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 in the encryption and decryption processes of some ciphers, akin to a password

Basic Ciphers

Caesar cipher

Click here to experiment with the 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)

Hex cipher

The Hex cipher, and other variants (such as Octal, Decimal, and Base64), rely on the fact that each ASCII character that you can type has a unique identifying number. For example,