One-Time Tape cipher machines
Mixers are a class of cipher machines that are based on the
Vernam Cipher, in which Plain text is mixed with
a random key stream (hence the name mixer).
If the mixer is used with a key tape that contains
evenly-spread truly random characters, the cipher is guaranteed
to be unbreakable.
Such a key tape is commonly referred to as a One-Time Pad (OTP)
or a One-Time Tape (OTT).
It is often thought that, like most cipher machines, mixers are
classified items. However, due to the way the mixer works, there is nothing
secret about the machine at all. Besides, when the machine is used
correctly, the code is unbreakable anyway.
Most mixer machines were therefore unclassified, although
circuit diagrams and user manuals may have been restricted at the time.
With machines of this class, it is the key tape that protects the
secret. This is the reason why the key tapes were only used once and were
destroyed immediately after use, so that they could not fall into the wrong
hands. Operational key tapes were always classified.
They often carried labels like NATO Secret.
Placing a classified key tape on a machine, makes the entire system classified.
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.
Many companies and countries claim the invention of the
cipher machine (mixer). Although the
Philips Ecolex was definitely not the
first machine in this class, its 'inventor' was payed for his patents
for many years. STK (now: Thales)
claims that it was a Norwegian invention,
but their patent of 1952 1 is predated by the
the British 5-UCO
and the British-Canadian Rockex,
all of which were developed during WWII and were
introduced in 1943.
Although all mixers are based on the so-called
an invention of Gilbert Sandford Vernam in 1918, and that Vernam
is also the (co)inventor of the One-Time Pad (OTP),
the first machine that was based on the
used a looped key tape and was therefore not a One-Time Tape machine.
This means that, based on the currently available information,
the Siemens T-43,
the British Rockex
and the 5-UCO
should be recognised as firsts.
Although this patent is frequenty mentioned in literature, for example
in , we have not been able to find it. If anyone has access to this
patent, please contact us.
- Norwegian National Security Authority (NSM), Årsmelding 2008
NSM Annual Report 2008 (Norwegian).
Noen kryptosuksesser. p. 15.
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© Crypto Museum. Created: Tuesday 04 August 2009. Last changed: Monday, 20 January 2020 - 20:53 CET.