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How the One Time Pad Works – CIA Instruction

One-time pad is a sheet of paper, on which random numbers arranged in groups or five (variations exist – Translator) are printed. Numbers on different sheets should vary. Sheet size depends on delivery and concealment methods.

cia_one_time_pad

 

PURPOSE: one-time pads are used for encoding and decoding messages. Such correspondence is resistant to disclosure.

Encoding is divided in two stages:

  1. letters of the original message are converted into numbers
  2. the same numbers are added to numbers from the notepad using simple math operations

Since the numbers on the cipher pad’s sheets are absolutely random, every sheet has to be destroyed  to avoid compromisation.

Many ciphers use math keys, generated using algorithms. Such keys are not random and can be decrypted by modern computers. A major issue of one-time pads is the necessity to replace them after all sheets are used. Usually one-time pads are small.

Real size

Real size

A single sheet of a one-time pad placed on the opposite side of a postal stamp, size 2 x 0.8 cm.

 

How to use the one-time pad

Every letter of the alphabet has its own number equivalent.

example

Replace letters with numbers from the table.

example_2

Divide the numbers from the one-time pad into pairs and write them in line (in this example we start from the last line in the cipher). Add the numbers from the message.
(Attention: use simple arithmetic addition. If sum exceeds 100, write last two digits in result)

example_3

Then the message is divided in groups of five and transmitted.

example_4

To decode the message,  the recipient uses the same page from his own one-time pad. Numbers are broken into pairs once again and subtracted.

example_5

Sometimes it’s needed add 100 to the number when subtracting. Replacing numbers with letters we can decode the original text.

example_6

Official designation – one-time encoding notepad (pad size and disguise should be noted)

Estimated 1948

 

Instruction dated on 1975

 

Source: Мелтон, Уоллес, Роберт, Шлезингер, Генри Кит. Искусство шпионажа. Тайная история спецтехники ЦРУ.  Москва.2013, c. 113 (Spycraft: The Secret History of the CIA’s Spytechs)

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