This page lets you generate a verification message signature, as well as verify a message as having been generated by an authenticated user.
This is very similar to signed messages in e.g. PGP.
You must be authenticated in order to generate a message signature.
Simply enter your message, then click the sign button. A signature that includes your username will then be generated.
Verification does not require authentication.
Enter the signature, then click the verify button. A verification message will display indicating whether the signature for the message is correct, and the message displayed.