WordPerfect through 5.1 used XOR encryption. With 5.1, as I recall, you had a dozen or so known bytes early on--nulls or spaces, I forget--so it was trivial to get any password of a dozen or fewer characters. With 4.2 it was simpler to build a lookup table of the keys--the key was formed by XORing the characters of the password with a logical shift after each character.

I believe that Sendero Luminoso made the mistake of trusting to WordPerfect's encryption, and suffered for that after one or more of its computers were seized in a raid.