The unbreakable code
Although every cipher, based on a mechanical or mathematical system of
permutations, can in theory be broken, there is one encryption method that
is truely unbreakable.
It is the so-called One Time Key (OTK). In the past it was often
implemented as a notepad full of random numbers and is therefore commonly
called One-Time Pad (OTP).
It also exists as One Time Tape (OTT).
The image on the right shows a typical OTP booklet as it was used by spies
of the former USSR (Russia) during the 1960s.
It consists of a stack of very thin small pages, each with a series of
random 5-digit numbers on them.
Each page was destroyed immediately after use.
OTPs like this, were often used in combination with Russian
spy radio sets
like the R-353,
often in relation to the number stations on the
short wave band.
The one shown here is from the internal collection
of the AIVD
 and was on display during the exhibition
Tijdrekken in 2011.
The theory behind the OTP is that the encryption-key has at least the same
length as the message (the plaintext) and consists of truely random numbers.
Each letter of the plaintext is 'mixed' with one element from the OTP.
This results in a ciphertext that has no relation with the plaintext when the
key is unknown. At the receiving end, the same OTP is used to retrieve the
original plaintext. For this to work, the following rules are mandatory:
- The OTP should consist of truely random numbers (noise).
- Precisely two copies of the OTP should exist.
- The OTP should only be used once.
- Both copies of the OTP are destroyed immediately after use.
Only if the above rules are strictly obeyed, the OTP is absolutely safe.
Adding numbers to the plaintext manually, is a time-consuming task.
It is therefore sometimes thought that OTPs are no longer considered practical.
However, with modern computer technology, the entire task of mixing and
unmixing plaintext with the key, can easily be automated (see below).
For a detailed description of the One-Time Pad and its history
complete with numerous examples, we would like to recommend the
paper Secure Communications with the One Time Pad Cipher
by Dirk Rijmenants .
Read now (off-site)
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 agency
found a partly used OTP booklet in his home, along with a fully operational
R-353 spy radio set,
a burst encoder and cassettes
The OTP shown above, dates back to the days of the 1960s, when the
Cold War was at its height. Eastern Block spies,
and in particular spies from East-Germany,
often used OTPs for their messages,
as it was absolutely safe and could not be
broken by the western intelligence agencies.
The major problem with OTPs however, is their distribution. A unique set
of OTP booklets needs to be issued and distributed to each individual spy
or agent abroad. As the OTP was destroyed immediately after use, sufficient
and timely supply of new OTPs had to be guaranteed.
OTPs were often smuggled into the country by using concealments.
An example of such a concealment is shown in the image on the right.
This tavel kit was used by an East-German agent who was cought in
The Netherlands during the 1960s. The OTP was confiscated by the
The travel kit contains the usual items that a typical traveller would
carry in those days for personal maintenance: a comb, a brush, a mirror,
parfume, a nail cutter, shaving gear, etc.
The travel kit also contains a secret storage compartment however,
that can be accessed by
removing the nail cutter and its holder.
Applying the right pressure and shifting the holder in the right direction, reveals an area in which the OTP is hidden.
The OTP itself, is stored inside the
flat orange transport bag show here.
By pulling a rope that is attached to the transport bag, the OTP is
released from its concealment. The storage compartment would not be noticed
during normal checks at customs. Even on the (rather limited)
X-Ray machines of those days,
the OTP would not be visible, as the bottom of the travel kit and the
lid of the concealment area are both made of metal.
Further images below.
The OTP and the travel kit shown on this page were kindly supplied
by the AIVD (Netherlands)
is the abbreviation of Binnenlandse Veiligheidsdienst
(Internal Security Service), the former name of the Dutch General
Intelligence and Security Service. In 2002, the agency was renamed
OTP systems come in many forms and flavours. The Russian OTP shown above,
contains only numbers. It requires the letters of a message to be converted
into numbers, before applying the OTP. It is also possible however, to use an
OTP based on letters.
An example of such a letter-based OTP is shown in the image on the right.
It's a stack of approx. 30 pages that are stapled together.
The cover at the left, contains a folded alphabet table that is used in the
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.
All OTP photos in this section are courtesy Detlev Vreisleben (Germany) 
and © Crypto Museum.
The use of OTPs in the military has always been very popular,
especially after the introduction of digital telegraph equipment,
also known as Teleprinters
Such machines use a 5-bit digital code
to identify each character,
and punched paper tape to store the messages.
By recording truely random data (noise) on a paper tape,
one could easily create a key tape.
The Vernam Principle was then used to 'mix' each
plaintext character with a (random) character from the key tape.
As mixing consists of a simple XOR-operation (modulo-2 addition),
the same key tape is mixed with the ciphertext at the receiving end,
in order to recover the plaintext again.
The image on the right shows a key-tape generator, produced by the Austrian company
In order to guarantee that both key tapes are identical, they are punched
simultaneously by the paper puncher on the right. Click for a larger view.
When used with punched paper tape,
the OTP is often referred to as One-Time Tape (OTT)
and the machine is commonly called a Mixer.
A good example of a mixer
is the ETCRRM that was used for many years
on the Washington-Moscow teleprinter hotline
during the Cold War.
Like the standard manual OTP cipher, OTT systems suffer from the same
key distribution problem. Especially in areas where communication takes
place on a large scale, such as in the Army,
one has to ensure that a sufficient supply of new key tapes is available
at all times.
Although this may seem a simple requirement, it often caused
major distribution problems with Army units in remote locations and
aboard ships. Key distribution would be seriously hampered, or indeed
be completely impossible, in the event of a war, e.g. when
operating behind enemy lines.
Key-tape shortages have led to several security incidents
over the years. Some operators used a previous key-tape again, or used
it in reverse direction, whilst others took a piece of (unused) key-tape
of, say, one metre in length, and taped it together as a loop.
For this reason, most Armies abandonned the use of OTT machines and
replaced them by cipher machines with a built-in key generator.
Such key generators generally consisted of a (pseudo) random number
generator (PRNG) that was seeded by a much shorter KEY. Good examples
of such machines are the Philips Ecolex X,
the Philips Aroflex
and the KL-51 (RACE).
Although such machines are often advertised as a more practical
implementation of the OTP, they do not meet the requirements of an OTP
and are never absolutely safe. They are just much more practicle.
Nevertheless, OTT machines remained in service for many years,
for messages that had to remain secret indefinitely, such as
at the highest level at NATO, with embassies and on the
Today, the principle of the OTP/OTT can easily be implemented with computers.
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