Solution to PlaidCTF 2022 Process Sample | by H4K47L4N1M

solution.py

# made by H4K47L4N1M | @lior5654

from typing import Text, List
from sage.all import *


# Constants

ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ_!?"
INV_ALPHABET = {c: i for i, c in enumerate(ALPHABET)}

N = len(ALPHABET)
R = IntegerModRing(N)

SAMPLE_ID = "_LLPEK!M!FHL?CPI?SBSHCYYH"
ENC = "XKGHB_LRYIHUC?CC_BUPWUJ?S"
SECRET_LEN = 25


def text_to_sequence(text: Text) -> Text:
    return [INV_ALPHABET[c] for c in text]


def sequence_to_text(sequence: List[int]) -> Text:
    return [ALPHABET[x] for x in sequence]



def recover(sample_id: Text, enc: Text) -> Text:
    sample_id = text_to_sequence(sample_id)
    enc = text_to_sequence(enc)
    res = []

    block_matrix = [
        [sample_id[t] for t in range(i, 25, 5)] for i in range(0, 5)
    ]

    block_matrix = Matrix(R, block_matrix).T


    for row_index in range(0, 5):
        goal_vector = vector(R, [enc[i] for i in range(row_index, 25, 5)])
        for v in block_matrix.solve_right(goal_vector):
            res.append(v)
    
    return "".join(sequence_to_text(res))


def main():
   print(recover(SAMPLE_ID, ENC))

if __name__ == "__main__":
    main()