# Computer Security

Topics: Cryptography, Encryption, Cipher Pages: 3 (488 words) Published: April 18, 2013
G53SEC Computer Security Spring 2012/2013

Coursework 1

Name ID

: :

Arvinth Gunasegaran 005917 09/04/2013

Due date :

Introduction/Technique
Cryptography is the act or practice of using techniques for secret communication over public communication channels. The purpose of this is to enable the transmission of messages secretly without being understood by third parties. Cryptography is achieved by means of encryption, which is the process of converting normal text to ciphered text using a key, either public or private. The ciphered text is then transmitted to the receiver, who can decrypt it back to normal text using either a public key (symmetric cryptography) or his or her private key (asymmetric cryptography). One of most famous symmetric encryption techniques is the Caesar cipher, or also known as the shift cipher. It is a type of substitution cipher that works by replacing each alphabet in plaintext into a corresponding alphabet some fixed number of positions either to the right or left of the alphabet. The first Caesar cipher shifted all characters to three positions to the right. However, a shift of any other number or to the left is also used.

Based on this, the objective of the coursework is to produce a reverse Caesar cipher encryption. The technique works by first choosing a fixed number to shift the ‘A’ character. Once the letter ‘A’ is shifted accordingly, the rest of the alphabets are filled in, in reverse. The example below shows a simple case of reverse Caesar with a shift value of 3.

A

B

C

D

E

F

G

H

I

J

K

D

C

B

A

Z

Y

X

W

V

U

T

In this case, the number to shift is 3. Hence, the letter ‘A’ is first shifted 3 places to the right. The rest of the alphabets are then listed in reverse order, which means the letter ‘A’ is followed by the letter ‘Z’, ‘Y’ and ‘X’, instead of ‘B’, ‘C’, and ‘D’ like in normal Caesar ciphers.

Program Explanation
The reverse Caesar...