Czechoslovak one-time pad
During the Cold War, the former federal state of
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 . 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.
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.
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.
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
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.
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.
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 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
- 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: Friday, 10 September 2021 - 11:26 CET.