I have this question from encrypto class / python coding. can you answer? Consid
ID: 3862924 • Letter: I
Question
I have this question from encrypto class / python coding. can you answer?
Consider the plaintext message below.
I WOKE UP LIKE THIS FLAWLESS
Encrypt this message using Vigenère’s cipher and a 5-letter keyword of your choice. While you are welcome to use a computer to verify your encryption, you must show all the steps of encryption by hand.
Solution.
(a) First, what is your 5-letter keyword? Keyword:
(b) Next, encrypt your message. Please show the setup for encryption (in other words, show how you align the keyword with the plaintext message in order to set up the message for encryption).
(c) Write out your ciphertext below.
(d) What is the keyspace of Vigenère’s cipher? Justify your answer.
Explanation / Answer
(a) First, what is your 5-letter keyword? Keyword:
FIRST
(b) Next, encrypt your message. Please show the setup for encryption (in other words, show how you align
the keyword with the plaintext message in order to set up the message for encryption).
In Vigenère’s cipher, he did not visualize the cipher in modular arithmetical terms.
Rather he viewed the cypher as a substitution cipher where a different alphabet was used for the next letter of
the message, with the alphabets repeating periodically --- according to some key.
For the given key word "FIRST", encrypt each letter of the message taken in the left-most column
to the letter in the keyword-letter column.
Thus, the first five letters of the message use the alphabets corresponding the the "F", "I", "R", "S", and "T" columns.
So, the Vigenere code with this keyword is really five Caesar shifts used in a cyclical fashion.
Decription is carried out working backwards from the keyword-columns to the left-most column.
Because we are really using five alphabets, the Vigenere encryption is sometimes called a polyalphabetic code
(c) Write out your ciphertext below.
For the above plain text ,i have used the keyword as FIRST and its cipher text is as follows
n efcx zx cadj byal ktroejaj
(d)What is the keyspace of Vigenère’s cipher? Justify your answer
The Vigenère cipher is a generalization of the shift cipher in different
letter of the plaintext are shifted by different amounts. Its key is a string k = k1 . . . kn
with ki {1, . . . , 26} (again each ki can be represented by a letter). The encryption
of a message m = m1 . . . m under key k is (m1 + k1 mod n)(m1 + k2 mod n). . .(m1 +
k(1) mod n)(m1 + k mod n).
1
Since the key space of the Vigenère cipher has size 26n
, except for small n, a bruteforce
attack is not feasible.