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Re: [openpgp] Question on computing v5 signatures

"brian m. carlson" <sandals@crustytoothpaste.net> Thu, 02 May 2019 23:52 UTC

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Date: Thu, 02 May 2019 23:52:19 +0000
From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: Heiko Stamer <HeikoStamer@gmx.net>, OpenPGP WG <openpgp@ietf.org>
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Subject: Re: [openpgp] Question on computing v5 signatures
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On Thu, May 02, 2019 at 07:13:51PM +0200, Werner Koch wrote:
> On Wed,  1 May 2019 21:29, HeikoStamer@gmx.net said:
> > I am wondering why a number of eight-octet size is used here. The
> > biggest field, AFAIS i.e. the hashed subpacket data area, is limited
> > by the included two-octet hashed subpacket length. So why 64 bit?
> 
> That seems to be a misunderstanding.  The original patch from Brian
> (9b846b7e from 2017-02-13) had this text:
> 
>   V5 signatures instead hash in a ten-octet trailer: the version of the
>   Signature packet, i.e., 0x05; 0xFF; and an eight-octet, big-endian
>   number that is the length of the hashed data from the Signature packet
>   (note that this number does not include these final ten octets).
> 
> He might have extended the counter to eight octets to better distinguish
> a V5 signature form a V4 signature.  Reading this I falsely concluded
> that the 32 bit counter of a V4 signature might overflow and thus added
> 
>   The four-octet big-endian number is considered to be an unsigned
>   integer modulo 2^32.
> 
> to the V4 signature desciption.  Obviously we both missed that a 32 bit
> counter is sufficient for a a max of 2*2^16+something octets.

Yes, I think I overlooked that. The goal was to avoid using a four-octet
length for the length of the actual data in a binary or text document
signature, but I misread and didn't realize that this is the length of
the signature packet, not the length of the data to be signed.
-- 
brian m. carlson: Houston, Texas, US
OpenPGP: https://keybase.io/bk2204

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