The crypto_sign_open() function verifies the signature in 'sm' using the receiver's public key 'pk'.
decrypted message, last 64 bytes are useless zeroes, must be of size at least sm.length
= signed message
= public key, slice size must be at least crypto_sign_PUBLICKEYBYTES, extra ignored
success flag
See Implementation
The crypto_sign_open() function verifies the signature in 'sm' using the receiver's public key 'pk'.