9. Suppose your spy colleague wanted to send you messages that you could be sure
ID: 3740955 • Letter: 9
Question
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 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? 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?
Explanation / Answer
a) This number, put at the end of each message, is an example of what cryptographic item?
Answer: This number at the end is called the signed bit.
b) Does it have the characteristic of being one way i.e can you deduce the original message?
Answer: No it cannot be deduced as the logic for that number generation will not be known except for the sender and receiver. So other people will not know the reason and logic behind this number generation.
c) Is it collision resistant?
Answer: No its not collision resistant as other signed bit generation logic can give the same number thought the logic is the same. In this case collision will happen.
d) Can you suggest some other way to indicate that message is from the sender without encryption?
Answer: Yes, we can share the IP address along with the message of the sender to authenticate if the message generated is form the intended person or not.
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