A proof of work function based on SHA256 hashes; finds a string with a given prefix whose hash value ends in the required number of zero bits

sha256_proof_of_work.c

/* compile with: cc -Wall -O3 sha256_proof_of_work.c -o sha256_proof_of_work -lcrypto */

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

#define PREFIX_BYTES 16
#define MAX_HASH_ATTEMPTS 10000000000L

uint8_t *sha256_proof_of_work(const uint8_t prefix[PREFIX_BYTES], int strength);


int main(int argc, char **argv) {
    uint8_t p[PREFIX_BYTES];
    uint8_t *result;
    FILE *f;
    int i, nbits = 10;
    
    if (argc == 2) nbits = atoi(argv[1]);
    if (nbits < 1) nbits = 1;
    if (nbits > 24) nbits = 24;
    
    printf("Searching for a hash with %d trailing zero bits\n", nbits);
    f = fopen("/dev/urandom", "r");
    fread(p, PREFIX_BYTES, 1, f);
    fclose(f);
    
    printf("Prefix: ");
    for (i=0; i<PREFIX_BYTES; i++) {
        printf("%02x", p[i]);
    }
    putchar('\n');
    
    result = sha256_proof_of_work(p, nbits);
    if (!result) {
        puts("No hash found");
        return 1;
    }
    printf("Output: ");
    for (i = 0; i < SHA256_DIGEST_LENGTH; i++)
        printf("%02x", *result++);
    putchar('\n');
    return 0;
}

uint8_t *sha256_proof_of_work(const uint8_t prefix[PREFIX_BYTES], int strength) {
    static uint8_t hash_in[SHA256_DIGEST_LENGTH];
    uint8_t hash_out[SHA256_DIGEST_LENGTH];
    int i, j, iszero;
    long nhashes;
    int zbytes = strength >> 3;
    int zbits = ~(~0 << (strength & 0x07));
    
    for (i=0; i<PREFIX_BYTES; i++) hash_in[i] = prefix[i];
    for ( ; i<SHA256_DIGEST_LENGTH; i++) hash_in[i] = 0;
    for (nhashes=0; nhashes<MAX_HASH_ATTEMPTS; nhashes++) {
        SHA256(hash_in, SHA256_DIGEST_LENGTH, hash_out);
        iszero = 1;
        for (i=SHA256_DIGEST_LENGTH-1; i>=SHA256_DIGEST_LENGTH-zbytes; i--) {
            if (hash_out[i]) {
                iszero = 0;
                break;
            }
        }
        if (iszero && (hash_out[i] & zbits) == 0) {
            return hash_in;
        }
        j = SHA256_DIGEST_LENGTH-1;
        while (!(++hash_in[j])) j--;
    }
    return NULL;  /* No hash found after MAX_HASH_ATTEMPTS; request another prefix? */
}