Aah, I was thinking you might be encoding stack traces or equivalents which you'd want to protect a lot more than the head of user submitted data which the user

Aah, I was thinking you might be encoding stack traces or equivalents which you'd want to protect a lot more than the head of user submitted data which the user chooses to re-submit through a seperate channel that is about as secure (esp for the purposes (email)) as the original channel. A hard coded key would be fine for this imho as you don't have to protect the user from the user's own data. Cool idea btw.