Teleprinter version of the OTP
One-Time Tape, or OTT, is an automated digital variant of the so-called
When correctly applied, OTT-encrypted messages are unbreakable.
OTT is best explained as a practical implementation
of the OTP Cipher for use with telegraph equipment,
also known as Teleprinters
using a 5-bit digital code (often ITA-2, baudot)
for the exchange of data.
A key tape is generated by recording the data of a random number generator
The Vernam Principle is then used to combine each
plaintext character with a single character from the key tape.
As a simple XOR-operation is used for this process
of 'mixing', the same key tape can be mixed again with the ciphertext
at the receiving end, in order to recover the plaintext again.
Machines that use OTT, are commonly called mixers.
A good example is the ETCRRM
that was used for many years
on the Washington-Moscow teleprinter hotline
during the Cold War.
Most mixers, or OTT machines, use data from a teleprinter machine
or from a paper-tape reader as input.
Such data is generally stored in 5-bit digital format,
commonly in ITA2 code (baudot),
but other data formats are also possible.
Plain text is either entered directly on the keyboard of the teleprinter
(online), or is stored on a punched paper-tape first and replayed
The above illustration explains how the mixer works.
Each letter of the Plaintext is added to a letter from
a Key tape, using an exclusive-OR, or XOR, operation.
In mathematics this is known as modulo-2 addition.
In cryptography it is known as the Vernam Cipher.
It has the advantage of being reversible: by adding the key stream
to the ciphertext, the original plaintext is retrieved.
➤ More about the Vernam Cipher
When using OTT equipment, or mixers,
a sufficient supply of key tapes
was mandatory in order to keep up with the constant flow of messages.
Key tapes were initially produced manually with so-called manual
tape punchers, but this had numerous drawbacks, such as the long time
it took to produce a single tape and the lack of randomness in the
human mind when pressing the buttons.
To overcome these drawbacks, the process was automated and machines
were developed for creating the (pseudo) random key streams and punching
them onto 5-level paper tape. Initially, mechanical methods were used
for producing the key stream, but as these too lacked sufficient randomness,
noise generators were introduced.
The image on the right shows the
5224, one of the first key tape generators
that were available on the market, made by
in Germany (now: Mils in Austria).
It has a built-in white noise generator and produces two tapes.
In order to ensure that both key tapes are identical, they are punched
simultaneously in a single tape puncher. Futhermore, the machine has 10
counters, to keep track of the number of zeros and ones that are generated.
In a truely random system, they should be distributed evenly.
Although noise generators were already in use during WWII for
creation of one-time keys for the SIGSALY transatlantic secure voice
link between the UK and the US,
most OTP and OTT systems that were used during the war
and shortly thereafter, used mechanically generated (and therefore
deterministic) pseudo random keys.
The Siemens T-43 mixer,
for example, used two Siemens T-52 Geheimschreibers
in series for the generation of its key tapes. If the codebreakers at
Bletchley Park had known this, it would have been relatively
easy for them to break the cipher.
In 1952, Dr. Werner Liebknecht, a developer at C. Lorenz AG in Stuttgart
(Germany) was the first one to publicly file a patent for a
Random Number Generator (RNG) based on a white noise source.
It produced evenly spread non-deterministic
numbers that were idealy suited for the generation of One-Time Tapes.
The patent was bought by Willy Reichert
in Trier (Germany) who used it
to build the first commercially available OTT generator on the market:
the Würfel (Dice).
➤ More about the 5224
Like with the manual OTP cipher, OTT systems suffer from the same
key distribution problem. Especially in applications where communication
takes place on a large scale, such as in the Army,
one has to ensure that a sufficient supply of fresh (unused)
key tapes is available at all times.
Although this may seem a simple requirement, it often caused
logistics 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
and compromises in the past.
Some operators re-used an old key-tape, or used
it in reverse direction. There are even stories about operators
who taped a one-metre piece of key-tape together and used it
as an endless 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 practical.
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, in diplomacy and on the
Today, the principle of the OTP/OTT is often implemented with computers,
but this poses a real security threat as there is
no such thing as a secure personal computer.
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© Crypto Museum. Created: Saturday 08 September 2012. Last changed: Friday, 30 December 2016 - 09:29 CET.