https://medium.com/@cedriczirtacic/macos-reverse-engineering-part-ii-unicorn-app-crackme-4a0fa73d77c1

generate_serial.m

- (IBAction)generate_serial:(id)sender {
    NSString *name = [self.name_field stringValue];
    unsigned char md5_serial[CC_MD5_DIGEST_LENGTH];
    NSString *final_serial = @"";
    
    if (name == nil || [name length] == 0) {
        NSAlert *alert = [[NSAlert alloc] init];
        [alert setMessageText:@"You must set a name!"];
        [alert setAlertStyle:NSAlertStyleWarning];
        [alert runModal];
        goto quit;
    }
    
    name = [name stringByAppendingString:@"+unicorn"];
    CC_MD5([name UTF8String], (CC_LONG)[name length], md5_serial);
    for(int i=0; i < (CC_MD5_DIGEST_LENGTH - 6); i++)
        final_serial = [final_serial stringByAppendingString:[NSString stringWithFormat:@"%X",md5_serial[i]]];
    [self.serial_field setStringValue:final_serial];
quit:
    ;
}