The Caesar cipher is a simple alphabet-shifting cryptography method. For example if the message is ‘hello’ and the key is 2, then each letter is replaced by the second letter following it in the alphabet and the message becomes ‘jgnnq’. To decypt the message, use the reverse operation : replace each letter by the one 2 letters before it in the alphabet (or 24 letters after it).
Simply write or paste your text in the first text box, press the “Shift Alphabet” and it will show you all the possible shifting. Select one of the shifted texts to see the key for encryption / decryption.
Origin detailed in this blog post.