Signature key and encryption key should be different #623

When declaring an SP there is no way to specify different values for signature and encryption key.
Having the same key for both is generally considered a bad practice and, moreover, can actually work only if using RSA keys.

From version v0.5.0 has been added the possibility to use EC keys, but this collides with the encyption verification feature that cannot be made using an EC key.

The best solution to this would be to have separate values for signature key and encryption key