A simple tool to calculate the number of bits differing between the SHA-256 hashes of two strings

bitChecker.c

/*
 * This is free and unencumbered software released into the public domain.
 * 
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 * means.
 * 
 * In jurisdictions that recognize copyright laws, the author or authors
 * of this software dedicate any and all copyright interest in the
 * software to the public domain. We make this dedication for the benefit
 * of the public at large and to the detriment of our heirs and
 * successors. We intend this dedication to be an overt act of
 * relinquishment in perpetuity of all present and future rights to this
 * software under copyright law.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
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 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
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/*
 * Checks the number of bits differing between two string inputs
 * Usage: bitChecker Foo Bar
 *
 * Compile using the following:
 * gcc bitChecker.c -o bitChecker -lcrypto
 */

#include <stdio.h>
#include <stdlib.h>
#include <openssl/sha.h>
#include <string.h>

// An array which quickly determines the number of bits in one byte. This trades off higher memory usage for faster execution speed.
// Just kidding, I just copied this shit from google because I was too lazy to write int count_bits(byte b) myself.
int bit_count[] = {
        0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
};

// Prints the hexadecimal representation of a SHA-256 hash
void print_sha256(unsigned char *hash)
{
        int i = 0;

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

	printf("\n");
}

// Calculates a SHA-256 hash from a string
void sha256_string(char *string, unsigned char * output)
{
	unsigned char hash[SHA256_DIGEST_LENGTH];
	SHA256_CTX sha256;
	SHA256_Init(&sha256);
	SHA256_Update(&sha256, string, strlen(string));
	SHA256_Final(output, &sha256);
}

int main(int argc, char* argv[])
{
	int i = 0;
	int totalBits = 0;
	unsigned char hash1[SHA256_DIGEST_LENGTH];
	unsigned char hash2[SHA256_DIGEST_LENGTH];
	
    if(argc < 3)
	{
		fprintf(stderr, "Usage: %s STRING1 STRING2\n", argv[0]);
		exit(1);
	}

	sha256_string(argv[1], hash1);
	sha256_string(argv[2], hash2);

	printf("SHA-256 hash of '%s':\n", argv[1]);
	print_sha256(hash1);

	printf("SHA-256 hash of '%s':\n", argv[2]);
	print_sha256(hash2);

	// The magic happens here
	// Iterates 256 times to go through every bit.
	for(i = 0; i < SHA256_DIGEST_LENGTH; i++)
	{
		// Calculating the XOR between both hashes returns the bits where both hashes differ
		unsigned char xorRes = hash1[i] ^ hash2[i];
		
		// We use the lookup table from above to determine how many bits differ for a given byte.
		totalBits += bit_count[xorRes];
	}

	printf("\n");
	printf("Total differing bits: %d\n", totalBits);

	return 0;
}