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The unbreakable code
The One-Time Pad, abbreviated OTP, is an encryption technique
in which each
character of the plaintext is combined with a character from a random key
stream. Originally described in 1882 by banker Frank Miller (USA), it
was re-invented in 1917 by Gilbert Vernam and Joseph Mauborgne.
The OTP is named after the sheets of paper (pads) on which the key stream is
usually printed. It can only be used once. When applied correctly, the OTP
provides a truely unbreakable cipher. An automated form, used in combination
with a teleprinter (telex),
is known as One Time Tape (OTT).
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The image on the right shows a page from a typical OTP booklet as it was
used during the Cold War
by Eastern-Bloc agents and spies,
in particular by agents
from the Soviet Union (USSR) and East-Germany (DDR).
The booklet consists of a stack of very thin sequentially numbered pages,
each of which contains a series of random 5-digit numbers.
Each page could only be used once and had to be destroyed immediately
after use.
OTPs of this type were often used in combination with
Eastern-Block spy radio sets, such as the DDR Type 2
and the Russian R-353 (PROTON).
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This section shows a selection of OTP systems from various
sources and countries. Although the exact operating procedure varies between
OTP systems, we will try to provide examples whenever possible.
Original OTP booklets are extremely rare as they were normally destroyed
after use. The surviving ones are generally held by the
intelligence services that either used or confiscated them.
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OTP systems on this website
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With a one-time pad (OTP), the encryption key has at least the same
length as the actual message (i.e. the plaintext) and consists
of truely random numbers or letters.
Each letter of the plaintext is 'added' to one element from the OTP
using modulo-addition.
When the key is unknown, this results in a ciphertext that has no relation
with the plaintext. At the receiving end, the same OTP is used to retrieve the
original plaintext. For this to work, the following rules are mandatory:
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- The OTP should consist of truely random characters (noise).
- The OTP (i.e. the key) must have at least the same length as the plaintext.
- Only two copies of the OTP exist.
- The OTP can be used only once.
- Both copies of the OTP are destroyed immediately after use.
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When the above rules are strictly obeyed, the OTP is absolutely safe.
Combining numbers with the plaintext manually, is a time-consuming task.
It is therefore sometimes thought that OTPs are no longer practical.
With modern computer technology however, the entire task of enciphering
and deciphering can easily be automated, just like it
was done in the past on teleprinter systems.
Although it may sound strange, manual OTP ciphers are still being used
today (2015) for sending secret messages to agents (spies) via the
Numbers Stations,
or One-Way Voice Links (OWVL),
that you may have heard on the short-wave radio bands.
For a detailed description of the One-Time Pad Cipher and its history,
complete with numerous examples, we would like to recommend the
excellent paper
Secure Communications with the One Time Pad Cipher,
by Dirk Rijmenants [3].
➤ Read now (off-site)
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The major disadvantage of the OTP, is the logistical problem of its
distribution. A unique pair
of OTP booklets must be issued and distributed to each individual agent
abroad. As the OTP will be destroyed immediately after use, sufficient
and timely supply of new OTPs has to be guaranteed.
During the Cold War, OTPs were often smuggled
into a country by means of a concealment, such as
the one shown in the image on the right. This regular travel kit was cleverly
converted into a concealment device by the
East-German Stasi.
➤ More information
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Another popular method for distributing and hiding OTP booklets,
was by printing them at very small size and hiding them
inside common objects
like ballpoints, photoframes and, as shown in the image on
the right, inside a walnut.
For many years, the walnut concealment was very popular with
KGB agents in Western Europe,
until it was discovered by the West-Germans.
➤ Walnut concealment
➤ Other concealments
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OTP systems come in many forms and flavours, but most of them consist
of either numbers or letters. An OTP based on numbers is also
known as a One-Time Figure Pad (OTFP). When sending text, each letter
of the plaintext has to be converted into a number before applying the OTP.
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An OTP based on letters is also known as a One-Time Latter Pad (OTLP).
An example of an OTLP is shown in the image on the right.
It consists of a stack of ~ 30 pages that are stapled together.
The conver (just visible at the left) holds a folded alphabet that is used
in the translation process.
Some OTPs are so small that they can easily be hidden inside a small object.
More examples and detailed photographs below. Some OTPs are so small that
they can be fitted inside a slide frame.
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All photographs in this section © Crypto Museum.
Objects courtesy Detlev Vreisleben (Germany) [1].
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When used properly, OTPs are inherently safe. But when they are used
incorrectly, e.g. by reusing them, they can be a cryptologic nightmare.
A good example of improper use is described by Matt Blaze in an article
about the book Compromised by former FBI counterintelligence agent
Peter Strzok [4]. He describes how secret OTP-encrypted messages were sent
to Russian illegals 1 in the US, via a
Numbers Station in Cuba
which had a good coverage throughout the US.
The station would broadcast message in morse code
and voice 24 hours a day, interleaving real messages with dummy traffic, so
that it was not revealed to an interceptor how often and how many real messages
were sent. However, the FBI discovered (as did others) that dummy traffic could
easily be discriminated from real traffic, as it did not contain the number 9.
This was probably caused by a faillure of the random number generator used
for the dummy traffic, or a bug in the software that handled it.
With this knowledge, the FBI was able to correlate the real messages to
the times that messages were decoded by a Russian illegal couple they had under
surveillance. It eventually led to the arrest of an several Russian illegals in
the US and Canada.
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In the espionage trade, an illegal is a foreign person who lives in a
country under a false or assumed identity, also known as a legend,
for the purpose of spying.
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OTP booklets, such as the one shown above,
have been captured during the Cold War by Western intelligence
agencies on a number of occassions. One documented example is the capture
of a Dutch man, who
acted as an East-German agent in The Netherlands, in 1969.
When he was finally exposed, the Dutch intelligence service
BVD
(now: AIVD)
found a partly used OTP booklet in his home, along with a fully operational
R-353 spy radio set,
a burst encoder and cassettes [5].
➤ More about the R-353 radio
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© Crypto Museum. Created: Friday 28 August 2015. Last changed: Saturday, 27 September 2025 - 17:14 CET.
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![Letter-based OTP, courtesy Detlev Vreisleben [2]](../img/301277/005/full.jpg)
