(Personal note) Code to decrypt CryptProtectData'ed data

CryptUnprotectData.cc

//
// Copyright (C) 2016 Arai, Junya
//
// This program reads CryptProtectData'ed hex string from stdin and decrypt it.
// To compile this with mingw32-g++ on Ubuntu 16.04, type
// x86_64-w64-mingw32-g++ --std=c++14 -o CryptUnprotectData.exe CryptUnprotectData.cc -lcrypt32 -mconsole -static
//

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <windows.h>
#include <wincrypt.h>

std::string read_stdin() {
  std::string ln, s;
  while (std::getline(std::cin, ln))
    s.append(ln);
  return s;
}

void remove_space(std::string* const s) {
  s->erase(std::remove_if(s->begin(), s->end(),
                          [](auto c) {return std::isspace(c);}),
           s->end());
}

std::vector<BYTE> to_bytes(const std::string& hex) {
  std::vector<BYTE> bin;
  for (size_t i = 0; i < hex.length() / 2; ++i) {
    const auto s = hex.substr(i * 2, 2);
    size_t     p;
    const int  v = std::stoi(s, &p, 16);
    if (p != 2) {
      std::cerr << "ERROR: Not valid hexadecimal character: " << s << std::endl;
      exit(EXIT_FAILURE);
    }
    bin.push_back(static_cast<BYTE>(v));
  }
  return bin;
}

std::string decrypt(const std::vector<BYTE>& bytes) {
  DATA_BLOB cipher = { static_cast<DWORD>(bytes.size())
                     , const_cast<BYTE*>(&bytes[0]) };
  DATA_BLOB plain;
  if(!CryptUnprotectData(&cipher, 0, 0, 0, 0, 0, &plain)) {
    std::cerr << "ERROR: CryptUnprotectData() failed.\n";
    exit(EXIT_FAILURE);
  }
  return std::string(&plain.pbData[0], &plain.pbData[plain.cbData]);
}

int main(int argc, char* /*argv*/[]) {
  if (argc != 1)
    std::cerr << "WARNING: The arguments are ignored\n";

  auto hex = read_stdin();
  remove_space(&hex);
  if (hex.length() % 2 != 0) {
    std::cerr << "ERROR: The length of ciphertext must be even\n";
    exit(EXIT_FAILURE);
  }

  std::cout << decrypt(to_bytes(hex)) << std::endl;

  return EXIT_SUCCESS;
}