Rework to use pysodium, document and test further

Still on key_pair, sign and verify

test/test_crypto.py

@@ -1,9 +1,31 @@
 """Cryptography primitives test module."""
 
-from hypercore_crypto import key_pair
+import pytest
+
+from hypercore_crypto import key_pair, sign, verify
+from pysodium import crypto_sign_PUBLICKEYBYTES, crypto_sign_SECRETKEYBYTES
+
+
+def test_key_pair_seed_length():
+    with pytest.raises(ValueError) as exception:
+        key_pair(b'wrong')
+        assert 'must be of length' in str(exception)
 
 
 def test_key_pair_length():
     public_key, secret_key = key_pair()
-    assert len(public_key) == 32
-    assert len(secret_key) == 64
+    assert len(public_key) == crypto_sign_PUBLICKEYBYTES
+    assert len(secret_key) == crypto_sign_SECRETKEYBYTES
+
+
+def test_sign():
+    message = b'mymessage'
+    _, secret_key = key_pair()
+    assert message not in sign(message, secret_key)
+
+
+def test_verify():
+    message = b'mymessage'
+    public_key, secret_key = key_pair()
+    signature = sign(message, secret_key)
+    assert verify(message, signature, public_key)