Suppose your spy colleague wanted to send you messages

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 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, 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?