linux-kernel - Re: [PATCH 4/8] akcipher: Move the RSA DER encoding to the crypto layer

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Date:	Fri, 26 Feb 2016 11:42:14 +0000
From:	David Howells <dhowells@...hat.com>
To:	Andrew Zaborowski <balrogg@...glemail.com>
Cc:	dhowells@...hat.com, Tadeusz Struk <tadeusz.struk@...el.com>,
	keyrings@...r.kernel.org, linux-security-module@...r.kernel.org,
	zohar@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH 4/8] akcipher: Move the RSA DER encoding to the crypto layer

Andrew Zaborowski <balrogg@...glemail.com> wrote:

> Without overhauling akcipher you could modify pkcs1pad so that sign
> takes the hash as input, adds the DER struct in front of it to build
> the signature, and the verify operation could at most check that the
> DER string matches the hash type and return the hash.  But I think
> RFC2437 suggests that you rather compare the signatures, not the
> hashes.

Whilst that is true about what RFC2437 shows, I wonder how strict it wants to
be about that rather than it just being a convenient way of describing the
algorithm.

The advantage of doing it the way the RFC suggests is that you get to use the
EMSA-PKCS1-V1_5-ENCODE operation twice, thereby saving code and only having
one place for bugs to occur instead of two - but you can argue this either
way.

That said, I would be okay with it returning just the message hash with the
padding stripped off, providing the padding is validated in the crypto layer,
if that's necessary.

David