initial

decrypt.py

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+"""Decrypt the LoHoSearch dataset CSV.
+
+The encrypted CSV must have three columns: question, answer, canary.
+Each question/answer is base64(XOR(plaintext, SHA256-derived keystream)).
+
+Decrypts every row's question and answer and writes them to an output CSV.
+"""
+
+import argparse
+import base64
+import csv
+import hashlib
+import sys
+
+ENCRYPTED_COLUMNS = ["question", "answer"]
+
+
+def derive_key(password: str, length: int) -> bytes:
+    """Derive a fixed-length key from the password using SHA256."""
+    hasher = hashlib.sha256()
+    hasher.update(password.encode())
+    key = hasher.digest()
+    return key * (length // len(key)) + key[: length % len(key)]
+
+
+def decrypt(ciphertext_b64: str, password: str) -> str:
+    """Decrypt base64-encoded ciphertext with XOR."""
+    encrypted = base64.b64decode(ciphertext_b64)
+    key = derive_key(password, len(encrypted))
+    decrypted = bytes(a ^ b for a, b in zip(encrypted, key))
+    return decrypted.decode()
+
+
+def read_rows(path: str):
+    with open(path, newline="", encoding="utf-8") as f:
+        reader = csv.DictReader(f)
+        return list(reader), (reader.fieldnames or [])
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description="Decrypt the LoHoSearch dataset CSV.")
+    parser.add_argument("encrypted", help="Encrypted CSV with question, answer, canary columns.")
+    parser.add_argument(
+        "--output",
+        default="LoHoSearch_decrypted.csv",
+        help="Path to write the decrypted rows (question, answer).",
+    )
+    args = parser.parse_args()
+
+    csv.field_size_limit(sys.maxsize)
+
+    enc_rows, enc_fields = read_rows(args.encrypted)
+
+    for col in ENCRYPTED_COLUMNS + ["canary"]:
+        if col not in enc_fields:
+            raise SystemExit(f"Encrypted CSV is missing column: {col}")
+
+    decrypted_rows = []
+    for enc in enc_rows:
+        canary = enc["canary"]
+        decrypted_rows.append(
+            {col: decrypt(enc[col], canary) for col in ENCRYPTED_COLUMNS}
+        )
+
+    with open(args.output, "w", newline="", encoding="utf-8") as f:
+        writer = csv.DictWriter(f, fieldnames=ENCRYPTED_COLUMNS)
+        writer.writeheader()
+        writer.writerows(decrypted_rows)
+    print(f"Wrote {len(decrypted_rows)} decrypted rows -> {args.output}")
+
+
+if __name__ == "__main__":
+    main()