9. 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). What CAIN property is that? Your colleague 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 Es in the message. If you get a message, and the number at the end is NOT an accurate count of the number of letter Es, that message is from an imposter. [a,b,c: 2 points, d: 4 points] a) This number, put at the end of each message, is an example of what cryptographic item? b) Does it have the characteristic of being one way i.e can you deduce the original message? c) Is it collision resistant? d) Can you suggest some other way to indicate that message is from the sender without encryption?