Re: [png-mng-misc] Retaining invalid dSIG chunks

[John B. <jb...@ac...>]

With regard to pngcrush I would expect it to remove dSIG.  I wouldn't expect
any other behavior.  The approach of not touching a signed image unless -rem
dsig is given seems correct and least likely to cause surprises.

With regard to other edit applications I would expect similar behavior,
ideally with a warning if a signed image is edited.

My reasoning is that this is what I would expect with a signed paper
document - only a photographic image (e.g. a fax) retains the signature and
even then the recipient of such an image may refuse to accept it.  Even
repagination invalidates my signature ;-)

The only exception to this that I can think of is letters to newspapers.  At
least in the cultures I am familiar with the editor reserves the right to
copy edit and the copy edits often go beyond simply grammatical errors -
e.g. the sentence structure is frequently changed.  Nevertheless the
signature on the published letter is, in some sense, reduced in authenticity
by this.

So my conclusion is that preserving dSIG under even minor edits which,
supposedly, aren't likely to invalidate the signature is a feature which has
no clear requirement.

John Bowler <jb...@ac...>