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 ciphertext stealing $($CTS$)$ mode, the implementation of which is

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

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

$)$ is $$ bits long, where $<.$ The encryption sequence is as follows:

$1.$ Encrypt the first $()$ blocks using the traditional cipher block chaining $($CBC$)$

technique.

$2.$ XOR $$ with the previous ciphertext block $$ to create $.$

$3.$ Encrypt $$ to create $.$

$4.$ Select the first $$ bits of $$ to create $.$

$5.$ Pad $$ with $()$ zeros at the end and exclusive$-$OR with $$ to create $.$

$6.$ Encrypt $$ to create $.$

Dr$.$ Ahmed I. Shawahna & Dr$.$ Mohammed S$.$ Hussein January $15,2024$

$3|$ P a g e

A$)$ Describe how to decrypt the ciphertext $(,...,,),$ that is$,$ show the decryption

sequence.

B$)$ If a single bit error occurs in the storage of ciphertext $$

$,$ which plaintext blocks, if any,

will be correctly restored by the decryption algorithm? Explain your answer.