Suppose your spy colleague wanted to send you messages that you could be sure came from him (and not an enemy trying to pretend to be him). Your colleague personally tells you: "Whenever I send you a message, the last thing in the message will be a number. That number will be a count of the number of letter E's in the message. If you get a message, and the number at the end is NOT an accurate count of the number of letter E's, then that message is from an imposter."