One-Time Tape machines
Mixers are a class of cipher machines that are based on the
Plain text is mixed with a random key stream,
hence the name mixer.
If the mixer machine is used as intended, the key tape contains
evenly-spread truely random characters and is therefore unbreakable.
Such a key tape is commonly referred to as a One-Time Pad (OTP)
or a One-Time Tape (OTT).
Mixer machines featured on this website:
Most of these machines use data from a teletype unit or from a paper-tape
reader as input. Such data is generally stored in 5-bit digital format
(e.g. in Baudot code), but other data formats are also possible.
Plain text is either entered directly on the keyboard of a teletype machine
(online), or is stored on a punched paper-tape first and sent later (offline).
The above illustration explains how the mixer works.
Eacht letter from the Plain-text is added to a letter from
a key tape, using an exclusive-OR (XOR) operation
(sometimes called 'module-2 addition).
The advantage of this operation is that it is reversable:
adding the key stream to the cipher text, reveals the plain text again.
For a detailed description of this principle,
read our page about the Vernam Cipher.
It is often thought that, like most other cipher machines, the mixers
were classified. 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 in 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.
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 was 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
and the British Rockex
should be recognised as firsts.
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