Re: IMA metadata format to support fs-verity

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2020-08-26 at 13:51 -0700, Eric Biggers wrote:
> Of course, the bytes that are actually signed need to include not just the hash
> itself, but also the type of hash algorithm that was used.  Else it's ambiguous
> what the signer intended to sign.
> 
> Unfortunately, currently EVM appears to sign a raw hash, which means it is
> broken, as the hash algorithm is not authenticated.  I.e. if the bytes
> e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 are signed,
> there's no way to prove that the signer meant to sign a SHA-256 hash, as opposed
> to, say, a Streebog hash.  So that will need to be fixed anyway.  While doing
> so, you should reserve some fields so that there's also a flag available to
> indicate whether the hash is a traditional full file hash or a fs-verity hash.

The original EVM HMAC is still sha1, but the newer portable & immutable
EVM signature supports different hash algorithms.

Mimi