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

This line will give the wrong results:

unsigned int nDataLen = sizeof(pbData);

It will always give you the size of a pointer. What you want instead is data.size().

Also, you don't need this part:

if(!CryptoPP::SHA256().VerifyDigest(abDigest, pbData, nDataLen))
{
    return SHA256(data);
}

It should always verify correctly, since you just calculated the digest based on the same data. And if it didn't, you'd go into infinite recursion.

To get readable output, you can convert it to hex. Here's an example for MD5 from the Crypto++ Wiki, it should work for you if you replace MD5 with SHA256:

CryptoPP::MD5 hash;
byte digest[ CryptoPP::MD5::DIGESTSIZE ];
std::string message = "abcdefghijklmnopqrstuvwxyz";

hash.CalculateDigest( digest, (byte*) message.c_str(), message.length() );

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

std::cout << output << std::endl;  

Answer 2

This outputs a base64 string using the CryptoPP::Base64Encoder:

#include "sha.h"
#include "filters.h"
#include "base64.h"

std::string SHA256HashString(std::string aString){
    std::string digest;
    CryptoPP::SHA256 hash;

    CryptoPP::StringSource foo(aString, true,
    new CryptoPP::HashFilter(hash,
      new CryptoPP::Base64Encoder (
         new CryptoPP::StringSink(digest))));

    return digest;
}

Answer 3

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;
}