The fact that modifying the observed public key doesn't change timing is not surprising: If the CPU is intended to verify the integrity of the microcode update

The fact that modifying the observed public key doesn't change timing is not surprising: If the CPU is intended to verify the integrity of the microcode update using public-key cryptography, then the public key used to verify the signature must already sit inside the CPU itself. Everything else would be pointless. The function of the public key inside the update is still somewhat mysterious. Conceivably, the signing public key is part of the microcode, and so it would be logical to have a field in the microcode update that can be used to change that signing key as part of the microcode update (so that subsequent updates can be signed with a new key). There are problems with that theory as well, especially that the author was unable to verify the supplied signature with the public key in the microcode update.