[andrewcooke]

so if you know the plaintext you get the user's key?

[clord]

Standard with one time pads (and symmetric encryption in general) — the key and plaintext are a shared secret.

[andrewcooke]

it's not standard with symmetric encryption in general; it's a "known plaintext attack". http://en.wikipedia.org/wiki/Known-plaintext_attack - with aes, for example, even if you know the plaintext there is no known way to obtain the key (faster than brute force search).

[quelsolaar]

The answer should be no. But I don't know.