Subject: Re: Briefing on CRL Path for IETF PKIX WG Meeting

From: Peter Sylvester <[email-masked]>
Date: Wed, 27 Oct 2004 18:19:27 +0200 (MEST)

I say it differently:

A CA is not a PKI. You say that according to X509 a DN is only
attributed to the same entity by one CA. That's in the text, but
this does not mean that in order to have a PKI working, you don't 
need more. 

At least you would probably agree that a CA cannot sign another
CA having the same DN (because this would be considered as a 
self signed cert). 

Now, a CA1 --> CA2 --> CA1  can legally occur as a pair of
cross certificates, how would you distinguish whether CA1
and CA1 are identical or not (in case the same keys are used
the situation is clear). otherwise in rekeying you check
CA1 self signed certs towards each other, if so, same CA1,
if you happen to find another CA creating a certificate
for the two CA1 keys, again same CA1, ... etc, you can go
recursively up and down through all cross certs in your PKI
(if you find them)

If you fail all these tests, i.e. all cases where by 
some direct application of the existing rules you must conclude
that the two CA1 are identical, then you say, ok, I assume that
these two CA1s are different but still valid candidates in my
PKI, i.e. you have two sets of CRLs (V1) ...

Anyway: If you allow this, you can never ensure that any CA in your PKI
cross certify any other CA, a feature that seems more 
interesting to have 

Of course you have a problem when cross certifying two 
existing PKIs, which still does not mean that one cannot
consider this situation as 'garbage in', it may happen that
by chance of path building you get a good path if you
don't see the other CA, but the question was whether one
should try and accept two different entities with the 
same DN. 

> Peter,
> 
> > denis, would you assume that it is a desirable feature in
> > a PKI that any CA could issue cross certs to any other in
> > a PKI, and that a PKI should not prohibit this.
> 
> What is the trick behind this question ?
>    ... and why this question relates to the topic of that thread ?
> 
> Nevertheless, my answer is the following:
> 
> Nobody can prevent a CA to issue a cross certificate.
> 
> However, it is another matter whether that cross certificate will be 
> accepted or not under the rules of the validation policy to build a valid 
> certification path.
> 
> Denis
> 
>

---

From: "Santosh Chokhani" <[email-masked]>
Date: Wed, 27 Oct 2004 17:35:23 -0400

Peter,

While X.609 and 3280 most likely do not say this explicitly (I have not done
an exhaustive search), a CA must not issue certificates to multiple distinct
entities (be it a CA or EE) with the same DN.

The issue of two CAs with the same name comes when PKIs cross certify or
even in a homogeneous PKI, control is distributed and there is no mechanism
to ensure CA name uniqueness.  Thus, CA name collision in a homogeneous PKI
is very unlikely.  

Please note that in cross certified PKI, use of name constraints will not
solve the problem of two or more CAs with the same name I am trying to
solve.

A minor nit, when a CA certifies another CA with the same name (a no-no),
that is not self-signed certificate; it is self-issued certificate.
-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On
Behalf Of Peter Sylvester
Sent: Wednesday, October 27, 2004 12:19 PM
To: [email-masked]; [email-masked]
Cc: [email-masked]; [email-masked]
Subject: Re: Briefing on CRL Path for IETF PKIX WG Meeting

I say it differently:

A CA is not a PKI. You say that according to X509 a DN is only attributed to
the same entity by one CA. That's in the text, but this does not mean that
in order to have a PKI working, you don't 
need more. 

At least you would probably agree that a CA cannot sign another CA having
the same DN (because this would be considered as a 
self signed cert). 

Now, a CA1 --> CA2 --> CA1  can legally occur as a pair of cross
certificates, how would you distinguish whether CA1 and CA1 are identical or
not (in case the same keys are used the situation is clear). otherwise in
rekeying you check CA1 self signed certs towards each other, if so, same
CA1, if you happen to find another CA creating a certificate for the two CA1
keys, again same CA1, ... etc, you can go recursively up and down through
all cross certs in your PKI (if you find them)

If you fail all these tests, i.e. all cases where by 
some direct application of the existing rules you must conclude that the two
CA1 are identical, then you say, ok, I assume that these two CA1s are
different but still valid candidates in my PKI, i.e. you have two sets of
CRLs (V1) ...

Of course you have a problem when cross certifying two 
existing PKIs, which still does not mean that one cannot consider this
situation as 'garbage in', it may happen that by chance of path building you
get a good path if you don't see the other CA, but the question was whether
one should try and accept two different entities with the 
same DN.

---

From: [email-masked] (Peter Gutmann)
Date: Thu, 28 Oct 2004 15:16:31 +1300

Peter Sylvester <[email-masked]> writes:

>At least you would probably agree that a CA cannot sign another CA 
>having the same DN (because this would be considered as a self signed 
>cert).

It can if it's a CA cert renewal (and what a cesspit that is, creating
huge and often highly surprising anomalies and holes in path processing 
big enough to drive a truck through).

Peter.

---

From: Richard Levitte - VMS Whacker <[email-masked]>
Date: Thu, 28 Oct 2004 07:26:31 +0200 (CEST)

In message <[email-masked]> on Wed, 27 Oct 2004 18:19:27 +0200 (MEST), Peter Sylvester <[email-masked]> said:

Peter.Sylvester> At least you would probably agree that a CA cannot
Peter.Sylvester> sign another CA having the same DN (because this
Peter.Sylvester> would be considered as a self signed cert).

I agree in a way, but the parenthesis makes me wonder if you're not
mixing things up.  There is a difference between self-issued CA
certificates (where two CA certificates have the same subject) and a
self-signed CA certificate (where the public key in one certificate
can be used to verify the signature in that same certificate).  The
former is typically present as soon as you do a CA certificate roll-
over.

But I agree that two different CAs (two different organizations, in
other words) "cannot" have the same DN.  I write "cannot", because
it's still technically possible, and whenever that happens, one can
only expect a small appocalypse.

Cheers,
Richard

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