Libsodium sign and verify

Question

The function

int crypto_sign(unsigned char *sm, unsigned long long *smlen,
            const unsigned char *m, unsigned long long mlen,
            const unsigned char *sk); 

prepends a signature to message m. However, I am unable to find in the documentation wether libsodium signs the message directly, or creates a hash first. If it uses a hash, what kind of hashing method uses?

Answer 1

It uses the EdDSA algorithm, which is similar to Schnorr signatures.

The message is indeed hashed (actually twice) using the SHA512 function in order to compute the signature.