Generate sha256 with OpenSSL and C++

I'm looking to create a hash with sha256 using openssl and C++. I know there's a similar post at Generate SHA hash in C++ using OpenSSL library, but I'm looking to specifically create sha256.

UPDATE:

Seems to be a problem with the include paths. It can't find any OpenSSL functions even though I included

#include "openssl/sha.h"

and I included the paths in my build

-I/opt/ssl/include/ -L/opt/ssl/lib/ -lcrypto 

Here's how I did it:

void sha256_hash_string (unsigned char hash[SHA256_DIGEST_LENGTH], char outputBuffer[65])
{
    int i = 0;

    for(i = 0; i < SHA256_DIGEST_LENGTH; i++)
    {
        sprintf(outputBuffer + (i * 2), "%02x", hash[i]);
    }

    outputBuffer[64] = 0;
}


void sha256_string(char *string, char outputBuffer[65])
{
    unsigned char hash[SHA256_DIGEST_LENGTH];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    SHA256_Update(&sha256, string, strlen(string));
    SHA256_Final(hash, &sha256);
    int i = 0;
    for(i = 0; i < SHA256_DIGEST_LENGTH; i++)
    {
        sprintf(outputBuffer + (i * 2), "%02x", hash[i]);
    }
    outputBuffer[64] = 0;
}

int sha256_file(char *path, char outputBuffer[65])
{
    FILE *file = fopen(path, "rb");
    if(!file) return -534;

    unsigned char hash[SHA256_DIGEST_LENGTH];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    const int bufSize = 32768;
    unsigned char *buffer = malloc(bufSize);
    int bytesRead = 0;
    if(!buffer) return ENOMEM;
    while((bytesRead = fread(buffer, 1, bufSize, file)))
    {
        SHA256_Update(&sha256, buffer, bytesRead);
    }
    SHA256_Final(hash, &sha256);

    sha256_hash_string(hash, outputBuffer);
    fclose(file);
    free(buffer);
    return 0;
}

It's called like this:

static unsigned char buffer[65];
sha256("string", buffer);
printf("%s\n", buffer);

std based

#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>

using namespace std;

#include <openssl/sha.h>
string sha256(const string str)
{
    unsigned char hash[SHA256_DIGEST_LENGTH];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    SHA256_Update(&sha256, str.c_str(), str.size());
    SHA256_Final(hash, &sha256);
	stringstream ss;
    for(int i = 0; i < SHA256_DIGEST_LENGTH; i++)
    {
        ss << hex << setw(2) << setfill('0') << (int)hash[i];
    }
    return ss.str();
}

int main() {
	cout << sha256("1234567890_1") << endl;
	cout << sha256("1234567890_2") << endl;
	cout << sha256("1234567890_3") << endl;
	cout << sha256("1234567890_4") << endl;
	return 0;
}

Using OpenSSL's EVP interface (the following is for OpenSSL 1.1):

#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>
#include <openssl/evp.h>

bool computeHash(const std::string& unhashed, std::string& hashed)
{
    bool success = false;

    EVP_MD_CTX* context = EVP_MD_CTX_new();

    if(context != NULL)
    {
        if(EVP_DigestInit_ex(context, EVP_sha256(), NULL))
        {
            if(EVP_DigestUpdate(context, unhashed.c_str(), unhashed.length()))
            {
                unsigned char hash[EVP_MAX_MD_SIZE];
                unsigned int lengthOfHash = 0;
    
                if(EVP_DigestFinal_ex(context, hash, &lengthOfHash))
                {
                    std::stringstream ss;
                    for(unsigned int i = 0; i < lengthOfHash; ++i)
                    {
                        ss << std::hex << std::setw(2) << std::setfill('0') << (int)hash[i];
                    }
    
                    hashed = ss.str();
                    success = true;
                }
            }
        }
    
        EVP_MD_CTX_free(context);
    }

    return success;
}

int main(int, char**)
{
    std::string pw1 = "password1", pw1hashed;
    std::string pw2 = "password2", pw2hashed;
    std::string pw3 = "password3", pw3hashed;
    std::string pw4 = "password4", pw4hashed;

    hashPassword(pw1, pw1hashed);
    hashPassword(pw2, pw2hashed);
    hashPassword(pw3, pw3hashed);
    hashPassword(pw4, pw4hashed);

    std::cout << pw1hashed << std::endl;
    std::cout << pw2hashed << std::endl;
    std::cout << pw3hashed << std::endl;
    std::cout << pw4hashed << std::endl;

    return 0;
}

The advantage of this higher level interface is that you simply need to swap out the EVP_sha256() call with another digest's function, e.g. EVP_sha512(), to use a different digest. So it adds some flexibility.

A more "C++"ish version

#include <iostream>
#include <sstream>

#include "openssl/sha.h"

using namespace std;

string to_hex(unsigned char s) {
    stringstream ss;
    ss << hex << (int) s;
    return ss.str();
}   

string sha256(string line) {    
    unsigned char hash[SHA256_DIGEST_LENGTH];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    SHA256_Update(&sha256, line.c_str(), line.length());
    SHA256_Final(hash, &sha256);

    string output = "";    
    for(int i = 0; i < SHA256_DIGEST_LENGTH; i++) {
        output += to_hex(hash[i]);
    }
    return output;
}

int main() {
    cout << sha256("hello, world") << endl;

    return 0;
}

Here's the function I personally use - I simply derived it from the function I used for sha-1 hashing:

char *str2sha256( const char *str, int length ) {
  int n;
  SHA256_CTX c;
  unsigned char digest[ SHA256_DIGEST_LENGTH ];
  char *out = (char*) malloc( 33 );

  SHA256_Init( &c );

  while ( length > 0 ) {
    if ( length > 512 ) SHA256_Update( &c, str, 512 );
    else SHA256_Update( &c, str, length );

    length -= 512;
    str += 512;
  }

  SHA256_Final ( digest, &c );

  for ( n = 0; n < SHA256_DIGEST_LENGTH; ++n )
    snprintf( &( out[ n*2 ] ), 16*2, "%02x", (unsigned int) digest[ n ] );

  return out;
}

I think that you only have to replace SHA1 function with SHA256 function with tatk code from link in Your post