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One - Time - Pad

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By Pete McCollum
This article is based on a section from my web page at: http://www.geocities.com/saipan59/clan_radio/otp.html . Pictures are available there.

This article presents an example of how message encryption was typically done by CIA communicators using a "one-time pad" (OTP). A retired CIA old-timer kindly provided the descriptions.

A one-time pad is essentially a pad of paper on which each page has a unique set of random letters. The sender and receiver have identical pads. Each letter on the pad is used to determine a single letter of the enciphered message. Since the letters on the pad are random, there is no formula that can be determined by studying the letters. Assuming that the pad is not compromised, and each page is used only once, then the OTP system is unbreakable. The disadvantage of the OTP system is that a copy of the pad must be
securely delivered to the person on each end of the communication.

The key letters on the pad, and the messages themselves, are typically written in 5-letter groups. This helped the communicators to collate and verify the length of the message, and if something was misunderstood, the receiving person could ask for a certain group to be repeated, etc.

"OTP is a very simple yet completely unbreakable symmetric cipher. To use a one time pad you need 2 copies of the "pad" which will vary in size from something around 8 x 10 inches, or approximately half that size. There are two pads issued to each user. One for encipher and one for decipher, and the key text is printed in red for encipher and black for decipher. Each page of the pad is sealed and must not be opened until actually enciphering or deciphering [if a page is not sealed, it must be assumed that the page has been compromised - Pete]. Typically the pads are set up in blocks of five letter random groups.The key text may not be reused and pages should be destroyed after each use."
To use the OTP, a method is needed for correlating a letter of plain text with the next letter of the key text (from the pad), to produce a letter of enciphered text. The method used is called a "Vigenere's Tableau", or Vigenere's square. The table has the alphabet in the left-most column, and also across the top. For each row, there is a shifted-reverse alphabet. So, the "A" row lists the alphabet backwards, beginning with Z. The "B" row begins with Y and ends with "DCBAZ", etc. The first 3 rows of the table look like this:
To encipher the first letter in a message, go to the row corresponding to the plain-text letter, then go to the column indicated by the first letter on your OTP. The letter at the row-column intersection is the enciphered letter. Note that the Vigenere's table itself does not contain any 'secret' information - it simply provides the mechanism for combining plain and key text into enciphered text.