Support signing a pre-calculated hash (#87)

* Split the hashing out of the sign method

This code change adds support to split the hashing of a message
and the actual signing of the message.

* Updating unit test and documentation

This commit updates the unit test and usage docs. In addition,
This change removes a redundant error check inside rsa.sign().

* Refactore unit tests and code comments

Removed the print statements from the unit test and refactored a
few code comments to improve readability.

* Rename hash function

The new hash function had the same name as a function in the
standard library. This commit changes the name to avoid conflicts.

* Rename hash function to compute_hash()

This commit renames the hash function to compute_hash().

1 files changed, 21 insertions, 0 deletions

diff --git a/tests/test_pkcs1.py b/tests/test_pkcs1.py
index a8afea7..a96c8be 100644
--- a/tests/test_pkcs1.py
+++ b/tests/test_pkcs1.py
@@ -111,3 +111,24 @@ class SignatureTest(unittest.TestCase):
         signature2 = pkcs1.sign(message, self.priv, 'SHA-1')
 
         self.assertEqual(signature1, signature2)
+
+    def test_split_hash_sign(self):
+        """Hashing and then signing should match with directly signing the message. """
+
+        message = b'je moeder'
+        msg_hash = pkcs1.compute_hash(message, 'SHA-256')
+        signature1 = pkcs1.sign_hash(msg_hash, self.priv, 'SHA-256')
+
+        # Calculate the signature using the unified method
+        signature2 = pkcs1.sign(message, self.priv, 'SHA-256')
+
+        self.assertEqual(signature1, signature2)
+
+    def test_hash_sign_verify(self):
+        """Test happy flow of hash, sign, and verify"""
+
+        message = b'je moeder'
+        msg_hash = pkcs1.compute_hash(message, 'SHA-256')
+        signature = pkcs1.sign_hash(msg_hash, self.priv, 'SHA-256')
+
+        self.assertTrue(pkcs1.verify(message, signature, self.pub))