alt Hacker NewsCaesar cipher is probably something even an untrained person could decode. Probably something more complex like a vigenere cipher that is still trivial to decode if you are at all familiar crptanalysis, but would look impossible to someone untrained.
Theoretically, with a long enough codeword to cipher ratio, vigenere is impossible to break (I think) (but sort of useless)
And yes, I've taught 8 year olds how to crack Caesar ciphers...