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Czechoslovak one-time pad

During the Cold War, the former federal state of Czechoslovakia used One-Time Pad (OTP) cipher systems for agent communication as well as for diplomatic traffic between capital Prague and their embassies abroad. For this purpose they issued OTP booklets, each with 40 pages or pads and 50 five-digit groups on each page [1]. The books were issued in identical pairs: red and blue.

The OTP booklets used by the Czechoslovakians measured 21 x 8 cm; not particularly small for a secret document and very difficult to hide. This is why they were probably used for diplomatic traffic between an embassy and the Home Office.

The books were printed in identical pairs and were delivered in a tamper-evident packaging, complete with a seal, a stamp and a sturdy plastic blister pack. Unauthorised opening would immediately be noticed by the user, in which case the key was compromised and the OTP would not be used; it was destroyed instead.
Two identical OTP booklets

The books were marked with the text PŘISNĚ TAJNÉ (top secret) and with the number 40 (pages). Each booklet further had a serial number printed at the front. Of the two identical books, the blue one was marked AS-1. It was used for the reception of encrypted messages. The red book was marked AS-2 and was used for sending encrypted messages. If an embassy wanted to send a message to the home office, they used a red book, whilst the home office used the matching blue book with the same serial number, and vice versa. Each party had multiple red and blue booklets.

Two identical OTP booklets Two identical OTP booklets Rear side Plastic retaining stubs Retaining stubs and seals at the edges Black light-blocking sheet Letter box perforation Removing an OTP sheet via the 'letter box'
The diagram below shows two OTP booklets, a red one and a blue one, that have the same serial number (038461 in this case). The booklet was actually a stack of papers that was sealed inside a transparent blister packaging with a brightly coloured back, so that they were easily identified.

Two identical OTP booklets (red and blue)

Both of these books remained in their protective packaging until they were needed, in which case a small pre-cut 'letter box' opening was made at the rear, by means of a knife. This reveals a protective perforated carton sheet that must be torn away, just like the black paper light shield immediately behind it. This black paper is present at both sides of the stack and protects the OTP against vetting. The pages of the OTP are retained at both sides by stubs in the blister packaging.

Index page

Next, the index page is taken out of the booklet. It contains the serial number of the OTP plus 40 five-digit identifiers, one for each page of the booklet. This way the user can check whether the book is complete and he can use it to strike out the pages that have been used and destroyed. It is important that the sequence of the pages is not altered in order to stay in sync with control.

page 01 with check number 58653

Each page of the booklet contains 50 random five-digit groups, arranged as 5 rows by 10 columns. A sequential page number (01-40) is printed at the bottom right. The check group is printed at the top right. For encoding and decoding, complete pages are always used. If a message is longer than one page, multiple pages are used. If the message is shorter, the rest of the page is discarded. For each new message, a fresh new page was taken from the stack.

The pages are printed with a matrix printer on extremely thin paper, much like the 30 grams sheets that were used during the 1960s to make carbon copies on an ordinary typewriter. They are perforated at both ends and are held in place by the plastic stubs in the blister packaging. By pulling-out a page through the 'letter box' at the rear, the page is effectively torn from the stack. The page is destroyed after use, so that the key can never be compromised afterwards.

  1. Miro Hornik, Red and blue Czechoslovakian OTP booklets - THANKS !
    Personal correspondence. August 2015.
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Crypto Museum. Created: Friday 28 August 2015. Last changed: Sunday, 31 July 2016 - 10:56 CET.
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