fix: decrypt otp.bin with the standard library

tests/test_crypto_aes.py

@@ -0,0 +1,54 @@
+"""otp.bin decrypts with the standard library alone.
+
+The build promises stdlib + pyyaml. AES needed cryptography, pycryptodome
+or an openssl binary, and verify.py raised RuntimeError on otp.bin for a
+contributor with none of them.
+"""
+
+from __future__ import annotations
+
+import ast
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+import crypto_verify  # noqa: E402
+
+STDLIB_OR_LOCAL = {"__future__", "hashlib", "struct", "collections", "pathlib", "sect233r1"}
+
+
+class StdlibAes(unittest.TestCase):
+    def test_fips_197_vector(self):
+        key = bytes.fromhex("000102030405060708090a0b0c0d0e0f")
+        cipher = bytes.fromhex("69c4e0d86a7b0430d8cdb78070b4c55a")
+        plain = crypto_verify._decrypt_block(cipher, crypto_verify._expand_key(key))
+        self.assertEqual(plain.hex(), "00112233445566778899aabbccddeeff")
+
+    def test_cbc_chains_blocks(self):
+        # NIST SP 800-38A F.2.2, CBC-AES128 decrypt, first two blocks.
+        key = bytes.fromhex("2b7e151628aed2a6abf7158809cf4f3c")
+        iv = bytes.fromhex("000102030405060708090a0b0c0d0e0f")
+        cipher = bytes.fromhex(
+            "7649abac8119b246cee98e9b12e9197d5086cb9b507219ee95db113a917678b2"
+        )
+        self.assertEqual(
+            crypto_verify._aes_128_cbc_decrypt(cipher, key, iv).hex(),
+            "6bc1bee22e409f96e93d7e117393172aae2d8a571e03ac9c9eb76fac45af8e51",
+        )
+
+    def test_no_third_party_import(self):
+        tree = ast.parse((REPO_ROOT / "scripts" / "crypto_verify.py").read_text())
+        modules = {
+            (node.module if isinstance(node, ast.ImportFrom) else alias.name).split(".")[0]
+            for node in ast.walk(tree)
+            if isinstance(node, (ast.Import, ast.ImportFrom))
+            for alias in node.names
+        }
+        self.assertLessEqual(modules, STDLIB_OR_LOCAL)
+
+
+if __name__ == "__main__":
+    unittest.main()