What is wrong with this sha 256 function?

Question

I'm starting to use crypto++ lib and maybe I have some misconceptions.

I don't realize why the following code produces a bad sha 256

# include <string>
# include <iostream>
# include "cryptopp/cryptlib.h"
# include <cryptopp/sha.h>
# include <cryptopp/hex.h>

using namespace std;

string sha256_hex(const string & str)
{
  byte digest[CryptoPP::SHA256::DIGESTSIZE];
  CryptoPP::SHA256().CalculateDigest(digest, (byte*) &str[0], str.size());

  string ret;
  CryptoPP::HexEncoder encoder;
  encoder.Attach(new CryptoPP::StringSink(ret));
  encoder.Put(digest, sizeof(digest));
  encoder.MessageEnd();

  return ret;
}

int main(int argc, char *argv[])
{
  auto sha256 = sha256_hex(argv[1]);
  cout << "str     = " << argv[1] << endl
       << "sha 256 = " << sha256_hex(sha256) << endl;

  return 0;
}

The following command

./test-sha256 hello

produces the following output

str     = hello
sha 256 = DD9F20FF4F1DD817C567DE6C16915DC0A731A4DF51088F55CEF4CD2F89CF9620

However according to this online calculator enter link description here, the correct sha 256 for "hello" would be 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824.

So my question is what am I doing wrong?

I have an additional question: how and when the memory used by the StringSink object should be freed?

Thanks in advance

Answer 1

Aren't you computing the hash of the hash here? You call sha256_hex twice, once with argv[1] as argument, and once with sha256 itself as argument.