kmn.c

/*
 * Keygen for the "KeygenMe or Not" challenge.
 * */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

void exit_error(char * e) {
    fprintf(stderr, "%s\n", e);
    exit(EXIT_FAILURE);
}

void auth(char * username, unsigned magicNumber) {
    unsigned username_length = strnlen(username, 32);
    unsigned uiSeed = (username[3] ^ 0x1337) + 0x5EEDED;
    unsigned uiAccumulator = 0;

    while(true) {
        if (uiAccumulator >= username_length) {
            /* I've commented the check from below in order to get the uiSeed. This is the keygen, basically */
            
            // if (magicNumber != uiSeed) {
            //     exit_error("magicNumber is not equal to uiSeed");
            // }
            
            /* I guess the block from below gives the title "Keygenme or Not" to this challenge. */

            // if (magicNumber == 6235464) {
            //     printf("Solution found!\n");
            // }

            printf("Solution found: %u\n", uiSeed);
            break;
        } else {
            /* Checks wheter username is a character between a ' '(space) and ''(empty) */
            if (username[uiAccumulator] < 0x20 || username[uiAccumulator] > 0x7F) {
                exit_error("err: username[uiAccumulator] < 0x20 || username[uiAccumulator] < 0x7F");
            } else {
                unsigned dividend = username[uiAccumulator] ^ uiSeed;

                uiSeed += dividend % 0x539;

                uiAccumulator += 1;
            }
        }
    }
}

int main(int argc, char *argv[], char *envp[]) {
    if (argc < 3) {
        auth("root-me.org", 6235464);
    } else {    
        auth(argv[1], atoi(argv[2]));
    }
    return EXIT_SUCCESS;
}