Need to have inter-spy communication? Need to make sure that no one can understand your messages? You need a cipher, specifically a Vigenere Cipher. Created in 1553 by Giovan Battista Bellaso (What an awesome name!) and named after
Blaise de Vigenère (eh) [1]. The Cipher was thought to be indecipherable for almost three centuries[2] and the French even called it "'le chiffre indéchiffrable' (French for 'the indecipherable cipher')" [1].

The Cipher works by combining the numerical values of the letters in the message with the numerical values of the letters in the key.

[1]http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
[2]http://www.math.cornell.edu/~mec/2003-2004/cryptography/polyalpha/polyalpha.html

Step 1: Your Message

When you want to send a message you only need two things:

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

Warning: Grammar-Nazis, you may want to stop reading here.

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

Step 3: Numeric Conversion

The tabula recta (the last picture) can be used to combine the rest of the steps, but can not (easily) be memorized. If you would rather use the table for the coding you can skip to step 6 for info.

Normal Method:
1. Take the first letter of your message (M)
2. Find it on the first table (13)
3. Write it down...simple right?
4. Repeat for the rest of your message.

Then do that process again for the key, but right it down in a separate area.

Step 4: Combining the Message and Key

1. Write the numbers of your message out in a line, if you must use more than one at least double space (leave extra space).
2. Write the first number of your key (9) below the first number of your message (13)
3. Write the second number of your key(14) below the second number of your message (5)
4. Continue as such, skipping spaces until you run out of key
5. Repeat the key from where you stopped (visible in the picture)

Step 5: Add 'em Up

Add the two numbers you wrote together. (Picture 1)

If they are over 26 (yellow in Picture 1) subtract 26 from the number. (Picture 2)

Step 6: Converting Numeric Values to Letters

Use the chart from before but in reverse, use the final numbers from the last step to get out letters.

To use the tabula recta:
1. Find the letter from your message in the top row
2. Find the letter from your key in the side row
3. Where the row and column meet is the ciphered letter.
4. Write it down and repeat until you reach the end. When you run out of key letters start from the beginning of the key.

Step 7: Decoding

Do the process in reverse.
1. Take the ciphered message and convert it to numeric values
2. Write the key's numeric values underneath the messages.
3. Subtract the key's values from the message's
4. If it is less than zero add 26
5. Convert the resultant numbers to letters.

For the Tabula Recta:
1. Find the Key's letter in left hand side
2. Go across the table until you find the ciphered letter.
3. Go up to the header and find the actual letter.
4. Repeat for the rest.

Can you solve this cipher?
Buwx guuposo jgce lav qgusjei aa casr :X
1. Go to http://trick.ly/9jz
2. Type in the password

Step 8: Variations

1. Instead of repeating the key, begin using the original message as a key.
ie. If your message is Alphabet and your key is Soup you would have the beginning like normal (a+s)(l+o)(p+u)(h+p), then start with the message (a+a)(b+l)...and so on.

2.Instead of using a key word, use a block of text such as a newspaper article, it significantly decreases the chance of breaking the code.