Subject: Re: TSA Response serialNumber Field

From: [email-masked] (Peter Gutmann)
Date: Thu, 25 May 2000 02:26:07 (NZST)

>I understand that using 20 octects allows to use the result of a hash function
>and thus does not mandate any sequentiality. It also has the further benefit
>to hide the number of time stamps generated.

Limiting it to a current hash size may cause problems in the future, when SHA-2
appears (accompanied by an edict that "All organisations shall transition to
use of SHA-2 by 2005") then the 20-byte upper limit is going to be a problem.

Peter.

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From: Denis Pinkas <[email-masked]>
Date: Thu, 25 May 2000 10:30:19 +0200

Peter,

This issue is not specific to the TSP. 

RFC 2459 does not include any text about the size of serial number.

The draft draft-ietf-pkix-ac509prof-03.txt proposes an upper limit
of 20 bytes. So you should also complain in the same way for that
draft. :-)

So there are two options:

1) Provide an upper limit, or
2) Provide a warning, without any limit (as you have suggested).

I would, personally, prefer the former since, in practice,
implementors will have to use an upper limit.

You should also notice that the document does not specify the use of
a hash. The hash is only one of many ways to generate serial
numbers. Since this is invisible to the outside world, TSAs can do
what they think is appropriate (and cannot be mandated to use
SHA-2).

Denis

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From: Paul Koning <[email-masked]>
Date: Thu, 25 May 2000 10:21:33 -0400 (EDT)

>>>>> "Denis" == Denis Pinkas <[email-masked]> writes:

 Denis> Peter, This issue is not specific to the TSP.

 Denis> RFC 2459 does not include any text about the size of serial
 Denis> number.

 Denis> The draft draft-ietf-pkix-ac509prof-03.txt proposes an upper
 Denis> limit of 20 bytes. So you should also complain in the same way
 Denis> for that draft. :-)

 Denis> So there are two options:

 Denis> 1) Provide an upper limit, or 2) Provide a warning, without
 Denis> any limit (as you have suggested).

 Denis> I would, personally, prefer the former since, in practice,
 Denis> implementors will have to use an upper limit.

Why?  If it's just a unique ID, then the only meaningful operation on
it is memcmp() for which a tightly bounded size is not at all needed.

   paul