Cryptography

Cryptography is the process of hiding messages; either by concealing them (eg. hiding them in an image), or by obfuscating them outright (eg. substitution cipher).

Basic Terminology

• Cipher: a method of encryption
• Plaintext: the legible text of a hidden message
• Ciphertext: the text after a message is concealed in it
• Encryption: The process of turning plaintext into ciphertext
• Decryption: The process of converting ciphertext back into plaintext
• Key: a string used by the cipher that is neither the plaintext nor the ciphertext

Basic Ciphers

Caesar cipher

The simplest example of a cipher is the Caesar cipher. The rules of the cipher are as follows:

Let n equal a value from 1 to 25
Shift each letter in the plaintext forward by n positions in the alphabet
The resultant string is the ciphertext

For example, to encrypt the string Game Detectives using the Caesar cipher, using an arbitrary n value of 2, then:

G -> H -> I
a -> b -> c
m -> n -> o
e -> f -> g
...

and the resultant ciphertext would be Icog Fgvgevkxgu. To decrypt this string back into Game Detectives, the process can simply be reversed by shifting each letter of the ciphertext 2 places backwards. (Note: another common name for the Caesar cipher is ROT<n> - ROT13 indicates that each letter is shifted halfway through the alphabet)