Add test case that verifies that ChaCha20 produces a reversible ciphertext

Author: Susmita-Chakrabarty

crypto/tests/test_chacha20.py

@@ -40,7 +40,7 @@ def test_invalid_nonce_size():
         assert "Nonce must be exactly 12 bytes" in str(e)
     else:
         assert False, "ValueError was not raised for long nonce"
-        
+
 def test_invalid_counter_values():
     """Test invalid counter values for ChaCha20."""
     for invalid_counter in [-1, -100, -999999]:
@@ -50,3 +50,12 @@ def test_invalid_counter_values():
             assert "Counter must be a non-negative integer" in str(e)
         else:
             assert False, f"ValueError not raised for counter={invalid_counter}"
+
+def test_encrypt_decrypt():
+    """Test encryption and decryption are symmetric."""
+    cipher = ChaCha20(VALID_KEY, VALID_NONCE)
+    plaintext = b"Hello, ChaCha20!"
+    ciphertext = cipher.encrypt(plaintext)
+    decrypted = cipher.decrypt(ciphertext)
+
+    assert decrypted == plaintext, "Decryption failed. Plaintext does not match."