Only for one-way algorithms (MD5 etc.).
Thanks for the correction! So, for example, base64. Is it a two-way algorithm?
What happened?
Two things: First, you assumed it was relatively easy for a cryptographer to decode it (or at least you made it look like you think it's easy, from what you wrote). Second, you should read more about the WOW! signal, because from my quick research, if it really is an extraterrestrial signal, it was made to resemble Hydrogen radiations, and Hydrogen is very abundant in our Universe. So (and again, if it really is from extraterrestrial lifeforms) it was not meat to be some sort of communication for us to decode, but rather a way to say "Hey, we are here and you know we're intelligent because we can mimic Hydrogen radiation".