private byte[] noPrefixSign(final byte[] data) throws GeneralSecurityException {
    return Ed25519.sign(data, publicKey, hashedPrivateKey);
  }
Constructs a Ed25519Sign with the {@code privateKey}.

@param privateKey 32-byte random sequence.
@throws GeneralSecurityException if there is no SHA-512 algorithm defined in {@link
    EngineFactory}.MESSAGE_DIGEST.
java_src/src/main/java/com/google/crypto/tink/subtle/Ed25519Sign.java