I encrypt messages for a another, goofier purpose. One of the people I am encrypting from is a compsci professor.
I use a Vigenere cipher, which should beat everything short of the Secret Werewolf Police, and possibly them, too. (It is, however, more crackable than a proper salted, hashed output.)
In a Vigenere, the letters in your input are moved by the numerical equivalent of the key, and the key repeats. Example:
Secret Statement/lie: Cats are nice. Key: ABC
New, coded statement: dcwt (down 1, 2, 3, 1) cuf ildg. Now, I recommend using long keys and spacing the output in five letter blocks to prevent easier soliving.
You can do this online:
http://sharkysoft.com/vigenere/
This will transmute "It seems unlikely the werewolf police will catch you," with the key "The movie ends the same way for all of us JRM." to:
Cbxrt ibzsz mdytd minyzw wxqdt cjeph bfqhr leuqh oxvbg tn.
(Letter grouping by me.)
Again Vigenere's are potentially crackable, but they are very hard. It's easier for the werewolf police to just come and eat anyone who puts up hashed or Vigenere ciphered predictions.
Vigeneres are potentially crackable, but they are very hard.
Unless your text is very short or your key very long, they are much easier than you apparently think. I have written programs that do a pretty good job of it.
(As MrMind says, if your key is as long as your text then this is a one-time pad, which indeed is uncrackable provided you have a reliable channel for sharing the one-time pad and don't make operational mistakes like reusing the key. But that's not a very common situation.)
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