After reading a lot a documents, from PDF-spec to technical
recommendations, I feel that the signature validation subject is still
unclear ...
In particular I submit a PDF that is NOT validated by
Acrobat Reader (de facto, the standard validator), but it is validated
by other tools (e.g. iText).
The Question: What is a ByteRange ?
*
The Official PDF-Spec says that ByteRange is an array of (pairs of)
integer.
* iText allows 2 or more pairs of integers.
The
following sample has 1 cert-signature plus 2 signatures
http:
//web.tiscali.it/irrational/exp-sign/X-Cert-AZ.pdf
All the
signatures are validated (integrity check) by iText method "verify"
*
A lot of Adobe docs say that ByteRange is made of 4 integers ONLY.
* Acrobat Reader won't validate the above pdf ; If you try, you can see
a generic message about "..damaged or suspect data in SigDict ..."
(Note: since the certificate used (for all the 3 signatures) has been
revoked, you should *temporarily* declared it as a trusted root
certificate)
If you inspect the above pdf, you can see that the 3rd
signature has the following "uncommon" byterange ;
/ByteRange [0 31771
56897 621 78000 4086] , that is the 3rd signature excludes the whole
block of the 2nd signature, plus of course its own /Contents block.
Do
you think this kind of signature is a valid signature or not ?
Do you
think Acrobat signature validation is correct or should Acrobat be more
permissive ?
Aldo
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