[openpgp] Re: Review of draft-ietf-openpgp-replacementkey-04

[Heiko Schäfer <[e-mail]>]

Hello Andrew, all,

On 8/7/25 1:46 AM, Andrew Gallagher wrote:
>> Why is the subfield "Record Length" field two octets?  a v6 fingerprint
>> is 32 octets long, and the imprint is the length of the signature's
>> digest.  The longest OpenPGP digest algorithms (SHA512 and SHA3-512) are
>> 64 octets long.  Even if we have some new key version with a 512-bit (64
>> octet) fingerprint,*and* the largest possible sig digest, we're still
>> only at 1+64+64=129 octets for the field.  The first octet will always
>> be zero.  why not make it one octet?
> I’ll admit this is a cautious design decsion. Yes, as far as we can tell now it seems unlikely that a digest algorithm of more than 512 bits will be necessary in the foreseeable future. But if it were, we could conceivably exceed the capacity of a single octet. Given the overall size of the target record, one spare octet is not that significant.

I'm leaning towards preferring a one byte length here.

To my understanding, it is currently considered unlikely that there will 
ever be a need for hash functions with digests larger than 512 bits.
Even 512 bit hashes are, as I understand it, already a design that 
embodies an excess of caution.

While defensive designs are attractive to me, having a two byte length 
here feels like overkill (that I expect would mostly seem baffling and 
odd to implementers).

But then, I'm not a domain expect. I'd be happy to defer to expert 
opinion that favors two byte lengths.

Thanks,
Heiko