Re: IMA metadata format to support fs-verity

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Aug 26, 2020 at 08:53:33PM -0400, Mimi Zohar wrote:
> 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.
> 

Read what I wrote again.  I'm talking about the bytes that are actually signed.

- Eric