Init. Commit

qualifiers/shardslope-crypto-easy/solve/solve.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+import argparse
+import json
+import pathlib
+import re
+
+
+def inv(value: int, prime: int) -> int:
+    return pow(value % prime, -1, prime)
+
+
+def int_to_bytes(value: int) -> bytes:
+    return value.to_bytes(max(1, (value.bit_length() + 7) // 8), "big")
+
+
+def recover_slope(share_a: dict, share_b: dict, prime: int) -> int:
+    x1, y1 = int(share_a["x"]), int(share_a["y"])
+    x2, y2 = int(share_b["x"]), int(share_b["y"])
+    return ((y2 - y1) * inv(x2 - x1, prime)) % prime
+
+
+def recover_secret(share: dict, slope: int, prime: int) -> int:
+    return (int(share["y"]) - slope * int(share["x"])) % prime
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "export",
+        nargs="?",
+        default="../public/escrow_export.json",
+        help="path to escrow_export.json",
+    )
+    args = parser.parse_args()
+
+    data = json.loads(pathlib.Path(args.export).read_text())
+    prime = int(data["field_prime"])
+    calibration = data["calibration_batch"]["shares"]
+    production_share = data["quarantine_batch"]["shares"][0]
+
+    slope = recover_slope(calibration[0], calibration[1], prime)
+    secret_int = recover_secret(production_share, slope, prime)
+    flag = int_to_bytes(secret_int).decode()
+    if not re.fullmatch(r"caplag\{[^{}]+\}", flag):
+        raise SystemExit("recovered secret is not a caplag flag")
+    print(flag)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())