Re: [RFC PATCH v3] akcipher: Introduce verify_rsa/verify for public key algorithms

[Herbert Xu <herbert@gondor.apana.org.au>]

On Fri, Jan 18, 2019 at 11:58:46PM +0300, Vitaly Chikunov wrote:
> Previous akcipher .verify() just `decrypts' (using RSA encrypt which is
> using public key) signature to uncover message hash, which was then
> compared in upper level public_key_verify_signature() with the expected
> hash value, which itself was never passed into verify().
> 
> This approach was incompatible with EC-DSA family of algorithms,
> because, to verify a signature EC-DSA algorithm also needs a hash value
> as input; then it's used (together with a signature divided into halves
> `r||s') to produce a witness value, which is then compared with `r' to
> determine if the signature is correct. Thus, for EC-DSA, nor
> requirements of .verify() itself, nor its output expectations in
> public_key_verify_signature() wasn't sufficient.
> 
> Make improved .verify() call which gets hash value as parameter and
> produce complete signature check without any output besides status.
> 
> RSA-centric drivers have replaced verify() with verify_rsa() which
> have old semantic and which they still should implement (if they want
> pkcs1pad to work). If akcipher have .verify_rsa() callback, it will be
> used for a partial verification, which then is finished in
> crypto_akcipher_verify().
> 
> Now for the top level verification only crypto_akcipher_verify() needs
> to be called.
> 
> For pkcs1pad crypto_akcipher_verify_rsa() is introduced which directly
> calls .verify_rsa() for its backend. Without this api PKCS1 can not be
> implemented.
> 
> Tested on x86_64.
> 
> Signed-off-by: Vitaly Chikunov <vt@altlinux.org>

Thanks for working on this!

We have been here before.  We changed the AEAD interface in a way
that is not dissimilar to what you want to do here.

So I think the basic plan should be:

1) Implement new top-level verify, alongside existing verify_rsa.
2) For existing drivers implement a wrapper over verify_rsa.
3) Convert *all* existing users to the new verify API.
4) Remove top-level verify_rsa API.
5) Convert existing drivers over to verify API.
6) Remove verify_rsa completely.

> +int crypto_akcipher_verify(struct akcipher_request *req,
> +			   const unsigned char *digest, unsigned int digest_len)

We should not add new fields outside of akcipher_request.  So
these fields need to go inside it.  However, I think we don't
actually need two new fields.  They could both go into the src
scatterlist.  All we need is a new field, say textlen to indicate
where the text stops and where the hash starts.  We could also
overlay textlen over dstlen as it would now be unused for verify.

The advantage of having it in one scatterlist is that for those
users that already have the two pieces together could simply provide
a single SG element (I don't know how likely that is in practice
though).

For others you would simply tack on an extra SG element.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt