[openpgp] Re: canonicalizing document for OpenPGP "Text Si gnature" (sig typeID 0x01) [was: Re: Re: Analysis d ocument]

Am 16.09.25 um 20:53 schrieb Daniel Kahn Gillmor:
> On Tue 2025-09-16 13:55:12 +0200, Falko Strenzke wrote:
>> I would happily create an erratum if only I understood what is meant by
>> this text if not reencoding. Is it meant to signify a persistent
>> */modification/* of the document to be signed? I can't believe that, as
>> that would mean a substantial side effect.
> I think it simply means "feed the UTF-8, CRLF form of the document into
> the digest algorithm for signing".
>
> As a concrete example, consider the standard tools iconv [0] and todos
> [1].  If the message is encoded in ISO-8859-1, feed it into the standard
> input of "iconv -f iso-8859-1 -t utf-8 | todos", and use the standard
> output of that pipeline in the signing process.
>
> [0]https://manpages.debian.org/stable/manpages/iconv.1.en.html
> [1]https://manpages.debian.org/stable/tofrodos/todos.1.en.html
>
> This leaves the original document in all its iso-8859-1, linefeed-only
> splendor, while still producing a signature over the canonicalized form.

That's what I call "UTF-8 re-encoding before signature generation and 
verification". So at least we could clarify that. Clearly, this is what 
I understood all the time. Thus there is no need for an erratum.

Best regards,
Falko

>
> A verifier can perform the a comparable transformation before verifying,
> since they might have received the same document encoded in many
> different possible ways, due to "helpful" intervening agents in whatever
> chain of copies took it from the signer to the verifier.
>
> The key insight here is that digital documents can appear in many
> different forms, and there is no difference between the "original" and a
> copy.  At the same time, digital documents are typically not transmitted
> so much as they are copied.
>
> Is the "original" transformed during the course of signing? It could be,
> if the signer decides to replace it with the canonicalized form!  If it
> is, it will be a new file.  If it isn't transformed, that should be OK
> too.  Both encodings of the document will have a verifiable signature,
> if it is sig typeID 0x01 (Text Signature).
>
>          --dkg
>
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Dr. Falko Strenzke

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