Subject: RE: Structuring Denis issues RE: Comments on <draft-ietf-pkix-crlaia-00.txt>

From: "Stefan Santesson" <[email-masked]>
Date: Fri, 13 May 2005 11:28:44 +0100

Denis,

Thanks for the clarification.
Yes, we agree on issue number 1 (remove SHOULD and MUST)

So the remaining issue is:

Problem: The security considerations talk about "mitigation" of the name
collision problem but gives inadequate advice.

Your proposed resolution

 a) warn the user that the CRL where this extension was found may not be
    the right one.
 
 b) warn the user that the CRL issuer certificate he might obtain using
    this extension may not be the right one.
 
 c) provide guidance on how to GUARANTEE that the CRL Issuer is indeed
    the one nominated by the CA that has issued the target certificate
    (i.e. when the CRL Issuer certification path and the target
    certificate certification path are identical).
 
 d) say that other possibilities exists, but need additional trust
    conditions (there are zillions of such possible trust conditions).

To complete the picture it would now be very helpful if you, for each of
these statements, could confirm or explain how these issues are a result
of using the AIA extension in CRLs. Or in other words, which of these
security considerations could you ignore (would go away) if you were NOT
using the AIA extension in a CRL to locate the CRL Issuer certificate.

Stefan Santesson
Program Manager, Standards Liaison
Windows Security

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From: Denis Pinkas <[email-masked]>
Date: Thu, 19 May 2005 17:17:34 +0200

Stefan,

> Yes, we agree on issue number 1 (remove SHOULD and MUST)

We also agree on a typo. :-|

>  a) warn the user that the CRL where this extension was found may not be

The AIA extension in found in a CRL that is not yet checked to be the right one.

>     this extension may not be the right one.

The AIA extension does not provide enough information so that the CRL issuer 
  certificate that is found there is indeed the right one, since at this 
time the certification path is not yet formed. There may be a 
man-in-the-middle attack.

>     certificate certification path are identical).

This would indeed be a useful guidance given the two above statements.

>     conditions (there are zillions of such possible trust conditions).

This would indeed be a useful guidance given the previous statement.

I am spending a *considerable* time on this issue and I believe that you 
have now perfectly understood what the problem is.

You know that I do not consider the AIA extension useful in the "basic" case.

Since the other cases (the "zillions" of other cases) rely anyway on 
additional local trust conditions, some local configuration values may be 
used to know where to fetch the missing CRL issuer certificates, hence why 
this extension is not really useful and may even be dangerous to be used if 
people simply use what they get from the pointer.

Denis