crypto_sign_ed25519_sk_to_pk

[sarciszewski]

Recently, libsodium implemented a function called crypto_sign_ed25519_sk_to_pk() which, given a secret key, would return the derived public key.

(This will be really advantageous to have for a PHP implementation of SQRL I'm developing which is currently set up to fall back to your implementation if the PHP extension is missing.)

I could submit a pull request if you'd prefer. :)