On Fri, May 12, 2023 at 01:18:45PM +0200, Ard Biesheuvel wrote: > On Fri, 12 May 2023 at 13:04, Matthew Garrett <mjg59@xxxxxxxxxxxxx> wrote: > > > > On Tue, May 09, 2023 at 06:21:44PM -0700, Eric Biggers wrote: > > > > > SHA-1 is insecure. Why are you still using SHA-1? Don't TPMs support SHA-2 > > > now? > > > > TXT is supported on some TPM 1.2 systems as well. TPM 2 systems are also > > at the whim of the firmware in terms of whether the SHA-2 banks are > > enabled. But even if the SHA-2 banks are enabled, if you suddenly stop > > extending the SHA-1 banks, a malicious actor can later turn up and > > extend whatever they want into them and present a SHA-1-only > > attestation. Ideally whatever is handling that attestation should know > > whether or not to expect an attestation with SHA-2, but the easiest way > > to maintain security is to always extend all banks. > > > > Wouldn't it make more sense to measure some terminating event into the > SHA-1 banks instead? Unless we assert that SHA-1 events are unsupported, it seems a bit odd to force a policy on people who have both banks enabled. People with mixed fleets are potentially going to be dealing with SHA-1 measurements for a while yet, and while there's obviously a security benefit in using SHA-2 instead it'd be irritating to have to maintain two attestation policies.
