Re: phasing out SHA1 for digest creation

On 12/6/24 02:11, Wiktor Kwapisiewicz via Gnupg-devel wrote:
> On 6.12.2024 01:54, Jacob Bachmeyer via Gnupg-devel wrote:
>> This could be as simple as including a nonce in the signature.
>
> Just for the record, due to the way of how OpenPGP hashes files, 
> there's plenty of other metadata influencing the final hash e.g. 
> signature creation time (I guess it's rather improbable that the 
> attacker would control that up to a second precision; it's not a high 
> entropy data though; also: some implementations embed nonce data in 
> notations).

So PGP is already resistant to such attacks and can be made entirely 
immune by simply adding a nonce to the signature, which the protocol 
already allows?

Does GPG already do this?  If not, can this message count as a feature 
request for secure nonces in signatures?  Even 64 bits should be enough 
to guard against collision-based forgeries, but I would suggest a nonce 
length equal to one half of the digest length.

(I initially wanted to propose making the nonce length equal to the 
digest length, but the pigeonhole principle suggests that a nonce that 
long *might* make signatures malleable with enough computation---an 
attacker *might* be able to use the nonce field to make a signature 
"fit" a different document hash.  I do not know if factoring a 4096-bit 
RSA key would be easier---I would expect such an attack to be 
computationally infeasible.)

Alternately, for the next PGP protocol version, including a nonce N in 
the calculation of the digest H and also signing {N,H} instead of just H 
should allow longer nonces without risking the signature integrity.  (I 
wonder if the SSH developers were thinking along those lines...)


-- Jacob



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