Joyetech firmware bruteforce offset

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

#define NONE    255

// Joyetech APROM
const char signature[] = { 0x0A, 0x0A, 0x4A, 0x6F, 0x79, 0x65, 0x74, 0x65, 0x63, 0x68, 0x20, 0x41, 0x50, 0x52, 0x4F, 0x4D, 0x0A };

void displayHelp( char * );
int decryptFile( char *, int  );

/**
*   MAIN
*/
int main( int argc, char *argv[] ) {
    if ( argc != 2 ) {
        displayHelp( argv[0] );
        return EXIT_SUCCESS;
    }

    int i; int res;
    for ( i=-128; i<=127; i++ ) {
        res = decryptFile( argv[1], i );
        if ( res != NONE ) {
            printf( "Offset found : %d\n", i );
            break;
        }
    }
    return EXIT_SUCCESS;
}

void displayHelp( char *appName ) {
    printf( "Usage : %s firmware\n", appName );
    printf( "%s find offset for decrypt\n", appName );
    printf( "\n" );
}

int decryptFile( char *inFirmware, int offset ) {
    // Ouverture du firmware chiffré
    FILE *fpIn = fopen( inFirmware, "rb" );
    if ( fpIn == NULL ) {
        printf( "Unable to open %s\n", inFirmware );
        return NONE;
    }

    // Offset de départ, c'est ce que l'on cherche à trouver
    signed char key = (char)offset;

    // Lit jusqu'à la fin du fichier, octet par octet
    char c; int i=0; int limit = sizeof( signature );
    size_t sizeOfChar = (size_t)sizeof( char );
    while ( fread( &c, sizeOfChar, sizeOfChar, fpIn) != 0 ) {
        c = c ^ key; // xor

        if ( c == signature[i] )
            i++;
        else
            i = 0;

        // Si on a trouvé la signature
        if ( i == limit ) {
            fclose( fpIn );
            return 1;
        }

        /* Incremente la clé */
        if ( (key >= -128) && (key < 127) )
            key++;
        else
            key = -128;
    }

    fclose( fpIn );
    return NONE;    // valeur arbitraire
}