Substitutions are quite simple. As the name implies, they substitute one thing for another to encrypt plaintext into ciphertext. The key is the arrangement of the characters (if we're dealing with an alphabet substitution) that tells us what is exchanged for what.
Consider the alphabet and a rotation cipher of 2 positions.
- abcdefghijklmnopqrstuvwxyz becomes
To encrypt, replace all letters in your plain text with the corresponding letter below it, as given in the box above. For example, using this method,
- marie huynh wrote this
- octkg jwapj ytqvg vjku
To decrypt, simply replace these letters with the corresponding ones above.