Re: [PATCH] crypto: pkcs7 - use constant-time digest comparison

[Ignat Korchagin]

On Sun, Feb 1, 2026 at 5:41 AM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> On Sat, Jan 31, 2026 at 07:55:03PM -0800, Daniel Hodges wrote:
> > This creates a timing side-channel that could allow an
> > attacker to forge valid signatures by measuring verification time
> > and recovering the expected digest value byte-by-byte.
>
> Good luck with that.  The memcmp just checks that the CMS object
> includes the hash of the data as a signed attribute.  It's a consistency
> check of two attacker-controlled values, which happens before the real
> signature check.  You may be confusing it with a MAC comparison.

On top of that the CMS object and the hash inside is "public", so even
if you have state-of-the-art quantum computer thing you can just take
the object and forge the signature "offline"

> - Eric