The power has been out for the three months. No one knows what happened. The stores ran out of food after the first week. The water stopped flowing soon after that. Many large towns and cities are becoming uninhabitable. There is no law and no one to call for help. There is talk of terrible atrocities being committed by armed gangs.
You are lucky, your town happens to be remote enough and small enough to manage the chaos. Folks from all walks of life are pulling together to make the best of this difficult time. An ad-hoc committee was elected to represent the town and you have been asked to help with security. Agreements were made to help a local farmer protect and harvest his crops in return for a share of the food.
You have one problem. Although you have a decent security program, you worry about using the handheld radios. A few nights ago, one of the checkpoints could hear some chatter on the same frequency. Your radios are not the expensive ones with “secure” frequencies. You worry that if you can hear others, then others can hear you. Some of the information passed to shifts is sensitive to the security needs of the community. How do you pass on information without risk of interception?
Today, you were discussing the possibility of bad guys listening in on your conversations, when someone mentions cryptography as a solution.
Now the old days in the military started flashing in your head… yes… we had this thing called a CEOI (Communications-Electronics Operating Instructions).
This person claims you can use a variation of the Vernam Cipher. A Vernam Cipher allows for the substitution of a plain text message with a random series of text or numbers from a “One Time Pad”. The resulting encrypted message is secure enough to be sent over the airwaves, without risk of being deciphered by an interceptor. Once it reaches its destination, the intended receiver will use the same “One Time Pad” to “unmix” the message.
A true Vernam Cipher, if used properly is thought to be unbreakable. The reason is because the cipher key constantly changes. This randomness makes it very difficult to decipher.
A poor mans version of this type of cipher can be something simple like a book. Each person intending to receive the secret message uses the book as a tool for decryption. As long as the bad guys don’t know that you are using the book as a tool, you should be secure.
Lets say you wish to send the following message over the radio to a friend:
GET BACK TO THE BUILDING
Using a book as a “One Time Pad” you simply look for each of the words in the book. As you find a word, note the page number, line number and word number in the line.
For simplicity, we will choose words all on the same page. “GET BACK TO THE BUILDING” encrypted will look like this:
11-1-25 / 11-32-8 / 11-1-10 / 11-8-2 / 11-7-7
The first three numbers (11-1-25) indicate the word you are looking for will be on page 11, line 1, and word 25.
You simply conduct this find and replace operation with each sequence of numbers. What you end up with is a message.
“Get back to the building”
Although this type of cipher is not a true Vernam Cipher, it will work as a field expedient method of secure communications. That is unless the bad guys find out what you are using for encryption. To keep them from figuring this out, never mark inside the book or leave any trace of encrypted messages in the book. If any of these clues were left in the book, it wouldn’t take much for the bad guys to figure it out.
The next article will dicuss a slightly more difficult, but secure version of the Vernam Cipher.