Subject: [Ietf-krb-wg] pkinit alg agility and ticket in checksum

From: Sam Hartman <[email-masked]>
Date: Fri, 29 Apr 2011 14:25:20 -0400

Folks, Painless Security has been implementing the pkinit algorithm
agility spec.
Currently we're targeting MIT Kerberos.

We've hit a bit of a snag for that implementation.

MIT has a plugin framework for preauthentication and pkinit is executed
in that framework.

The algorithm agility draft includes a new KDF that takes several inputs
such as the principals involved, the as-req, and the  ticket.

Unfortunately, the ticket is not actually done at the point MIT handles
padata. The authorization data plugins are called after the padata
plugins.

It's my understanding that there is a reason padata is handled before
authdata. I think it has to do with PAC handling and needing to know
whether pkinit is used to produce the PAC.
Luke,  can you remind me here?

Handling aspects of authdata after padata is certainly not unique to
MIT. For example, Heimdal handles SignedPath after it handles pkinit.

This opens up an architectural issue of what the ordering should be
between padata and authdata (and ticket extensions and enc_padata).

Here are the constraints I know:

1) Anonymous tends to influence what authdata you want to include

2) apparently some authdata want to know what preauth were used

3) Some padata such as pkinit produce authdata

Then there's the question of what the pkinit agility draft should do.
Certainly, integrity protecting the ticket is nice.
However being able to implement without re-architecting your
implementation is also nice.
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From: Luke Howard <[email-masked]>
Date: Sat, 30 Apr 2011 00:17:26 +0200

> Luke,  can you remind me here?

If PKINIT is used, the PAC contains additional information (see [MS-PAC] 2.6). There are probably also some interactions with S4U that need to be considered but if I recall correctly, S4U2Self is not implemented using the pre-authentication provider framework.

-- Luke