Preparation
Write a one-time pad. You will need this to encrypt your messages. A one-time pad is a pad of paper with a random sequence of letters written on it. You will need two exact copies - one for yourself, one for your agent (the person receiving the message). Lets' say our one-time pad is abgsdpeycnghf.
Give one copy to your agent. Keep the other.
Encrypting a message
Write the message at the top of a piece of paper. Our message is hello.
Convert it into number. Hello becomes 8 5 12 12 15.
Write down the letters of the one-time pad underneath. You only need as many letters as there are in the message. Using our one-time pad, these are abgsd. This is the key.
Convert the key into numbers. The key is now 1 2 7 19 4.
Add together the corresponding numbers to get the codetext. Add the first letter of the message to the first letter of the key, second to second, etc. Our codetext is 9 7 19 31 19.
For every number over (not equal to) 26, subtract 26. Our codetext becomes 9 7 19 5 19.
Convert back into letters. Our codetext message is IGSES.
Destroy the part of the one-time pad that you used. This is why it is a one-time pad: each part is used once. If it's written on paper, tear it off and burn it. Our one-time pad is now peycnghf.
Send the message.
Decrypting a message
Your agent sends you a message back. You need to decrypt it. Say the message is WTNGPFM.
Write the message at the top of a piece of paper. Convert it into numbers and write these underneath. The codetext is now 23 20 14 7 16 6 13.
Write the necessary letters from the one-time pad underneath. This is the key. Our key is peycngh.
Convert the key into numbers. Our key becomes 16 5 25 3 14 7 8.
Subtract the corresponding numbers of the key from the message. 23-16 = 7, 20-5 = 15, 14-25 = -11, etc. Our message becomes 7 15 -11 4 2 -1 5.
For any negative number, or zero, add 26. The message is now 7 15 15 4 2 25 5.
Convert into letters. The message is goodbye.
Destroy the part of the one-time pad that was used. Our one-time pad is now f.
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