication
response in a WWW-Authenticate (Proxy-Authenticate) header in a 401 -
Unauthorized (407 - Proxy Authentication Required) response, MUST
choose one of the available mechanisms and construct a new request as
described below. This request MAY contain the headers from the
original request (except for Content-Length which should be either
removed or be 0), MUST contain Authorization (Proxy-Authorization)
header containing a "SASL" <auth-scheme> token and MUST NOT contain
the body of the original request (if any). We will reference any such
request as a "SASL request". The "SASL" <auth-scheme> token MUST
include <sasl-sid> value provided by the server and <sasl-mechanism>
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directive with the chosen SASL mechanism name. If the chosen
mechanism allows for "initial response" type messages, the client
MUST also include the initial response in a <sasl-credentials>
directive.
Note that unless noted otherwise, all authentication requests sent by
the client MUST include "Expect: 102-continue" header as described in
4.2.4, unless the client knows that the authentication exchange is
complete (successfully or not). The client MAY also include "Cache-
Control: no-store" and "Pragma: no-cache" as described in 4.3.8.
If the client is able and willing to negotiate a SASL security layer,
it MUST establish an end-to-end tunnel using the CONNECT method as
described in section 5.3 of [RFC2817] before starting an
authentication exchange. The Authorization header MUST NOT be used in
a CONNECT request. However, in order to save round trips, Proxy-
Authorization header MAY be used in a CONNECT request.
4.3.3 Server behavior upon receiving a "SASL" <auth-scheme> token
The server (proxy), upon receiving an authorization request
containing a "SASL" <auth-scheme> token with a <sasl-mechanism>
directive, checks if it supports/accept the authentication mechanism.
If the provided mechanism is not supported or accepted, the server
MUST reply with 406 - Not Acceptable response.
The server (proxy), upon receiving an authorization request
containing a "SASL" <auth-scheme> token with a <sasl-credentials>
directive, checks if the client is authenticated. If the client is
not authenticated, the server responds with a 102 - Authentication
Required (407 - Proxy Authentication Required) response containing a
new <sasl-challenge> directive with a "SASL" <auth-scheme>
authentication response token in a WWW-Authenticate (or Proxy-
Authenticate) header. The server MAY also choose to reply with 401 -
Unauthorized response that contains WWW-Authenticate (or Proxy-
Authenticate) header without the <sasl-challenge> directive, in which
case the client shall interpret the response in accordance with
Section 10.4.2 of [RFC2616]. The server MAY also choose to reply
with 432 - Transition Needed response, that indicates that the user
name is valid, but the entry in the authentication database needs to
be updated in order to permit authentication with the specified SASL
mechanism.
If the client is authenticated, the server MUST at least include the
<sasl-sid> directive with its "SASL" <auth-scheme> authentication
response token. If the chosen SASL mechanism requires that further
challenge/response data (i.e. "server returns success with additional
data" in [RFC2222]) be sent by the server, the server MUST respond
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with a 102 - Authentication Required (407 - Proxy Authentication
Required) response containing also a <sasl-credentials> directive
with its "SASL" <auth-scheme> authentication response token in a WWW-
Authenticate (or Proxy-Authenticate) header. Unless the server fails
authentication, the client MUST reply to this with a new SASL request
containing Authorization header with just a <sasl-sid> directive with
the value provided by the server. The server will reply to this with
235 - Authentication Completed (236 - Proxy Authentication Completed)
response and at this point authentication is complete and SASL
security layer may be in effect (see Section 4.4.2).
<<Alternatively we can include additional data with 235 response.
Advantage: saves round trip.
Disadvantage: if the server fails mutual authentication, the client
can send a cancellation request with the current model. If we are to
include additional data in 235 response, the client is only able to
close connection.
Disadvantage (if used together with response body): In order to
protect response we need to send status line and SASL data in the
clear, followed by encrypted headers and body>>
If the client is authenticated and the server does not need to send
any further challenge information, the server replies with 235 -
Authentication Completed (236 - Proxy Authentication Completed)
response.
Upon receipt of a 235/236 response the client shall consider
authentication successful and may retry the original request (with
the body of the request, if any) that would be protected by the
negotiated security session (see Section 4.4.2). The client MAY
include WWW-Authenticate (or Proxy-Authenticate) header containing a
"SASL" <auth-scheme> token, however no <sasl-credentials>, <sasl-
challenge> or <sasl-mechanisms> directive shall be included.
<<We might be able to return response body in 235 for certain
responses, i.e. the responses for requests with no body. This can
save another round trip.>>
The presence of the WWW-Authenticate (or Proxy-Authenticate) header
in a response message, that contains a "SASL" <auth-scheme> token
with a <sasl-credentials> directive, indicates successful
authentication of the client. In particular, the presence of the WWW-
Authenticate (or the Proxy-Authenticate) in a 4xx (except 401) or 5xx
response indicates that the authentication was successfully
completed, but the request itself failed.
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<<Is this still valid?>>
However, the presence of the WWW-Authenticate (or Proxy-Authenticate)
header with <sasl-credentials> directive in a reply with status code
102 - Authentication Required, 401 - Unauthorized or 407 - Proxy
Authentication Required is a silly state and must be treated as a
server error by the client.
<<Do we need a new 1xx status code for proxy authentication?>>
4.3.4 Final part of handshake
The client, upon receipt of a 432 - Transition Needed response, MAY
retry authentication using SASL PLAIN mechanism. This SHOULD be done
with appropriate TLS protection in place. An interactive client MUST
not perform PLAIN authentication automatically and MUST warn the user
before proceeding.
The client, upon receipt of a "SASL" <auth-scheme> authentication
response and a <sasl-challenge> directive in a WWW-Authenticate
(Proxy-Authenticate) header for a 102 - Authentication Required (407
- Proxy Authentication Required) response, calculates its credentials
and responds with a new SASL request containing a <sasl-credentials>
directive and a "SASL" <auth-scheme> token in an Authorization
(Proxy-Authorization) header. If a WWW-Authenticate (Proxy-
Authenticate) header is received on a message that is not a 102 -
Authentication Required (407 - Proxy Authentication Required)
response, no further credentials are required.
4.3.5 Subsequent requests
When making any further requests within the given realm, the client
SHOULD include a "SASL" <auth-scheme> authorization request in an
Authorization (Proxy-Authorization) header that does not contain a
<sasl-credentials> directive. The client MUST include the <sasl-sid>
directive in the SASL authorization requests.
The same HTTP server may serve data governed by multiple realms that
may have separate associated authentication databases. If the client
left the authentication realm it was originally authenticated in, the
server may force the client to re-authenticate in the new realm. In
this case a new authentication exchange is started as described in
4.3.1. However there is a change in how the security layer is
established (see section 4.4.2). If a security layer is currently
active and the new authentication exchange negotiate no security
layer, the current security layer MUST NOT be removed. However, if a
new security layer is negotiated, it MUST replace the existing one.
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<<There is currently no agreement if security layer has remain the
same or has to be dropped on reauthentication. SASL document says
that the layer has to remain the same, but Magnus thinks that this
can be used for an attack when the same server is hosting multiple
realms, which are served by competing companies>>
<<Do we want "reauthentication not allowed" status code, or should we
drop it altogether, as encryption keys MAY be available to the second
realm during reauthentication itself?>>
4.3.6 Example sequence diagram
Client Server
----------------- Initial Request ----------------------->
<------ 401 WWW-Authenticate SASL (mechanisms,realm,id) --
--- Authorization (mechanism,id),Expect (102-continue) -->
<------ 102 WWW-Authenticate SASL (id,challenge) ---------
--- Authorization (id,credential),Expect (102-continue) ->
<------ 102 WWW-Authenticate SASL (id,credential) --------
--- Authorization (id,credential),Expect (102-continue) ->
(0 or more times depending on the SASL mechanism)
<------ 235 WWW-Authenticate SASL (id) -------------------
----------------- Initial Request (retry) --------------->
<------ 200 WWW-Authenticate SASL (id) -------------------
Subsequent requests:
Client Server
--- Authorization (id) ---------------------------------->
<------ 200 WWW-Authenticate SASL (id) -------------------
4.3.7 Pipelining considerations
Clients MUST NOT send any other data until the authentication
exchange is complete. As a consequence, clients MUST NOT use
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pipelining until any on-going authentication exchanges are completed
(whether successful or not).
Clients MAY put multiple HTTP requests inside a single SASL block
when a SASL security layer has been negotiated (see also section
4.4.2).
4.3.8 Caching considerations
In order to prevent caching of a HTTP response containing a piece of
a multistep SASL exchange, the client MUST send both "Cache-Control:
no-store" and "Pragma: no-cache" (for compatibility with older proxy
servers) together with an "Authorization" header in all intermediate
request. There are two exception to this rule:
1). the client is sending a OPTIONS/POST/PUT/DELETE request, that
have
non cacheable responses.
2). the client established end-to-end tunnel with CONNECT.
<<From HTTP 1.1 document:
Note that section 14.8 normally prevents a shared cache from
saving
and returning a response to a previous request if that request
included an Authorization header.
So, everything discussed in this section might not be required.>>
For the same reason, the server MUST send a "Cache-Control: no-store"
header together with the "WWW-Authenticate" header in all
intermediate responses.
4.3.9 "Web farm" considerations
Implementation and configuration of the SASL negotiation mechanism
described in this memo requires special considerations in the case of
web farm environments where several servers may serve user requests
since authentication state information otherwise may be lost. In
particular, if a persistent connection to a particular server cannot
be guaranteed, means for sharing of authentication negotiation state
must be available.
4.3.10 Other considerations
Clients MAY abort authentication exchanges at any time, by specifying
"*" in <sasl-credentials> and including <sasl-sid> of the
authentication exchange being cancelled. If the server receives such
a request, it MUST reject the exchange with a 401 - Unauthorized
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reply. After this, both the client and the server MUST return to
their previous state.
There MUST NOT be more than one WWW-Authenticate or Proxy-
Authenticate header field containing a SASL authentication response
in a response.
There MUST NOT be more than one Authorization or Proxy-Authorization
header field containing a SASL authorization request in a request.
4.4 Request/response encoding
4.4.1 SASL challenge/response Encoding
The <sasl-challenge> directive and the <sasl-credentials> directive
contain SASL challenges and responses respectively. The challenges
and responses MUST be base64 [RFC2045] encoded before being placed in
these fields. The base64 string may in general be arbitrarily long.
Clients and servers MUST be able to support challenges and responses
that are as long as are generated by the authentication mechanisms
they support, independent of any line length limitations the client
or server may have in other parts of its protocol implementation.
4.4.2 Security layer
If a protection mechanism is negotiated as part of the SASL security
session, then it MUST be applied to all subsequent requests and
responses sent between the server and client. Any negotiated security
layer takes effect immediately following the <message-body> that
concludes the authentication exchange for the client, and the
<message-body> of 235/236 response for the server. I.e., for later
requests (and responses) all data - including the status line and
headers - will be protected by the security layer.
4.4.3 Interaction with TLS
A client may not perform an HTTP/1.1 "Upgrade" to TLS [RFC2817] while
conducting a SASL negotiation, but is free to do so after, or before,
the SASL negotiation takes place.
This document allows for both TLS and SASL security layer to be
active at the same time. No matter in which order they were
negotiated, any data would be converted by SASL security layer first
and than by TLS, i.e. the protocol stack would look like this
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+---------+
| SASL |
+---------+
| TLS |
+---------+
| TCP |
+---------+
4.5 Error handling
4.5.1 Client errors
HTTP/1.1 status codes which apply to SASL-based mechanisms are:
-102 - Authentication Required
This is a new status code introduced by this document that is used
to perform SASL authentication exchange. It signifies that SASL
authentication is in progress and the client is required to send
more authentication data to the server. See section 4.3 for more
details.
-235 - Authentication Completed
This status code indicates that SASL authentication with the server
is complete and the client may retry sending the original request.
-236 - Proxy Authentication Completed
This status code indicates that SASL authentication with the proxy
is complete and the client may retry sending the original request.
-401 - Unauthorized
An HTTP/1.1 server will use this status code when credentials
supplied by a client could not be validated.
-406 - Not Acceptable
An HTTP/1.1 server will use this status code when a client suggests
an authentication mechanism which is not supported or accepted by
the server.
-407 - Proxy Authentication Required
An HTTP/1.1 server (proxy) will use this status code when credentials
supplied by a client could not be validated, in addition to the use
described in Section 4.3 above.
-432 - Transition Needed
This status codes indicates that the user name is valid, but the
entry in the authentication database needs to be updated in
order to permit authentication with the specified SASL mechanism.
This typically is done by authenticating once using the PLAIN
authentication mechanism. This SHOULD be done with appropriate TLS
protection in place.
An interactive client MUST warn the user before proceeding with PLAIN
authentication.
This status code can be sent, for example if a user has an entry in
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a system authentication database such as Unix /etc/passwd, but does
not have credentials suitable for use by the specified mechanism.
4.5.2 Server errors
When a client does not support any of the security mechanisms
suggested by a server, or is otherwise unable to complete a SASL
mechanism handshake with a server, it shall close the connection.
(In the latter case instead of closing connection the client MAY also
cancel SASL exchange by specifying "*" in <sasl-credentials> as
described in section 4.3.10). User-oriented clients SHOULD provide
the user with information about the failed handshake, and MUST fail
in a controlled, predictable manner.
4.6 Authorization identity
An authorization identity is one kind of access control factor. It
is the name of the entity that requests that operations be performed.
Access control policies are often expressed in terms of authorization
identities; e.g., entity X can perform operation Y on resource Z.
The authorization identity bound to an association is often exactly
the same as the authentication identity presented by the client, but
it may be different. SASL allows clients to specify an authorization
identity distinct from the authentication identity asserted by the
client's credentials. This permits agents such as proxy servers to
authenticate using their own credentials, yet request the access
privileges of the identity for which they are proxying. Also, the
form of authentication identity supplied by a service like TLS
[RFC2246] may not correspond to the authorization identities used to
express a server's access control policy, requiring a server-specific
mapping to be done.
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4.7 Examples
Note: In the examples, some lines are wrapped for readability
reasons.
4.7.1 Example 1 - Server requires authentication
This example illustrates a client requesting a URL and a server
responding with a list of supported SASL mechanisms. The client
selects one of these and responds with a new request containing an
initial-response type <sasl-credentials> directive. The server then
issues a <sasl-challenge> directive back to the client which once
again responds with a <sasl-credentials> directive in the
Authorization header field.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@host.com",
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="CRAM-MD5",
id="jfkasdgru42705"
Expect: 102-continue
S: HTTP/1.1 102 Authentication Required
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705",
challenge=PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9u
Lm1jaS5uZXQ+
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="jfkasdgru42705",
credentials=dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQ
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zODkw
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
Client retries original request after that:
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
...Requested Document follows...
4.7.2 Example 2 - Initial response
In this example a client knows in advance that a certain SASL
mechanism is required. The mechanism allows for an initial-response
type message and the client therefore includes a <sasl-credentials>
directive in its Authorization header. The server accepts the
credentials and responds with the requested information.
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="SECURID",
credentials=AG1hZ251cwAxMjM0NTY3OAA=
Expect: 102-continue
The client doesn't know if authentication is complete at this point,
as certain SASL mechanisms have variable number of steps.
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
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Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
...Requested Document follows...
4.7.3 Example 3 - One mechanism only
In this example a server supports only one SASL mechanism, that
allows for sending of initial challenge to a client.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="CRAM-MD5",
realm="testrealm@host.com",
id="jfkasdgru42705",
challenge=PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9u
Lm1jaS5uZXQ+
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="jfkasdgru42705",
credentials=dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQ
zODkw
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
...Requested Document follows...
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4.7.4 Example 4 - Server sends additional data
In this example a server sends additional data with end-of-
authentication response. (C.f. the "success with additional data" in
[RFC2222]) The example also demonstrates the use of an
integrity/privacy layer. Note that the client is using the CONNECT
method, as it is willing to negotiate integrity/privacy protection
provided by the DIGEST-MD5 SASL mechanism.
In examples, "CP:" and "SP:" indicate lines sent by a client and a
server respectively with SASL security layer active.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@example.com",
id="0001"
C: CONNECT classified.example.com:80 HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="DIGEST-MD5",
id="0001"
Expect: 102-continue
S: HTTP/1.1 102 Authentication Required
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",
challenge=cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNl
PSJPQTZNRzl0RVFHbTJoaCIscW9wPSJhdXRoIixhbGdv
cml0aG09bWQ1LXNlc3MsY2hhcnNldD11dGYtOA==
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
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Authorization: SASL
id="0001",
credentials=Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJ
lYWxtPSJlbHdvb2QuaW5ub3NvZnQuY29tIixub25jZT
0iT0E2TUc5dEVRR20yaGgiLG5jPTAwMDAwMDAxLGNub
25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9
ImltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9
uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxND
NhZjcscW9wPWF1dGg=
Expect: 102-continue
S: HTTP/1.1 102 Authentication Required
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",
credentials=cnNwYXV0aD00YjJiYjM3ZjA0OTEwNTA1Nzc3YzJmNjM
4YzkyMjcyNQ==
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="0001"
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001"
CP: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
SP: HTTP/1.1 200 OK
WWW-Authenticate: SASL
id="0001"
...Requested Document follows...
CP: ...Any subsequent request for a data on the same server,
unless the server requests reauthentication...
4.7.5 Example 5 - Abort
The following example shows how a client can abort authentication
exchange.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
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S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@example.com",
id="0001"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="DIGEST-MD5",
id="0001"
Expect: 102-continue
S: HTTP/1.1 102 Authentication Required
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",
challenge=cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNl
PSJPQTZNRzl0RVFHbTJoaCIscW9wPSJhdXRoIixhbGdv
cml0aG09bWQ1LXNlc3MsY2hhcnNldD11dGYtOA==
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="0001",
credentials=*
S: HTTP/1.1 401 Authentication Canceled
...
4.7.6 Example 6 - Client requires authentication
The following example is almost identical to Example 1, but here the
client requires authentication to the server.
C: OPTIONS http://classified.example.com/classified.html HTTP/1.1
Authorization: SASL
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanism="DIGEST-MD5,GSSAPI,CRAM-MD5",
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realm="testrealm@host.com",
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="CRAM-MD5",
id="jfkasdgru42705"
Expect: 102-continue
S: HTTP/1.1 102 Authentication Required
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705",
challenge=PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9u
Lm1jaS5uZXQ+
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="jfkasdgru42705",
credentials=dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQ
zODkw
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
Upon receipt of a 235 response the client submits the request it
originally intended to submit:
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
...Requested Document follows...
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4.7.7 Example 7 - Client uses POST request
In this example the client is willing to perform a POST request but
the server requires authentication.
In examples, "CP:" and "SP:" indicate lines sent by a client and a
server respectively with SASL security layer active.
C: POST http://classified.example.com/update_classified.php
HTTP/1.1
Host: classified.example.com
Content-Type: ...
Content-Length: ...
...request body...
<<How can the server tell the client that it will not accept POST
requests in the clear? As the client already send POST request body
once in clear, there is no match point of protecting it. However a
response from the server might still be worth protecting.>>
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,OTP",
realm="testrealm@example.com",
id="0001"
C: CONNECT classified.example.com:80 HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
C: POST http://classified.example.com/update_classified.php
HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="OTP",id="0001",credentials=AHRpbQ==
Expect: 102-continue
S: HTTP/1.1 102 Authentication Required
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",challenge=b3RwLW1kNSAxMjMga2UxMjM0IGV4dA==
C: POST http://classified.example.com/update_classified.php
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HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="0001",credentials=aGV4OjExZDRjMTQ3ZTIyN2MxZjE=
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL id="0001"
CP: POST http://classified.example.com/update_classified.php
HTTP/1.1
Host: classified.example.com
Content-Type: ...
Content-Length: ...
...request body...
SP: HTTP/1.1 200 OK
WWW-Authenticate: SASL
id="0001"
...Response to POST, if any...
CP: ...Any subsequent request for a data on the same server,
unless the server requests reauthentication...
4.8 Interoperability with existing HTTP/1.1 clients and servers
A client supporting a certain SASL-based authentication mechanism
allowing for initial responses MUST NOT include a <sasl-credentials>
directive with a "SASL" <auth-scheme> authorization request in an
Authorization or Proxy-Authorization header unless it knows that the
server supports the SASL mechanism in question. The client MAY use an
OPTIONS request to find out about the server's SASL capabilities.
A server supporting SASL-based authentication SHOULD include a
"Basic" and a "Digest Access" <auth-scheme> token in a WWW-
Authenticate or Proxy-Authenticate header field, if these
authentication methods are acceptable to the server. This ensures
proper interworking with clients only capable of performing a "Basic"
or "Digest Access" authentication. Since these authentication
mechanisms does not offer strong security, the risk of downgrading
attacks should be carefully considered (see also the "Security
Considerations" section in this memo and Section 4.1 and 4.2 in
[RFC2617]).
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4.9 Preferences
Servers MUST list authentication mechanisms in WWW-Authenticate
(Proxy-Authenticate) header field in preferred order.
4.10 SASL mechanism recommendations
It is RECOMMENDED that an SASL mechanism that supports the
negotiation of a security layer with integrity protection be used,
and that this protection be enabled to avoid the connection being
hijacked after authentication has taken place. [RFC2222] discusses
some of the security issues related to SASL mechanisms.
5 IANA considerations
5.1 GSSAPI/SASL service name
For use with SASL [RC2222], a protocol must specify a service name to
be used with various SASL mechanisms, such as GSSAPI. For HTTP, the
service name shall be "http".
6 Security considerations
6.1 Introduction
This memo describes a method to integrate the SASL framework in
HTTP/1.1. SASL as such allows a wide variety of mechanism, each with
their own security characteristics. Being descriptive rather than
prescriptive, this memo does not mandate any particular SASL
mechanism, and a complete threat analysis can therefore not be given.
The following sections represent an attempt to discuss threats that
can be regarded to be generic in the sense that they apply to the
integration itself rather than specific SASL mechanisms. Security
services offered by, and security considerations applying to,
particular SASL mechanisms can be found through the IANA SASL
mechanism registry.
6.2 Active attacks
6.2.1 Man-in-the-middle
Users of SASL in HTTP/1.1 SHOULD recognize that certain man-in-the-
middle attacks are possible since the negotiation of the particular
SASL security mechanism to be used is not necessarily protected. For
example, if the server suggests SASL mechanisms A, B and C in a
"SASL" <auth-scheme> message where A is a "strong" mechanism (for
some definition of "strong") but B and C are "weak" or provide fewer
security attributes than A, then an attacker could simply remove A
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from the list. This forces the client to choose a "weaker" mechanism
and neither side will detect the changes made by the attacker.
To mitigate these attacks, servers SHOULD only suggest SASL
mechanisms that will provide adequate security for the task at hand.
Similarly, the SASL <auth-scheme> token may be removed from the WWW-
Authenticate (Proxy-Authenticate) header, thus forcing use of either
the Basic or Digest Access method. For this reason, and unless other
precautions (such as only accepting certain SASL mechanisms) are
taken, it is RECOMMENDED that this authentication mechanism be used
only in conjunction with a transport, e.g. TLS, providing protection
against these attacks (server authentication and integrity protection
of messages).
6.2.2 Denial of service
Since HTTP/1.1 requests and responses are not protected against
modification per se, an attacker may, by removing SASL elements from
HTTP/1.1 headers hinder a client from accessing a certain service.
This is however a generic threat and not specific to the mechanism
described herein.
6.2.3 Replay
Use of the "Cache-Control: no-store" and "Pragma: no-cache" headers
when indicated in requests and responses ensures that proxies do not
inadvertently store and/or deliver SASL handshake messages that
otherwise could be used in replay attacks.
6.3 Passive attacks
Unless a transport security providing confidentiality is employed,
the method described in this memo is susceptible to passive attacks
where an attacker wants to find out about the mechanisms that are
supported by a particular client.
6.4 Other considerations
Section 8.2 of [RFC2817] contains relevant security considerations
for the CONNECT method.
Note that SASL mechanisms offering confidentiality and integrity
protection of messages are only usable in conjunction with the
CONNECT method as described, since a proxy otherwise would be unable
to handle the messages properly.
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7 Acknowledgements
Text for Section 4.6 was borrowed from [RFC2829]. Thanks to Keith
Burdis, Raif S. Naffah, Mark Nottingham, Joe Orton and John P Speno
for providing useful feedback and suggestions.
8 Copyright
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
9 References
9.1 Normative references
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3," IETF RFC 2026, October 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies," IETF
RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," IETF RFC 2119, March 1997.
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[RFC2222] Myers, J., "Simple Authentication and Security Layer," IETF
RFC 2222, October 1997.
[RFC2234] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF," IETF RFC 2234, November 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P., Berners-Lee, T., "Hypertext Transfer Protocol --
HTTP/1.1," IETF RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., Stewart, L., "HTTP Authentication: Basic and
Digest Access Authentication," IETF RFC 2617, June 1999.
[RFC2817] Khare, R., Lawrence, S., "Upgrading to TLS Within
HTTP/1.1," IETF RFC 2817, May 2000.
9.2 Informative references
[RFC2246] Dierks, T., and C. Allen, "The TLS Protocol Version 1.0,"
IETF RFC 2246, January 1999.
[RFC2829] Wahl, M., Alvestrand, H., Hodges, J., and R. Morgan,
"Authentication Methods for LDAP," IETF RFC 2829, May 2000.
10 Authors' addresses
Magnus Nystrom Email: magnus@rsasecurity.com
RSA Security
Box 10704
121 29 Stockholm
Sweden
Alexey Melnikov Email: mel@messagingdirect.com
ACI WorldWide/Messaging
59 Clarendon Road,
Watford, Herts,
United Kingdom, WD17 1FQ
Robert Zuccherato Email: robert.zuccherato@entrust.com
Entrust, Inc.
1000 Innovation Drive
Ottawa, Ontario
Canada, K2K 3E7
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Appendix A. Changes since previous revisions
Changes since -05
Replaced "Cache-Control: no-cache" with "Cache-Control: no-store" as
per Mark Nottingham comment.
ABNF corrections from Joe Orton and John P Speno.
More corrections from Joe Orton.
Changed 401 to a new status code 102 used solely for authentication.
Added Transition Needed status code (432). Should check if this code
conflicts with anything.
Added new "Expect: 102-continue" header.
Reworked section 4.3 to describe more error cases and more detailed
implementation instructions.
Disallow TLS Upgrade during SASL authentication (it is fine before or
after). Clarified order of security layers.
Clarified that Authorization header with SASL response MUST NOT be
used with CONNECT.
Relaxed restriction for mixing SASL session ids on the same
connection in certain cases.
Added new 235/236 status codes for successfully completed
authentication.
Clarified that the body of the original request MUST NOT be sent
until authentication is complete. Updated examples to reflect that.
Added an example with a POST request.
Changes since -04
Reworked the Introduction section.
Updated example 4.7.4 to include Authorization header in CONNECT
request. This saves a round trip.
Added text that the client must use OPTIONS to find out which SASL
mechanisms are supported by the server. Added an example.
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Added text regarding the server requiring reauthentication when the
client leaves the realm it authenticated in.
Some clarification about the CONNECT method. Added text that a
CONNECT request should start the authentication exchange.
Incorporated comments from Raif S. Naffah and Keith Burdis.
Changes since -03
Fixed several errors in examples due to change from "sasl-mechanism"
to "sasl-mechanisms".
More comments from Keith Burdis.
Changes since -02
Added discussions about CONNECT and session protection.
Added "Proxy servers considerations" section. Updated examples to
include headers that prevent caching.
Added Web farm considerations section that talks about a next
response going to a different backend web-server.
Incorporated many suggestions/corrections from Keith Burdis.
Editorial changes. Cleanup some SHOULDs and MUSTs.
Changes since -01
Added examples
Split ABNF into client and server side. ABNF cleanup.
Many editorial changes.
Appendix B. Major Open Issues
235/236 status codes for successful authentication.
401 vs. new 1xx response code for authentication exchange.
Use "Expect" header to avoid sending request body with every
authentication step for POST/PUT methods.
When using a security layer, the status line should be transmitted
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twice: in cleartext and in encrypted block? (Another proposal is to
return 100/failure response code in the clear and the success in the
encrypted block).
(The following issue is actually closed, but additional information
may reopen it) Persistent connection is required when negotiating a
security layer. Not requiring it when a security layer is not
required adds additional complexity. Not requiring persistent
connections will also change how session ids are being used and when
they are destroyed.
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