Deterministic signatures digest prefix

[Richard Ulrich]

Hi,

we sign some files such as grub config, kernel, initrd for a Live DVD so that
grub can verify them when the system boots. Most of the DVD ISO builds
reproducibly. At the moment I am trying to get also those signatures
reproducible.
By using faketime, I harmonized the timestamp that is part of the signature. The
main difference I see at the moment is the "Digest prefix"
Even with lots of searching and reading all sorts of documentation and forum
posts, I was not able to figure out how to make the digest prefix constant.

This is one of the commands I use to produce the signature:

faketime -f "2025-05-29 00:00:00" gpg --local-user ccc --digest-algo SHA512 --
detach-sign boot/vmlinuz

The actual private key is on a YubiKey, but I don't think that makes a
difference.

Then I examine the signature with:

sq packet dump --hex boot/vmlinuz.sig

With kind regards
Richard
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