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Re: Multiple challenges in a single WWW-Authenticate header field

From: Thomas Broyer <t.broyer@gmail.com>
Date: Fri, 4 Dec 2009 08:47:43 +0100
Message-ID: <a9699fd20912032347s5adcac93x2f938b6f4bb7ed54@mail.gmail.com>
To: "HTTP Working Group (ietf-http-wg@w3.org)" <ietf-http-wg@w3.org>
[re-sent to list, sorry for the dupe Eran]


On Fri, Dec 4, 2009 at 5:08 AM, Eran Hammer-Lahav <eran@hueniverse.com> wrote:
> draft-ietf-httpbis-p7-auth defines the WWW-Authenticate as:
>
>     WWW-Authenticate   = "WWW-Authenticate" ":" OWS WWW-Authenticate-v
>     WWW-Authenticate-v = 1#challenge
>
> Importing challenge from RFC 2617:
>
>      challenge   = auth-scheme 1*SP 1#auth-param
>
> Which means a single header can contain multiple comma-separated challenges:
>
> WWW-Autenticate: Basic realm="X1", Digest realm="X1", domain="http://example.com", Basic realm="X2"
>
> Is this right?

Yes

> It seems odd to rely parsing multiple challenges on the presence of a space between the auth-scheme and list of auth-param.

Hence the following note in RFC 2617:
  Note: User agents will need to take special care in parsing the WWW-
  Authenticate or Proxy-Authenticate header field value if it contains
  more than one challenge, or if more than one WWW-Authenticate header
  field is provided, since the contents of a challenge may itself
  contain a comma-separated list of authentication parameters.

> I am not sure why multiple challenges are even allowed,

WWW-Authenticate: Basic realm="sent in the clear"
WWW-Authenticate: Digest realm="more secure", <other auth-params>
WWW-Authenticate: HMACDigest realm="even more secure", <other auth-params>
WWW-Authenticate: WSSE realm="some other scheme"
(where HMACDigest is
http://tools.ietf.org/html/draft-sayre-http-hmac-digest and WSSE is
described at http://www.xml.com/pub/a/2003/12/17/dive.html )

or
WWW-Authenticate: GoogleLogin realm="use your Google Account", <other
auth-params>
WWW-Authenticate: Cookie realm="or a local HTML form", <other auth-params>
WWW-Authenticate: OpenID realm="or your OpenID", <other auth-params>
(where Cookie is
http://tools.ietf.org/html/draft-broyer-http-cookie-auth and OpenID is
something like http://www.ietf.org/mail-archive/web/dix/current/msg00581.html
or http://wiki2008.openid.net/OpenID_HTTP_Authentication )

The goal is that the client uses the most secure scheme it supports
(in the first example, browsers would use Digest or Basic, while
HttpLib2 would use HmacDigest)

> but either way, they should be separated using something other than a comma.

Using commas is the way to go in HTTP so that you can have multiple
WWW-Authenticate headers (the exception being "Set-Cookie
in-the-wild").

But parsing WWW-Authenticate is really not that hard:
http://code.google.com/p/httplib2/source/browse/httplib2/__init__.py#228



-- 
Thomas Broyer
/tɔ.ma.bʁwa.je/
Received on Friday, 4 December 2009 07:48:17 GMT

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