Another version of the columnar transposition cipher is the cipher using a key word. In this cipher, we encrypt as follows: Given a key word, we remove all the duplicate characters in the key word. For example, if the key word is ‘SECRET’, we remove the second ‘E’ and use ‘SECRT’ as the key word. To encrypt, we form a table as follows: In the first row, we write down the key word. In the following rows, we write the plaintext. Suppose we want to encrypt the text ‘ATTACKATDAWN’. We make a table as follows:
| S | E | C | R | T |
| A | T | T | A | C |
| K | A | T | D | A |
| W | N | X | X | X |
Then we read off the columns in alphabetical order. We first read the column under ‘C’, followed by the columns under ‘E’, ‘R’, ‘S’ and ‘T’. We get the cipher text TTX TAN ADX AKW CAX. To decrypt, we reverse the process. Note that, since we know the length of the keyword, we can find the length of the columns by dividing the length of the message by the length of the keyword. Given the ciphertext ‘HNDWUEOESSRORUTXLARFASUXTINOOGFNEGASTORX’ and the key word ‘LANCE’, find the plaintext.
To decrypt the given ciphertext using the columnar transposition cipher with the key word LANCE, we will follow these steps: ### Step 1: Remove duplicate letters from the key word The key word given is LANCE. It does not have any duplicates, so the key word remains LANCE. ### Step 2: Create the table layout The key word "LANCE" has 5 letters, so we will divide the ciphertext into columns based on the number of letters in the key word (5 columns). The length of the ciphertext is 40 characters, so we will have 8 rows (40 / 5 = 8). Ciphertext: HNDWUEOESSRORUTXLARFASUXTINOOGFNEGASTORX ### Step ___ __________ _______ _____ ____ _________ ___ _________ _____ _____ __________ _______.
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i) Bob uses ElGamal cyrptosystem with parameters p = 47, g = 5 and the secret value
x = 3. What values will he make public?
ii) Alice wants to send Bob the message M = 15. She chooses k = 5. How will she compute
the cipher text? What information does she send to Bob?
iii) Explain how Bob will decrypt the message.
a) Considering the bytes 10001001 and 10101010 as elements of the field where
is the polynomial
find their product and quotient.
b) Find a recurrence that generates the sequence 110110110110110.
a) Decrypt the ciphertext 101000111001 which was encrypted with the Toy block cipher once
using the key 101010010. Show all the steps. (5)
b) A 64 bit key for the DES is given below
11000111 10000101
11110111 11000001
11111011 10101011
10011101 10010001
i) Check whether the key is error free using the parity bits.
ii) Find the keys for the second round.
Another version of the columnar transposition cipher is the cipher using a key word. In this cipher, we encrypt as follows: Given a key word, we remove all the duplicate characters in the key word. For example, if the key word is ‘SECRET’, we remove the second ‘E’ and use ‘SECRT’ as the key word. To encrypt, we form a table as follows: In the first row, we write down the key word. In the following rows, we write the plaintext. Suppose we want to encrypt the text ‘ATTACKATDAWN’. We make a table as follows:
| S | E | C | R | T |
| A | T | T | A | C |
| K | A | T | D | A |
| W | N | X | X | X |
Text:"KSTYZKESLNZUV", encrypted with Vigenère cipher with key "RESULT"
Use Miller-Rabin test to check whether 75521 is a strong pseuodprime to the base 2.
Find the inverse of 13 (mod 51) using extended euclidean algorithm