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1. [6 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: Block (LBRB) is 8 bits; Encryption Key (LK||RK) is 8 bits; Ciphertext -LC|| RC where LC-LB RK and RC RB LK; For example, if plaintext=A7 (Hexa) and Key = 6D then L.C« AD-1010 11010111-7 (Hexa); and RC = 760111 0110 = 0001 = 1 (Hexa); so Ciphertext - 71 (Hexa). To decrypt, it does the reverse operation: Given ciphertext of CELCIRC, it finds plaintext BLBURB, by finding LBHLCORK and RB RCLK
Alice KDC Bob KEK: KA = DC (hexa) KEK: KA=DC (hexa); KEK: kB = A4 (hexa); kB = A4(hexa); Alice sends a message to KDC requesting a session key between Alice and Bob Generate a random session key: kses = 7B (hexa); YA = eka(kses) = ?? YB = ekB(kses) = ?? KDC sends ya = ?? to Alice KDC send YB = ?? to Bob Decrypt yA to derive Decrypt ys to derive kses using ka = kses using kB = Message to send, m =E3 (Hexa) Encrypyt m using session key, y = ekses(m) Alice sends y to Bob Decrypt y using session key to get m = Verify that this is the message sent by Alice