decrypt.py

"""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()