HashVerificationFilter and "message hash or MAC not valid" exception

Question

In Crpto++ one can easily use pipeline to hash an input.

std::string out;
CryptoPP::SHA256 sha;
CryptoPP::StringSource ss(input,true,
                          new CryptoPP::HashFilter(sha,
                              new CryptoPP::HexEncoder(
                                  new CryptoPP::StringSink(out))));

Now inorder to verify a gives message x produces the same hash output, I would like to use HashVerificationFilter. I have tried it but it doesn't work. Anyone know the correct syntax ?

const int flags = CryptoPP::HashVerificationFilter::THROW_EXCEPTION | CryptoPP::HashVerificationFilter::HASH_AT_END;
CryptoPP::SHA256 sha;
try
{
    CryptoPP::StringSource ss(input + out, true,
                              new CryptoPP::HashVerificationFilter(sha, NULL , flags));
}
catch(const CryptoPP::Exception& e)
{
    std::cerr << e.what() << std::endl;
    exit(1);
}

I get the output :

HashVerificationFilter: message hash or MAC not valid

Answer 1

std::string out;
SHA256 sha;
StringSource ss(input,true,
    new HashFilter(sha,
        new HexEncoder(
            new StringSink(out)
)));

You HexEncode your hash. You need to decode it before passing it to the filter:

StringSource ss(input + out, true,
    new HashVerificationFilter(sha, NULL , flags)
);

Or, remove the encoding filter:

std::string out;
SHA256 sha;
StringSource ss(input,true,
    new HashFilter(sha,
        new StringSink(out)
));