$3$ Task $3$: Padding

For block ciphers, when the size of a plaintext is not a multiple of the block size, padding may be required.

All the block ciphers normally use PKCS#$5$ padding, which is known as standard block padding. We will

conduct the following experiments to understand how this type of padding works:

Use ECB, CBC$,$ CFB$,$ and OFB modes to encrypt a file $($you can pick any cipher$).$ Please report

which modes have paddings and which ones do not. For those that do not need paddings, please

explain why.

Let us create three files, which contain $5$ bytes, $10$ bytes, and $16$ bytes, respectively. We can use

the following "echo $-n"$ command to create such files. The following example creates a file $f1.txt$

with length $5($without the $-n$ option, the length will be $6,$ because a newline character will be added

by echo$)$:

$echo $-$n $"12345">$ fl$.$txt

We then use "openssl enc$-$aes$-128-$cbc$-$e$"$ to encrypt these three files using $128-$bit AES with CBC

mode. Please describe the size of the encrypted files.

We would like to see what is added to the padding during the encryption. To achieve this goal, we

will decrypt these files using "openssl enc$-$aes$-128-$cbc$-$d$".$ Unfortunately, decryption by default

will automatically remove the padding, making it impossible for us to see the padding. However,

the command does have an option called $"-$nopad", which disables the padding, i$.$e$.,$ during the

decryption, the command will not remove the padded data. Therefore, by looking at the decrypted

data, we can see what data are used in the padding. Please use this technique to figure out what

paddings are added to the three files.

It should be noted that padding data may not be printable, so you need to use a hex tool to display

the content. The following example shows how to display a file in the hex format:

$ hexdump$-$C pl$.$txt

$00000000$

$ xxd pl$.$txt

$00000000$:$313233343536373839494$a$4$b $4$c$0$a $123456789$IJKL.

Submission for Task $3(20$ points$)$

$(1)$ Which modes have paddings and which ones do not? How did you know if a mode has a padding

through the encrypted file? For those that do not need paddings, please explain why. Please also list the

commands that you used in step $1.(5$ points$)$

$(2)$ Please describe the size of the encrypted files which are encrypted in step $2?(5$ points$)$

$(3)$ What paddings are added to the three files in step $2?$ Please attach the screenshots of decrypted files

with paddings $($i$.$e$.,$ decrypt the files with $"-$nopad"$).(10$ points$)$