Question $5(14$ points$)$:

Bob wishes to encrypt some plaintext and stores the resulting ciphertext on his hard drive.

Specifically, he wants the ciphertext to be the same length as the original plaintext. For this

purpose, he employed the clphertext stealing $($CTS$)$ mode, the implementation of which is

shown in the figure below. Initially, the plaintext is divided into independent blocks of length $S$

bits, giving the plaintext blocks $P_1,P_2,$dots,$P_N.$ Assume that the last block of plaintext $($i$.$e$.,$

$\{$:$P_N)$ is $L$ bits long, where $(N-2)P_{N-1}{C}_{N-2}{Y}_{N-1}{Y}_{N-1}{E}_{N-1}L{E}_{N-1}{C}_N{P}_N(S-L)E_{N-1}{Y}_N{Y}_N{C}_{N-1}L.$ The encryption sequence $isas$ follows:

Encrypt the first $(N-2)$ blocks using the traditional cipher block chaining $(CBC)$

technique.

XOR $P_{N-1}$ with the previous ciphertext block $C_{N-2}to$ create $Y_{N-1}.$

Encrypt $Y_{N-1}to$ create $E_{N-1}.$

Select the first $L$ bits $of{E}_{N-1}to$ create $C_N.$

Pad $P_N$ with $(S-L)$ zeros $at$ the end and exclusive$-OR$ with $E_{N-1}to$ create $Y_N.$

Encrypt $Y_{N}to$ create $C_{N-1}.$