Re: [pkix] Clarification on "zero" hash value in SigPolicyHash (CAdES)

Peter Gutmann <pgut001@cs.auckland.ac.nz> Tue, 16 July 2019 09:32 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Stefan Santesson <stefan@aaa-sec.com>, Niklas Matthies <pkix@nmhq.net>, "pkix@ietf.org" <pkix@ietf.org>
Thread-Topic: [pkix] Clarification on "zero" hash value in SigPolicyHash (CAdES)
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Date: Tue, 16 Jul 2019 09:32:15 +0000
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Subject: Re: [pkix] Clarification on "zero" hash value in SigPolicyHash (CAdES)
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Stefan Santesson <stefan@aaa-sec.com>; writes:

>However, option 3 is the absolute worst from an implementation perspective as
>it is the hardest to programmatically distinguish from a real hash value.

I would say it's the best from an implementation perspective, you don't need
to make any code changes, it's a normal looking hash value that's guaranteed
not to match anything.  Existing implementations that expect a hash there will
continue to work as normal.

>My conclusion is that the standard is so ambiguous that any receiving
>implementation should be able to handle all 3 alternatives.

A slightly different interpretation is that since that part of the standard is
essentially unimplementable, it's likely no-one has ever implemented it, so it
can be safely dropped.  Given earlier evidence that it's only there for
backwards compatibility with something no-one can identify, this enhances the
case for dropping it from the standard.

Peter.