CaesarCerydra

Crypto - CaesarCerydra Writeup

解密脚本

def is_prime(n):
    if n < 2:
        return False
    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            return False
    return True

def main():
    primes = []
    for num in range(2048, 4096):
        if is_prime(num):
            primes.append(num)
    ciphertext = "GXAKMS{34hE_djwwvk_UfGJkw_WFU_LUs_RWF8sn_Kp0rWI}"
    base = 14
    for r in primes:
        plaintext = []
        for i, c in enumerate(ciphertext):
            term1 = base
            term2 = (i + 1) * r
            term3 = i * (i + 1) * (2 * i + 1) // 6
            offset_mod26 = (term1 + term2 + term3) % 26

            if 'A' <= c <= 'Z':
                dec_char = chr((ord(c) - ord('A') - offset_mod26) % 26 + ord('A'))
                plaintext.append(dec_char)
            elif 'a' <= c <= 'z':
                dec_char = chr((ord(c) - ord('a') - offset_mod26) % 26 + ord('a'))
                plaintext.append(dec_char)
            else:
                plaintext.append(c)

        plaintext_str = ''.join(plaintext)
        if plaintext_str.startswith("XSWCTF{") in plaintext_str:
            print(f"r = {r} -> {plaintext_str}")
            break

if __name__ == '__main__':
main()

注：base=14是因为base = 33550336 % 26 = 14

感谢出题人审阅 辛苦了