The only true "free alternative" to Enigma that offers absolute security is Instead of protecting the EXE with a heavy armor, Alex moved his "license check" and "core algorithm" to a simple web server.
You must modify and recompile your source code. It doesn't work on already-compiled files. Also, the free version adds a nag screen? (Historically, no nag for SDK use, but check current EULA). enigma protector alternative free