To generate the random charter unto limit + iphone

 NSString *chars = @"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    NSMutableString *randomString = [NSMutableString stringWithCapacity: string_length];
    for (int i=0; i<string_length; i++)
    {
        [randomString appendFormat:@"%C", [chars characterAtIndex: arc4random()%[chars length]]];
    }