Question

Problem 4. Let a message M of 128 bits be encrypted with 128-bit AES using cipher feedback (CFB) mode with s=32. Draw a diagram for both encryption and decryption. Identify the number of output ciphertext blocks in encryption part.

Answer 1

AES in CFB mode with a segment size of 32 bits will produce 4 output ciphertext blocks for a 128-bit message. The encryption and decryption process involves encrypting the IV and XORing it with plaintext to encrypt, and with ciphertext to decrypt.

Step-by-step explanation:

The question pertains to the encryption and decryption process using Advanced Encryption Standard (AES) with a 128-bit message in cipher feedback (CFB) mode, specifically with a segment size of 32 bits. In CFB mode, AES will take an initial vector (IV) and encrypt it, then the first 32 bits of the resulting block are XORed with the first 32 bits of the plaintext to produce the first segment of the ciphertext. This segment of ciphertext then becomes the next IV for the process to encrypt the next segment. Since the message M is 128 bits and the segment size is 32 bits, there would be 4 output ciphertext blocks after encryption. Similarly, during decryption, the IV is encrypted and then XORed with the ciphertext to recover the plaintext. The process continues with the next ciphertext block replacing the IV for each step until the message is recovered.