Lab 3- Classic Cryptography

In this lab, we need to
- Explain What is Symmetric and Asymmetric Cryptography
- Implementing Caesar Cipher for Symmetric Cryptography.
- Implementing Vigeneré Cipher for Symmetric Cryptography.

Symmetric encipherment

Caesar Cipher

The Caesar Cipher is formed by shifting the letters of the original alphabet. For example by replacing each letter of the alphabet with the letter three places down the alphabet. It is monoalphabetic as only one letter in plaintext is exchanged for one letter of ciphertext.

plaintext : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

ciphertext : D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

A caesar chiper with a key 3

For an example the plain text in figure 4.2 below can be encrypted using key 3 to get the cipher text in figure below

plaintext : THE ATTACK TONIGHT START AT EIGHT,

REGROUP AT STATION A

ciphertext : WKHDWWDFNWRQLJKWVWDUWDWHLJK

WUHJURXSDWVWDWLRQD

A brute-force cryptanalysis is easily performed: by simply trying all the keys available.

The characteristics which enables the use of brute-force cryptanalysis:

1. The encryption and decryption algorithms are known
2. There are only 26 keys to try.
3. The language of the plaintext is known and easily recognizable. The pattern is obvious.

Task 1 – Deciphering simple substitution cipher text

Remind! Use a spreadsheet program to help you in encrypting your plaintext or decrypting your ciphertext

1. By using key 9 encrypt the following phrase

* Information Technology Security
* The target is the building on the right of the zoo
* The Truck is heading toward the dock

2. The following cipher text which is intercepted by the Special Forces during a routine surveillance on a terrorist headquarters. It was known that the cipher text is an English text containing a word ‘FALL’. Decrypt the cipher text.

RCTRZAVGGNJOCVORZXVIGZVMIOJNOVIYVBVDIVIYWZXJHZNOMJIBZM

Vigeneré Cipher

The cipher text encrypted using Caesar cipher method is easily broken by using a brute force attack. An attacker can easily try every combinations of character to break the code as the number of possibility is just 26. In order to improve the deciphering process we can used the vigeneré cipher method. The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. It is a simple form of polyalphabetic substitution.

A key is needed to encrypt a plaintext; a key can be a word or a phrase. To have a strong cipher text it is advised to use different key on each encryption.

Task 2 – Encrypt and decrypt using vigeneré cipher

1. Encipher the message ‘CHAPTER 2 AUTHENTICATION AND BASIC CRYPTOGRAPHY ‘using Vigenere cipher and the enciphering key ‘MONEY’.
2. The following ciphertext was intercepted by the MI6 Forces during a routine surveillance on a terrorist headquarters. It is believe that the first cipher text can be decrypted into (as picture below)

and the second ciphertext intercept