[dila]

The correct key will decrypt some fixed ciphertext into fixed plaintext. The known plaintext is apparently the digits of pi as hex digits.

#include <iostream>
#include <sstream>
#include <stdint.h>

int main(int argc, char* argv[])
{
  if (argc != 2) {
    std::cout << "Usage: " << argv[0] << " <key>" << std::endl;
    return 0;
  }

  uint64_t k;
  std::stringstream ss;
  ss << argv[1];
  ss >> std::hex >> k;
  if (!ss) {
    std::cout << "Invalid key!" << std::endl;
    return 0;
  }

  uint32_t s = 0;
  for (int i = 0; i < 10000; ++i) {
    s = s * 1664525 + 1013904223;
    int j = s % 64;
    uint64_t a = (k >> j) & 1;
    uint64_t b = (k & 1) << j;
    k ^= a ^ b;
  }

  uint64_t r = k ^ 0x1221777f1c3e4341;
  if (r == 0x3141592653589793) {
    std::cout << "Correct key!" << std::endl;
  } else {
    std::cout << "Wrong key!" << std::endl;
  }

  return 0;
}