excelhashreverse.c

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>

#define ITERATIONS 1000000

uint16_t get_password_hash(const char *password, int password_len) {
    register uint16_t hash = 0;
    register int char_index = password_len;
    do {
        hash ^= (uint16_t) ((uint8_t) password[--char_index]);
        // bit 15 is always zero, so don't need to worry about it
        hash = (hash >> 14) | ((hash & 0x3FFF) << 1);
    } while (char_index > 0);
    return hash ^ ((uint16_t) password_len) ^ 0xCE4B;
}

int reverse_hash(uint16_t hash, char *password_out, int password_len) {
    hash ^= ((uint16_t) password_len) ^ 0xCE4B;
    if (hash & 0x8000) return 0;
    for (int char_index = 0; char_index < password_len; char_index++) {
        hash = (hash >> 1) | ((hash & 0x0001) << 14);
        uint16_t c = hash & 0x007F;
        password_out[char_index] = (char) ((uint8_t) c);
        hash ^= c;
    }
    return hash == 0;
}

volatile uint16_t output_hash;

int main() {
    srand(time(NULL));
    
    /*
    printf("Check: %04X == FEF1\n", get_password_hash("abcdefghij", 10));
    
    clock_t total = 0;
    char input[15];
    
    for (int i = 0; i < ITERATIONS; i++) {
        int length = 10 + (rand() % 6); // 10 to 15 inclusive
        for (int j = 0; j < length; j++) {
            input[j] = rand() % 256;
        }
        clock_t start = clock();
        output_hash = get_password_hash(input, length);
        clock_t end = clock();
        total += end - start;
    }
    const double secs = ((double) total) / ((double) CLOCKS_PER_SEC);
    const double secs_per_it = secs / ITERATIONS;
    printf("Benchmark: %d ns per iteration\n", (int) (secs_per_it * 1e9));
    */
    
    uint16_t input_hash = 0xFEED;
    printf("input hash: %04X\n", input_hash);
    
    const int n = 8;
    char rev_password[n];
    if (!reverse_hash(input_hash, rev_password, n)) {
        printf("reverse_hash failed (try adjusting n).\n");
        return 0;
    }
    printf("reversed password: ");
    for (int i = 0; i < n; i++) {
        printf("%02X", (uint8_t) rev_password[i]);
    }
    printf("\n");
    
    printf("hash of reversed password: %04X\n", get_password_hash(rev_password, n));
}