Generating a SHA256 hash with Crypto++, using a string as input and output?

Question

I need an example of how to use Crypto++ to generate a SHA256 hash from a std::string and output a std::string. I can\'t seem to figure it out. Everything I\'ve tried gives me i

Answer 1

Your code will expect a null-terminated string from the buffer you supply to the string constructor! Which means the result will almost certainly be wrong.

To enforce the digest size and, use the following instead:

return std::string((char*)abDigest, CryptoPP::SHA256::DIGESTSIZE);

Also with respect to printing it the following correctly produces the test vector BA7816BF8F01CFEA414140DE5DAE2223B00361A396177A9CB410FF61F20015AD for the string "abc"

std::string string_to_hex(const std::string& input)
{
  static const char* const lut = "0123456789ABCDEF";
  size_t len = input.length();

  std::string output;
  output.reserve(2 * len);
  for (size_t i = 0; i < len; ++i)
  {
    const unsigned char c = input[i];
    output.push_back(lut[c >> 4]);
    output.push_back(lut[c & 15]);
  }
  return output;
}

std::string SHA256(std::string data)
{
  CryptoPP::byte const* pbData = (CryptoPP::byte*)data.data();
  unsigned int nDataLen = data.length();
  CryptoPP::byte abDigest[CryptoPP::SHA256::DIGESTSIZE];

  CryptoPP::SHA256().CalculateDigest(abDigest, pbData, nDataLen);

  // return string((char*)abDigest);  -- BAD!!!
  return std::string((char*)abDigest, CryptoPP::SHA256::DIGESTSIZE);
}

void test_cryptopp() {
  std::cout << string_to_hex(SHA256("abc")) << std::endl;
}