a. [9 points]
Shared session key establishment using a Key Distribution Center (KDC). Using the
following table, illustrate how Alice can initiate a secure session with Bob with the help
of KDC. Here, KEKs are the long term key establishment keys used to transport the
session keys across the network securely. Assume the encryption process to be as
follows: Divide the key into two halves: LK and RK; Divide the plaintext into two
halves: LT and RT; Then ciphertext= LC||RC where LC=LK XOR RT; and RC = RK
XOR LT; where LC, RC, LT, and RT are each 4 bits; Plaintext and ciphertext are each 8
bits. For example, if plaintext=A7 (Hexa) and Key = 6D; then LC=6 ? 7 = 0110 ? 0111
= 0001 = 1 (Hexa); and RC = D ? A = 1101 ? 1010 = 0111 = 7 (Hexa); so Ciphertext =
17 (Hexa). To decryptt, it does the reverse operation: Given ciphertext of C=LC||RC, it
finds plaintext T=LT||RT, by finding LT=RC ? RK and RT = LC ? LK. In the above
example, LT = 7 ?D = 0111?1101=1010=A; RT=1 ? 6 = 0001 ? 0110 = 0111=7; So
Plaintext=A7 (Hexa).

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