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It is important when transmitting data over a network that it is kept secure. Encryption encodes data so that only those who have the encryption key or password can decrypt it.

Encryption methods include:

• Transposition – The letters or words of the plain text are switched around in some way, fixed by a given rule (the key). An example would be to reverse the order of the letters. So, "a simple example" becomes "elpmaxe elpmis a".
• Substitution – Each letter you want to encrypt is substituted by another letter or symbol. So, if each letter is substituted by the letter preceding it in the alphabet, "a simple example" becomes "z rhlokd dwzlokd".

Encryption helps to serve three purposes:

• Helps to identify authentic users
• Prevents alteration of the message
• Prevents unauthorised users from reading the message

How is data encrypted?

Plain text is encoded with the use of an encryption key (rules required to decrypt the data) before data transmission. This cipher text (encoded message) can now be sent over a network. On receipt, the data is decrypted using the appropriate decryption key. If data is intercepted during transmission, the data is meaningless without the key.

The Caesar cipher

The Caesar cipher is one of the earliest known and simplest ciphers. It is a type of substitution cipher in which each letter in the text is 'shifted' a certain number of places down the alphabet. For example, with a shift of 1, A would be replaced by B, B would become C, and so on. The method is named after Julius Caesar, who apparently used it to communicate with his generals.

Caesar cipher is a very simple encryption method and is easily cracked if one studies the frequency of repeating letters. To improve the strength of the encryption method a more complex key could be used.

Instead of shifting by 1, a pattern of shifts could be used: For example, 1,4,5,1. This pattern would be repeated and would be more difficult to crack using frequency analysis.

Mathematically, using a simple shift of 1 means a character can be 1 of 26 possibilities. Using a key with 4 parts, a character can now be one of 26 * 26 * 26 * 26 = 456976 combinations. Much more difficult to crack!