$4$ Task $4$: Error Propagation $-$ Corrupted Cipher Text

To understand the error propagation property of various encryption modes, we would like to do the

following exercise:

Create a text file that is at least $1000$ bytes long.

Encrypt the file using the AES$-128$ cipher.

Unfortunately, a single bit of the $55$th byte in the encrypted file got corrupted. You can achieve this

corruption using the bless hex editor.

Decrypt the corrupted ciphertext file using the correct key and IV$.$

Submission for Task $4(20$ points$)$

$(1)$ Please answer the following question before you conduct this task: How much information can you

recover by decrypting the corrupted file, if the encryption mode is ECB,$CBC,CFB,$ or OFB $,$

respectively? $(5$ points$)$

$(2)$ Please find out whether your answer is correct or wrong after you finish this task. Please provide

justification. Specifically, please attach the screenshot of decrypted ciphertext in Step $4$ and calculate

how many bits are flipped compared with the plaintext. $(15$ points$)$

$5$ Task $5$: Programming using the Crypto Library

In this task, you are given a plaintext and a ciphertext, and your job is to find the key that is used for the

encryption. You do know the following facts:

The aes$-128-$cbc cipher is used for the encryption.

The key used to encrypt this plaintext is an English word shorter than $16$ characters; the word can

be found from a typical English dictionary. Since the word has less than $16$ characters $($i$.$e$.128$

bits$),$ pound signs $($#: hexadecimal value is $023)$ are appended to the end of the word to form a

key of $128$ bits.

Your goal is to write a program to find out the encryption key. Please the English word list attached in this

lab assignment. The plaintext, ciphertext, and IV are listed in the following:

Plaintext $($total $21$ characters$)$: This is a top secret.

Ciphertext $($in hex format$)$: $1$b$4$faa$3403$db$410533$b$2$e$99$b$4$a$4$dcf$2$a

$93$a$5$af$11$b$35988$e$62$cb$57$d$03$e$221$bfc$6$

IV $($in hex format$)$: aabbccddeeff$00998877665544332211$

Submission for Task $5(20$ points$)$

$(1)$ Please write a program to find out the encryption key. Please attach the screenshot of your program.

You can use any programming language that you are familiar with to conduct this task. $(10$ points$)$

$(2)$ What is the key used in this task? $(10$ points$)$