Q1. (a) Briefly explain how an integrity check value (ICV) may be used in network communication to detect errors.
(b) When using an ICV, explain why
(i) a message might be transmitted correctly but the receiver believes the message contains an error (a false negative).
(ii) a message might be transmitted incorrectly but the receiver believes the message is correct (a false positive).
(c) Briefly explain why an ICV cannot be used to detect modifications.
Q2. Consider the following two algorithms A and B for computing an ICV:
Step A1) Split the data into 7-bit chunks
Step A2) For each chunk add an 8th bit, choosing the value of the bit to be the same as the 1st bit
Using algorithm A, the 21-bit message 0001110 1010011 1110110, for example, would be transmitted as: 00011100 10100111 11101101
Step B1) Split the data into 7-bit chunks
Step B2) For each chunk add an 8th bit, choosing the value of the bit so that each 8-bit chunk contains an even number of bits with the value 1 (and hence an even number of bits with the value 0)
Using algorithm B, the 21-bit message 0001110 1010011 1110110 would be transmitted as 00011101 10100110 11101101.
For each algorithm A and B explain:
(a) How the receiver checks whether the message has been transmitted without error.
(b) Under what circumstances a false negative occurs.
(c) Under what circumstances a false positive occurs.
Hence:
(d) Carefully explain which algorithm you think is better for computing ICVs.
3. Consider the following protocols, Probe and Copy:
Probe 1. Send "Hello" message
Probe 2. Recipient returns a string of 512 random bits to the sender
Copy 1. Send message
Copy 2. Recipient returns a copy of the message to the sender
Assuming machines A and B are each running implementations of the Probe and Copy protocols, explain:
(a) How an attacker M can use Probe to cause A to send a message to B.
(b) How M can establish an endless loop of messages between A and B.
(c) Whether the attacker needs to be on-path to execute these attacks?