Blaise de Vigenère (eh) . The Cipher was thought to be indecipherable for almost three centuries and the French even called it "'le chiffre indéchiffrable' (French for 'the indecipherable cipher')" .
The Cipher works by combining the numerical values of the letters in the message with the numerical values of the letters in the key.
Step 1: Your message
1. Your message
2. The Key- The key should be known by everyone you want to be able to decipher the message. Longer is more secure than short and random letters are more secure than a word. It should also be easy for everyone to remember.
3. For the first few times you are going to want some scratch paper to work the cipher out on, if you get good you can do it in your head.
My message: Meet at the Statue of Liberty at four on Friday.
My Key: Instructables
Why I picked my key: It is relatively long and it is not a word (in standard dictionaries).
Step 2: Step 1: Misspellings
This is a very simple step but a one that greatly enhances the security of your message.
Why?: One hundred years ago this wouldn't have been nearly as important but in the age of computers it is significantly more important. By misspelling the words you foil some of the attempts a computer can make at solving the cipher using linguistic patterns.
Before: Meet at the Statue of Liberty at four on Friday
After: Meeet att thee Stattue of Libertea at for on Fryday