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Subject: Re: IMA metadata format to support fs-verity
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Eric Biggers <ebiggers@kernel.org>,
        Chuck Lever <chuck.lever@oracle.com>
Cc:     linux-fscrypt@vger.kernel.org, linux-integrity@vger.kernel.org,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Date:   Wed, 26 Aug 2020 20:53:33 -0400
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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