Allow to set saltLength for RSA-PSS

[tex0l]

Currently, the saltLength used for PSS signatures is the cryptopp default one of 32 bytes.

However, it is very common to specify it (the RFC has this parameters available https://www.rfc-editor.org/rfc/rfc3447#section-9.1.1). Here are examples in JS that allow to:

• https://www.npmjs.com/package/node-forge#rsa
• https://nodejs.org/api/crypto.html#cryptosignalgorithm-data-key-callback which default to the maximum salt length (key size in bytes, minus hash length, minus 2, which gives 478 bytes for a 4096bits (512 bytes) key with SHA256 (32 bytes).

Would it be possible to set such a parameter upon signing?

I made it work like this with a hard-coded salt value of 478 bytes (see above why 478):

struct PSS_CUSTOM : public SignatureStandard
{
    typedef PSSR_MEM<false, P1363_MGF1, 478> SignatureMessageEncodingMethod;
};

and then instead of setting PSSR I set PSS_CUSTOM. It is compatible with my reference implementation.

EDIT: I wasnt able to pass a saltLength as an argument, as the saltLength of 478 I set in my example is defined statically and not dynamically.. I don't see how it'd be possible with cryptopp to pass it dynamically, I'm not fluent enough in cpp... Do you think of anything? This is really blocking for me, as I need to make this implementation compatible with the node.js default implementation of PSS which used the max salt length.

EDIT2: apparently, the saltLength can only be set as a template parameter at compile time.. see https://groups.google.com/g/cryptopp-users/c/UFVcV9Ml_Cs/m/NFYEfxhF0CEJ would it be possible to define a max PSS length for relevant hashing algorithms and key sizes?