Have some third party (or several) that LW would trust hold on to it in secrect.
Nitpick: cryptography solves this much more neatly.
Of course, people could accuse you of having an efficient way of factorising numbers, but if you do karma is going to be the least of anyone's concerns.
Factorization doesn't enter into it - to precommit to a message that you will later reveal publically, publish a hash of the (salted) message.
Here's the new thread for posting quotes, with the usual rules: