Init. Commit

finals/scalaralias-crypto-easy/solve/solve.py

@@ -0,0 +1,75 @@
+#!/usr/bin/env python3
+import importlib.util
+import json
+import re
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+def load_verifier():
+    path = ROOT / "public" / "verify_receipt.py"
+    spec = importlib.util.spec_from_file_location("scalaralias_verify", path)
+    module = importlib.util.module_from_spec(spec)
+    assert spec.loader is not None
+    spec.loader.exec_module(module)
+    return module
+
+
+verifier = load_verifier()
+
+
+def malleate_receipt(receipt: dict) -> dict:
+    changed = json.loads(json.dumps(receipt))
+    changed["signature"]["s"] = str(int(changed["signature"]["s"]) + verifier.Q)
+    return changed
+
+
+def binding_key(receipt: dict) -> bytes:
+    return verifier.hashlib.sha256(
+        b"KiteLedger sealed export binding\0" + verifier.canonical_json(receipt)
+    ).digest()
+
+
+def keystream(key: bytes, nonce: bytes, length: int) -> bytes:
+    out = b""
+    counter = 0
+    while len(out) < length:
+        out += verifier.hashlib.sha256(
+            key + b"scalaralias-stream" + nonce + counter.to_bytes(4, "big")
+        ).digest()
+        counter += 1
+    return out[:length]
+
+
+def xor_bytes(left: bytes, right: bytes) -> bytes:
+    return bytes(a ^ b for a, b in zip(left, right))
+
+
+def open_export(receipt: dict, sealed: dict) -> str:
+    if not verifier.verify_receipt(receipt):
+        raise ValueError("receipt does not verify")
+
+    key = binding_key(receipt)
+    nonce = bytes.fromhex(sealed["nonce_hex"])
+    ciphertext = bytes.fromhex(sealed["ciphertext_hex"])
+    tag = bytes.fromhex(sealed["tag_hex"])
+    expected = verifier.hashlib.sha256(key + b"export-tag" + nonce + ciphertext).digest()[:16]
+    if expected != tag:
+        raise ValueError("sealed export tag mismatch")
+    plaintext = xor_bytes(ciphertext, keystream(key, nonce, len(ciphertext))).decode()
+    match = re.search(r"caplag\{[^}]+\}", plaintext)
+    if not match:
+        raise ValueError("flag not found")
+    return match.group(0)
+
+
+def main() -> None:
+    receipt = json.loads((ROOT / "public" / "receipt.json").read_text())
+    sealed = json.loads((ROOT / "public" / "sealed_export.json").read_text())
+    print(open_export(malleate_receipt(receipt), sealed))
+
+
+if __name__ == "__main__":
+    main()