add ed25519 to curve25519 conversion helpers

[abom]

Don't know if it's the right place to add these helpers or If I did any unintended mistakes or more tests need to be added!

If this need to be moved to Util module or somewhere else, just let me know.

https://libsodium.gitbook.io/doc/advanced/ed25519-curve25519 should be read for anyone who uses this (we already used this in one of our projects).

Resolves #204.

[tarcieri]

The methods should at least indicate that they're converting from the Edwards ("Ed25519") to Montgomery ("Curve25519", "X25519") forms.

Unfortunately this gem uses the now outdated "Curve25519" naming which is a bit ambiguous (as Curve25519 is also often used to describe the curve regardless of what form it's expressed in).

All that said, maybe to_curve25519_private_key and to_curve25519_public_key?

[abom]

Yes, that would be less ambiguous for sure, updated method names and added (x25519) in the doc.

[tarcieri]

It'd be good to find some test vectors and ensure the output is correct for both methods

[abom]

added test vectors (generated by pynacl).

[tarcieri]

Looks good, thanks!

[nlsrchtr]

Hi @tarcieri and @abom,

thanks for your work here. I was trying this on main branch with the following code:

signing_private_key = RbNaCl::Signatures::Ed25519::SigningKey.new(SecureRandom.random_bytes(32))
signing_public_key = signing_private_key.verify_key
private_key = signing_private_key.to_curve25519_private_key
public_key = private_key.public_key

and was running into undefined method "to_curve25519_private_key" for #<RbNaCl::Signatures::Ed25519::SigningKey:eddda15d> (NoMethodError).

Is there anything I'm missing here? Thanks for your help!

[abom]

Hi @nlsrchtr

I tested your example and it's working, please make sure you're using the correct version of rbnacl. I think you need to make sure it does not load it from another path. Also, a complete backtrace would help.