SEEDS Lab $-$ Secret Key Encryption Lab $($Can be found online$)$

It is well$-$known that monoalphabetic substitution cipher $($also known as monoalphabetic cipher$)$ is not

secure$,$ because it can be subjected to frequency analysis. In this lab, you are given a cipher$-$text that is encrypted

using a monoalphabetic cipher; namely, each letter in the original text is replaced by another letter, where the

replacement does not vary $($i$.$e$.,$ a letter is always replaced by the same letter during the encryption$).$ Your job is

to find out the original text using frequency analysis. It is known that the original text is an English article.

In the following, we describe how we encrypt the original article, and what simplification we have made.

Step $1$: let us generate the encryption key, i$.$e$.,$ the substitution table. We will permute the alphabet

from a to z using Python, and use the permuted alphabet as the key. See the following program.

#$!/$bin$/$env python$3$

import random

s $=$ "abcdefghijklmnopqrstuvwxyz"

list $=$ random.sample$($s$,$ len$($s$))$

key $=.$join$($list$)$

print$($key$)$

Step $2$: we use the tr command to do the encryption. We only encrypt letters, while leaving the space

and return characters alone.

$ tr $$abcdefghijklmnopqrstuvwxyz$$sxtrwinqbedpvgkfmalhyuojzc$\backslash$

$<$ plaintext.txt $>$ ciphertext.txt

We have created a ciphertext using a different encryption key $($not the one described above$).$ It is included in

files.zip, which can be downloaded from Canvas. Your job is to use the frequency analysis to figure out the

encryption key and the original plaintext.

We have created a ciphertext using a different encryption key $($not the one described above$).$ It is included in

files.zip, which can be downloaded from Canvas. Your job is to use the frequency analysis to figure out the

encryption key and the original plaintext.

$ tr $$aet$$XGE$<$ in$.$txt $>$ out.txt