[johnrob]

That's a very difficult feature to implement, since the encryption has to be 2 way. It's much easier to achieve this with passwords, because all the system needs to do is validate supplied passwords (as opposed to reproducing the passwords themselves).

[Lewisham]

You could use a passphrase in the head of your email.

"Open sesame" or something equally corny.

However, you'd need an inversion of what is proposed here. Instead of emailing you an entry from the past then asking for your entry for today, it would need to ask for today's entry (plus passphrase), then reply back with an entry from the past instead.

I don't think the latter version is as compelling as the former, but encryption is sort of a must have.