Add unit test for negative counter validation

Author: Susmita-Chakrabarty

crypto/tests/test_chacha20.py

@@ -40,3 +40,13 @@ 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]:
+        try:
+            ChaCha20(VALID_KEY, VALID_NONCE, counter=invalid_counter)
+        except ValueError as e:
+            assert "Counter must be a non-negative integer" in str(e)
+        else:
+            assert False, f"ValueError not raised for counter={invalid_counter}"