Question

I am trying to hash a string then use that hash as the name of a file.

My problem: is that all the C++ hashers/crypters I have come across hash a std::string or char* & return the hashed string as an unsigned char*?

How do I convert that unsigned char* to a char* or std::string so I can then write it to a file or filename? Or do I not need to convert it to a normal string to use it?

tstring hashString( tstring str )
{
    // Post: 

    unsigned char hashStr[SHA256_DIGEST_SIZE];
     std::string messageStr = str;

    SHA256::getInstance()->digest( messageStr, hashStr );
    //TCHAR *hashStrSigned = reinterpret_cast <TCHAR*>(hashStr);

    // can I just use this hashStr to create a file? Or do I have to convert it to char* to use?

    Handle newF = CreateFile( (LPTSTR)hashStr, GENERIC_ALL, 0, NULL, CREATE_ALWAYS, 
                              0, NULL );

    return tstring(hashStrSigned);
}

Answer 1

The result of the hash likely contain bytes that map to chars that are not allowed in a filename (e.g. null-char, ‘+’, ‘?’, etc….). So “hashStr” in your code isn’t likely to be a string, but just an array of bytes that aren’t null terminated.

Try this little function to convert from “binary hash to string suitable for file name”

void HashToString(unsigned char* shaHash, std::string* pStr)
{
    char szHash[SHA256_DIGEST_SIZE*2+1];
    char* pszOut = szHash;

    for (int x = 0; x < SHA256_DIGEST_SIZE; x++)
    {
        sprintf(pszOut, "%.2X", shaHash[x]); // write out as hex chars
        pszOut += 2; // advance 2 chars
    }
    *pszOut = '\0'; // null terminate

    *pStr = std::string(szHash);
}

In your above code example, you’d call it as follows:

std::string str;
HashToString(hashStr, &str);
HANDLE newF = CreateFileA(str.c_str(), ...);