Question

At 4:35 AM, a covert channel with a bandwidth of 2 bit/sec gains access to a 2048 byte plaintext message that reads “Bomb Location X at 6:05 AM”. How long will it take to transmit the plaintext of this message? If the covert channel has no access to the encryption key used for the communication, is the scenario a computationally secure one for the transmitter?

Answer 1

1).

The time taken will be equal to total message length divided by the bandwidth of the channel.

Given, bandwidth = 2bits/sec,

message length = 2048 bytes

but one byte = 8 bits

Hence message length = 2048 * 8 bits = 16384 bits

Therefore time taken to send the message = 16384 / 2 = 8192 sec = (8192 / 60) min = 136.53 min

2). Let us assume the covert channel gains access to the key and decodes the message.

Then the transmitter had to transmit the message, so total time for transmission as calculated in part 1 = 136.53 min

And total time for bomb to explode= 90 min (Time difference between 4:35AM to 6:05AM)

So it is computationally secure as the covert channel won't be able to transmit it within the given time of 90 min and by the time the message reaches the destination, bomb would have exploded.