$(1$ point$)$ This question concerns block cipher modes. We will use a simple affine cipher, which can be

expressed in $C$ as follows.

char cipher$($unsigned char block, char key$)$

$\{$

return $($key$+11*$block$)\%256$;

$\}$

The inverse of this cipher is shown below.

char inv$\_$cipher$($unsigned char block, char key$)$

$\{//163$ is the inverse of $11$ mod $256$

return $({163}^{**}(block-$key$+256))\%256$;

$\}$

Note that the block size is $8$ bits, which is one byte $($and one ASCII character$).$ We will work with the fixed key

$008.$

We now decrypt various ciphertexts using modes for this cipher. In every case in which the mode requires an

IV$,$ the IV will be OxAA. In the case of CTR mode, we use a $($nonce $||$ counter$)$ arrangement in which the nonce

is the left $5$ bits of $0$xAA and the counter is a $3$ bit counter that begins at $0.$ In all of the problems given below,

one character is one block. Each character of the plaintext should be regarded as its corresponding ASCII

code. The ciphertext is given in hexadecimal.

a$)$ Decrypt the ciphertext $"303$E$24110012"$ using $CTR$ mode. Please enter your answer in ASCII characters

$($aka words$).$

b$)$ Decrypt the ciphertext $"80338$BEEF$9"$ using ECB mode. Please enter your answer in ASCII characters

$($aka words$).$

c$)$ Decrypt the ciphertext $"3BF8D72D83"$ using $CFB$ mode. Please enter your answer in ASCII characters

$($aka words$).$

d$)$ Decrypt the ciphertext $"5BE1595A0D3B"$ using $CBC$ mode. Please enter your answer in ASCII

characters $($aka words$).$

e$)$ Decrypt the ciphertext $"37CD6733"$ using OFB mode. Please enter your answer in ASCII characters $($aka

words$).$