Substitution cipher

In a substitution cipher, also known as a Caesar Cipher, the secret key is the replacement of certain or all of the plaintext with another character, thus creating the ciphertext. For example, let's say you are writing the sentence, "the quick brown fox jumps over the lazy dog." We, as humans, will see it in its natural form, plaintext. If we were to use a key such as A=Z, B=Y, C=X and so on, we would have the following:

If we encrypt the message: the quick brown fox jumps over the lazy dog, the ciphertext will result in: gsv jfrxp yildm ulc qfnkh levi gsv ozab wlt. Reversing the ciphertext using the preceding chart will result in the plaintext.

The key is anything you choose. In the previous example, I decided the key would be the alphabet in reverse.