Editing Final Report 2011 (section)

===Cipher Methodology and Analysis===
A cipher is any general system for hiding the meaning of a message by replacing each letter in the original message with another letter<ref name=TheCodeBook>''The Code Book'', author Simon Singh, The Fourth Estate, 2000.</ref>. The two general types of ciphers are substitution ciphers and transposition ciphers. Substitution ciphers are those in which each letter of the plaintext is replaced by another letter to form the ciphertext, while transposition ciphers are those in which letters within the message retain their values but change position<ref name=TheCodeBook>''The Code Book'', author Simon Singh, The Fourth Estate, 2000.</ref>.

A simple example of a substitution cipher is the Caesar Cipher. The cipher is formed by shifting each plaintext letter three places along the alphabet to form the ciphertext letter as shown in the figure below.
<center>[[File:Caesar Cipher.png|Caesar Cipher]]</center>
<center>'''Figure 2 - Caesar cipher encryption process'''</center>


A simple example of a Caesar Cipher encryption would to be to encrypt the word “face”. By moving each letter along three places (refer to the figure above), the plaintext first letter ‘f’ goes to ciphertext ‘I’, ‘a’ goes to ‘D’, ‘c’ to ‘F’ and ‘e’ to ‘H’  . Thus the plaintext “face” gets transformed to the ciphertext “IDFH”. Decryption can be performed by trivially reversing the process.

Ciphers typically involve a general method that specifies what sort of algorithm is used in encrypting the plaintext, and a key that specifies the exact values used in the algorithm<ref name=TheCodeBook>''The Code Book'', author Simon Singh, The Fourth Estate, 2000.</ref>. For example, in the above Caesar Cipher, the algorithm can be considered a shift of the alphabet with the key being 3, resulting in the specific instruction of a shift in the alphabet of 3.

Cipher analysis on substitution ciphers is traditionally performed through a process called [http://en.wikipedia.org/wiki/Frequency_analysis Frequency Analysis]. This process uses linguistics and statistics, recognising that each letter in a language has its own unique characteristics that can be used to identify it. For example, in the English language, the letter ‘e’ has the characteristic of occurring most commonly; on average 13% of the time<ref name=letterfreq>Lewand, Robert, English Letter Frequencies, http://pages.central.edu/emp/LintonT/classes/spring01/cryptography/letterfreq.html</ref>. Thus it would make sense to replace the most commonly occurring ciphertext letter with the plaintext letter ‘e’.