Initial commit

experiments/test1_into_bytes.py

@@ -0,0 +1,56 @@
+"""Test 1: does cryptography's into-API (CipherContext.update_into) accept an
+immutable bytes object as the output buffer, despite bytes being read-only?
+
+Python's buffer protocol will hand out a buffer descriptor for bytes (pyo3
+requests PyBUF_FULL_RO), so the only thing stopping the write is an explicit
+readonly check in cryptography's Rust binding. This script finds out, and
+verifies correctness against the normal bytes-returning update() call.
+"""
+
+from cryptography.hazmat.primitives.ciphers import Cipher
+from cryptography.hazmat.primitives.ciphers.algorithms import ChaCha20
+
+KEY = bytes(range(32))
+NONCE = bytes(16)
+DATA = b"The quick brown fox jumps over the lazy dog"
+
+
+def fresh_encryptor():
+    return Cipher(ChaCha20(KEY, NONCE), mode=None).encryptor()
+
+
+def main():
+    expected = fresh_encryptor().update(DATA)
+
+    # control: writable bytearray must work
+    buf = bytearray(len(DATA))
+    n = fresh_encryptor().update_into(DATA, buf)
+    assert bytes(buf) == expected and n == len(DATA)
+    print(f"control  bytearray       : ok ({n} bytes written)")
+
+    # the hack: immutable bytes as the output buffer.
+    # Fresh object: refcount 1, zero-filled, cached hash not yet computed,
+    # so no other code can observe the pre-mutation state.
+    out = bytes(len(DATA))
+    print(f"before   bytes({len(DATA)})         : {out[:16].hex()}...")
+    try:
+        n = fresh_encryptor().update_into(DATA, out)
+    except Exception as e:  # noqa: BLE001 -- probing behavior, any error is the answer
+        print(f"bytes-hack               : REJECTED  {type(e).__name__}: {e}")
+        return
+    print(f"bytes-hack               : ACCEPTED ({n} bytes written)")
+    print(f"after    bytes({len(DATA)})         : {out[:16].hex()}...")
+    print(f"mutated in place         : {out != bytes(len(DATA))}")
+    print(f"matches update() result  : {out == expected}")
+
+    # and via an explicit memoryview (readonly flag should propagate)
+    try:
+        fresh_encryptor().update_into(DATA, memoryview(bytes(len(DATA))))
+    except Exception as e:  # noqa: BLE001 -- probing behavior, any error is the answer
+        print(f"memoryview(bytes)        : REJECTED  {type(e).__name__}: {e}")
+    else:
+        print("memoryview(bytes)        : ACCEPTED")
+
+
+if __name__ == "__main__":
+    main()