One-time tape cipher machine
- not in collection
5-UCO was an electronic One-Time Tape
(OTT) cipher machine,
developed in the UK
during WWII and used for Top Secret Ultra
messages during the war. After the war, the machine was used by the
British Commonwealth 1 , the US and
The machine was suitable for fully synchronous secure
telegraphic data over the HF radio bands.
5-UCO is also known as
5-UCO was an extremely large machine that consisted of a 6-foot tall 19"
rack in the which the various electric, electronic and mechanical parts
were mounted, plus an external teleprinter.
5-UCO is the abbreviation of 5-Unit COntrolled,
indicating that it was intended for 5-bit teletype circuits. It provided
full Traffic-Flow Security (TFS) and could be operated over (commercial)
land lines as well as over noisy HF radio links.
The machine was driven by a central 160V DC motor that was mounted at
At the center are two punched-paper-tape readers: one for the transmision
key tape and one for the reception key tape.
An additional DC motor was mounted to the rear
of the RX tape reader. It was used for the RX data synchronisation system.
The image above shows a complete BID/30/1 setup. At the left
is the 19" rack that contains the 5-UCO device.
It is connected to the teleprinter
that is placed on the table at the right.
This image was supplied by GCHQ and is reproduced here by kind
permission of Director GCHQ
5-UCO was developed during WWII by Colonel G. ff Bellairs,
Dr. G. Timms and Mr. D.C. Harwood, around 1943 .
During the war it was used for distributing
to commanders in the field without the risk of any messages
being decrypted by the enemy. After the war it was kept in use
for several years,
also by US Intelligence Agencies, because of its synchronisation
capabilities, its Traffic-Flow Security (TFS)
and its ability to be used over tandem HF radio links.
Nevertheless, the system was only used for traffic of the highest
level of secrecy, mainly because of its operational cost. In 1960 it
was estimated that just the key tape supply cost GBP 5000 per year
for a single 5-UCO machine .
In the mid-1960s, the 5-UCO (BID/30) was replaced by the NSA-developed
TSEC/KW-26 stream cipher
(codenamed Romulus and Orion).
In this context, the expression 'British Commonwealth' is used to
identify the following countries of the Commonwealth of Nations:
Great Britain, Canada, Australia and New Zealand.
BID means British Inter Departmental. Systems with a BID designator are
generally used by more than one single governmental agency or department.
The image below is probably the only one of its kind that has survived .
It shows a complete BID/30/1 setup at the left, connected to an external
Creed 7 teleprinter on the table at the right.
The BID/30 unit (5-UCO) consists of
the 6-feet tall 19" rack on the left, divided into 9 sections.
The machine is mechanically driven by a 160V DC motor (6A) that has two
additional windings: one known as the advance winding and the other
one known as the retard winding.
To the right of the motor are two Creed 6S tape readers and some
mechanical gears. The central motor drives the TX key-tape reader
and, through a differential gear, the RX key-tape reader.
Mounted just above the Power Supply Unit (8A and 9A) is an alarm bell (7A)
that went off if one of the sensing pins in the TX tape reader got stuck .
Operators then had to repair the unit before continuing. This operation
was checked daily by means of a test tape that simulated the problem.
On the control panel, between the 4 large meters, are 4 indicator lamps:
2 red ones and 2 green ones. Two of these lamps flashed when the receiver's
advance/retard motor was activated, whilst the other two stayed on to show
whether the last correction was advance or retard.
Under normal conditions the lamps alternated randomly, but if this pattern
changed, it provided an indication to the operator that the machine at
the other end was running too slow or too fast .
In the years following WWII, the alliance of West-European states,
known as the Western Union (WU), used a number of British and American
cipher machines for secure communication beteen the member states.
This situation continued after the WU was dissolved into the
North Atlantic Treaty Organisaton (NATO)
in 1949. Typex machines
were used on the links between NATO headquarters and the UK,
whilst CCM machines were
used for communication between the other member states and NATO.
However, these machines were not approved for TOP SECRET traffic.
For traffic at the highest level of classification, NATO used
the American SIGTOT
and the British 5-UCO, but both were continuously
in short supply .
Furthermore, the 5-UCO was considered too large to
be of practical use in the field, not to mention the high price of
US$ 12,000 .
This situation changed when in 1953 first the
was developed and later the Dutch Ecolex.
Both machines were approved
for COSMIC and NATO messages of all classifications in 1954
and would soon replace the 5-UCO and SIGTOT machines.
The price of these machines was also a bit lower:
US$ 3000 - US$ 6000 for the Ecolex
and just US$ 1200 for the ETCRRM
Like most other cipher machines in the
mixer class, 5-UCO uses
the so-called Vernam Cipher,
a method of 'mixing' each character
from the plaintext tape with one character from the random-key tape
using a binary bitwise XOR operation (a.k.a. modulo-2). At the receiving end,
the same random character from an identical key tape was mixed with
the encrypted character, revealing the original plaintext character.
If the characters on the key tape are truely random, and only two
identical key tapes exist (which are used only once and are destroyed
immediately after use), this code can not be broken.
It is the only method for keeping a message secret indefinitely.
➤ More about the Vernam Cipher
In order to keep the machine 'in sync' with its counterpart at the other end,
a crystal-operated timebase is used. The signal from the crystal oscillator
at the top (1A) is fed to a divider circuit (2A) and then to a phase
comparator (3A) where it is synchronised with the signal from the sprocket
wheel of the TX tape transport system. The output of the phase comparator is
used to drive either the advance or retard windings of the
central motor, as illustrated in this drawing:
An additional small reversible DC motor was mounted to the rear of the
rightmost tape reader. Through the differential gear, this motor automatically
kept the data stream synchronised with the distant station. A T-bar,
mounted in between the two tape readers, also allowed manual advance/retard
correction. This was needed during the startup of a session or after the
signal was lost due to bad HF conditions. Once it was manually synchronised,
the timebase took over.
In transmission mode (TX), the signal from the main teleprinter hall
(or an individual teletype unit) arrives at the 5-UCO where it activates
the start/stop clutch and engages the TX key-tape reader. The 5-bit teletype
signal is then 'mixed' with the 5-bit data from the tape reader (XOR) and
converted to a serialised data stream that is subsequently transmitted,
as shown above.
When receiving (RX), the process is reversed. The incoming signal
is used to synchronise the timebase, which in turn controls the
speed of the RX key-tape reader. The received serial data stream is
then converted to 5-bit code and mixed with the 5-bit data from the
tape reader (XOR), after which the resulting plaintext is sent to the
connected teleprinter, as shown above.
In order to maintain synchronisation, the tapes were running constantly,
even if no data was actually entered from, say, the external teletype.
In such cases, the all-spaces character was constantly encrypted
and sent. As the key tape lasted approx. 3 hours, it was important to
use it as efficiently as possible. For this reason, messages were generally
not entered directly on the teletype, but on a punched paper
tape which was prepared in advance of the actual transmission.
5AControls and readouts
6AMain motor and two Creed 6S punched paper tape readers
8AFuses and power distribution
9A0/80/160V DC Power Supply Unit (PSU)
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© Crypto Museum. Created: Friday 13 March 2015. Last changed: Wednesday, 21 April 2021 - 21:49 CET.