HackTheBox Brainy’s Chyper Challenge

#!/usr/bin/python3
import json

def run_bf(code):
    mem = [0] * 30000
    dp = cp = 0
    output = ""
    brackets = {}
    stack = []
    for i, c in enumerate(code):
        if c == '[': stack.append(i)
        elif c == ']':
            start = stack.pop()
            brackets[start] = i
            brackets[i] = start
    while cp < len(code):
        c = code[cp]
        if c == '>': dp += 1
        elif c == '<': dp -= 1
        elif c == '+': mem[dp] = (mem[dp] + 1) % 256
        elif c == '-': mem[dp] = (mem[dp] - 1) % 256
        elif c == '.': output += chr(mem[dp])
        elif c == '[' and mem[dp] == 0: cp = brackets[cp]
        elif c == ']' and mem[dp] != 0: cp = brackets[cp]
        cp += 1
    return output

def decrypt_rsa_crt(c, p, q, dp, dq):
    def mod_inverse(a, m):
        def egcd(a, b):
            if a == 0: return b, 0, 1
            g, x, y = egcd(b % a, a)
            return g, y - (b // a) * x, x
        _, x, _ = egcd(a, m)
        return x % m
    m1 = pow(c, dp, p)
    m2 = pow(c, dq, q)
    qinv = mod_inverse(q, p)
    h = (qinv * (m1 - m2)) % p
    m = m2 + h * q
    return m

def main():
    with open('brainy.txt', 'r') as f:
        bf_code = f.read().strip()
    output = run_bf(bf_code)
    print("[+] Brainfuck decoded")
    params = {k: int(v) for k, v in 
             (pair.split(':') for pair in 
              output.strip('{}').split(','))}
    print("[+] Extracted RSA parameters")
    m = decrypt_rsa_crt(
        params['c'], params['p'], params['q'],
        params['dp'], params['dq']
    )
    print("[+] Decryption complete")
    try:
        hex_str = hex(m)[2:]
        if len(hex_str) % 2: hex_str = '0' + hex_str
        flag = bytes.fromhex(hex_str).decode()
        print("Flag:", flag)
    except:
        print("[-] Error decoding result")

if __name__ == "__main__":
    main()