Add unit test for ChaCha20 key size validation

Author: Susmita-Chakrabarty

crypto/tests/test_chacha20.py

@@ -7,4 +7,20 @@
 VALID_NONCE = B"\x00" * 12
 assert len(VALID_NONCE) == 12, "VALID_NONCE must be exactly 12 bytes"
 
-secure_rng = random.SystemRandom()
+secure_rng = random.SystemRandom()
+
+def test_invalid_key_size():
+    """Test invalid key sizes."""
+    try:
+        ChaCha20(b"short_key", VALID_NONCE)
+    except ValueError as e:
+        assert "Key must be exactly 32 bytes" in str(e)
+    else:
+        assert False, "ValueError was not raised for short key"
+
+    try:
+        ChaCha20(b"A" * 33, VALID_NONCE)
+    except ValueError as e:
+        assert "Key must be exactly 32 bytes" in str(e)
+    else:
+        assert False, "ValueError was not raised for long key"