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OTT UK Rockex →
Online one-time tape cipher machine
- not in collection
5-UCO, also known as HST and as BID/30, 1
was an electronic One-Time Tape
(OTT) cipher machine,
or mixer, 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 2 , the US and NATO.
The machine was suitable for fully synchronous secure
teleprinter traffic over HF radio bands.
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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 Code, 3
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
the bottom. 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.
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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 [1].
This image was provided by GCHQ and is reproduced here with kind
permission from Director GCHQ.
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5-UCO was developed during WWII, around 1943, by Colonel G. ff Bellairs,
Dr. G. Timms and Mr. D.C. Harwood [4].
During the war it was used for distributing
Ultra Intelligence
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 TFS function,
and its ability to be used in tandem over narrowband HF radio links.
Despite its capabilities, the system was only used for traffic of the highest
level of secrecy, mainly because of its high operational cost.
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In 1955, the price for a single 5-UCO unit was USD 12,000,
whilst the machine was permanently in short supply [7].
In 1960 it was estimated that the key tape supply cost GBP 5000 per year
for a single 5-UCO machine [2].
In the mid-1960s, the 5-UCO (BID/30) was therefore gruadually
replaced by the NSA-developed
TSEC/KW-26 stream cipher
(codenamed Romulus and Orion).
The 5-UCO was so secret, that it was initially thought that all machines
had been destroyed when they were decommissioned in the second half of
the 1960s. However, in 2018 a nearly complete UCO-5 (BID/30/1) was
discovered in the vaults of GCHQ. The machine was on public display
the following year, at the temporary exhibition
Top Secret: From Ciphers to Cyber Security
at the Science Museum in London (UK).
The photographs below were made there in November 2019.
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BID means British Inter Departmental. Systems with a BID designator are
generally used by more than one governmental agency or department.
➤ More
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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.
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According to some sources, 5-UCO is the abbreviation of
5-Unit Controlled. The accounts differ. Note that
different users/services used different names for the device.
➤ More
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Unless otherwise specified, the images above are Copyright Crypto Museum (2019)
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The image below is probably the only one of its kind that has survived [1].
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,
numbered from top to bottom 1A to 9A (printed on the right vertical
bar of the rack). Note the presence of a morse key
on unit 6A. It was used for clear communication with the other end.
The rear of the rack (only partly visible here) also hold a number of
sections, numbered 1B to 7B.
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.
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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 [5].
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 (5A), between the large meters, are 4 indicator lamps:
2 red and 2 green ones. Two 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.
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When the pattern changed however, it was an indication to the operator
that the machine at the other end was running too slow or too fast, in
which case the operator had to use the T-bar to make corrections [5].
The image above shows a close-up of unit 6A, which holds the motor
(left) and the two Creed tape readers. The differential gear with the
T-bar for manual advance/retard correction is visible in between the
tape readers. To its right is a regular morse key,
which is used for clear communication with the other end.
Move the mouse over the image to highlight it.
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At the centre of the machine, just under the control panel,
are two Creed 6S6 tape readers
that accept standard 5-hole paper tape.
The leftmost reader is for the plaintext tape, whilst the rightmost
one must be loaded with the keytape.
Both tape readers are engraved with the text '7 UNIT'.
This refers to the serial data word format, which is 7 bits wide
(1 start bit, 5 data bits and 1 stop bit).
Note that is different from regular tape readers which
support 7.5-bit data words (7½ UNIT) with
1 start bit, 5 data bits, 1½ stop bit.
➤ Creed 6S6 tape reader documentation
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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 [6].
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 [7].
This situation changed when in 1953 first the
Norwegian ETCRRM
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$ 3,000 for the Ecolex II
and just US$ 1,200 for the ETCRRM
[7].
➤ More about NATO
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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 XOR operation (modulo-2 addition). 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 one-time tape cipher machines
➤ More about the Vernam Cipher
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In order to keep the machine 'in sync' with the station at the other end,
a crystal-controlled 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 keytape 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.
In receiving mode (RX), the process is reversed. The incoming signal
is used to synchronise the timebase, which in turn controls the
speed of the RX keytape 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), which is then serialised again and delivered to
the connected teleprinter, as shown above.
To maintain synchronisation, the tapes were running constantly,
even if no data was actually sent. This is known as Traffic Flow Security
(TFS).
In such cases, the all-spaces character was constantly encrypted
and sent. As the key tape lasted ~ 3 hours, it was important to
use it as efficiently as possible. For this reason, messages were generally
not entered directly on the teleprinter, but on a punched paper
tape which was prepared in advance of the actual transmission.
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My recollections of BID/30
8 April 2015 [5]
The equipment was rack mounted and about 6 ft high.
At the centre, there was a sealed gearbox driven by a 160V/DC motor.
This had 2 extra windings. External to the gearbox and rear mounted were
3 distributors, a toothed wheel and a small DC motor.
Front mounted were two Creed 6S6 auto heads, and a manual advance/retard.
A crystal oscillator and divider circuit fed a phase comparator;
the other input was from the toothed wheel pick-off coil.
The output was fed to either the advance or retard winding and so locked
the motor speed and transmit random key tape to the crystal reference.
Through a differential gear the motor also drove the receive random key
tape auto head. This was kept in sync with the distant terminal, automatically
by a small reversible motor, or during set up/lost sync by the manual
advance/retard handle.
Asynchronous 7 1/2-unit baudot from teleprinter or auto head in traffic
hall was fed via a clutch to the start/stop distributor.
This routed the 5 code elements to 5 storage capacitors.
Each bit was added modulo 2 to the key bit.
The synchronous transmit distributor re-assembled this as a serial
7-unit cipher output. In the absence of traffic the "all space" character
was enciphered.
Incoming receive cypher reversed this process and fed plain text to local
or distant teleprinter or printing reperforator. An "all space trap" sent
steady mark output. If the incoming element changeovers were out of step
with the key tape element changeovers then the auto advance/retard motor
would correct this error. Red and green lights showed advance or retard
in progress (flashing when motor operated) and (steady light),
the last operation.
During start up, a half turn on the manual advance/retard changed the key
one character. If the equipment lost sync after an outage, then the handle
was pulled out, rotated slowly to find the "in sync", indicated by
plain text or mark on the local teleprinter.
It was then dropped into the nearest element detent.
Key tapes lasted about 3 hours and were destroyed after use.
We learnt to remove them from the single sided spool and tear the tape
in one operation. New boys usually failed their first attempts and ended
up unwinding them into the waste sack!
The start/stop distributor, DS1, was quite noisy and when the traffic
hall did its finals at 23:59, the clatter from a row of BIDs would awake
any technician grabbing 40 winks.
Alarm circuits monitored for stuck peckers on the transmit auto head.
These were checked daily with a test tape. Faults were usually polarized
relays needing adjustment and dirty distributors or worn brushes.
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Looking for a machine called HST
22 June 2014 [8]
I was trained as an Electronic Technician (Cipher Mechanic) in 1953 and
worked on two machines that I have never subsequently seen or heard mentioned.
One was called ROCKEX and was a free-standing unit utilising a bank of thyratron
electronic valves and the other was a large fixed electronic unit utilising
one-time perforated tape for encryption on a high-speed telegraphic link
to war office command HQs around the world. It was called HST.
The HST machine transmitted encrypted messages in real time between
continents, and used a one-time perforated tape that had to be synchronised
with the machine at the other end.
It was a vertical stacked machine, about 5 ft high,
with modular racks of pentodes etc., and a type of handle on the front that
enabled the operator to sync the tape (on the front of the machine) manually
with the receiving unit. We were told that there were only ever 2 random tapes
manufactured: one held in London and the other
mailed out to the particular foreign station.
I used this machine at GHQ Nairobi Cipher Office from 1954-1956 to transmit
to London, Melbourne, Singapore, Aden, etc., and handled topsec traffic
re: Archbischop Makarios detention and also the Suez fiasco, etc.
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We received the above story by e-mail from Anthony (Tony) Bellchambers
on 22 June 2014. Although we were able to help him with information about
Rockex, we had no idea about the other machine, which he called HST.
After reading his description again, we figured that he might be talking
about the 5-UCO of which we had just received a photograph from GCHQ.
After sending him the photograph — this page did not yet exist — he confirmed
that it was indeed the one we was talking about: BID/30.
In March 2015, after this page had come online, he replied:
Yes, that is indeed the machine. Very many thanks! It certainly rolled back
the years to the hours I spent trying to find which valve in the frequency
divider was faulty!
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Looking for a machine called 'racks'
5 January 2017 [9]
When I was in the Royal Navy there was a system being used in 1962 called,
I think, "racks". Basically there were stocks of reels which consisted of
5 unit tape and I, at the sending station, would take a reel from stock,
put it on the rack and put the start of the tape (blue line I think)
on an auto head (probably a Creed).
The receiving station would have the same tape and he would put it on the
same position as me, and then I would send the word mark and a letter in
morse code, then a long dah (morse code) and we would both start the tape
together.
Its a long time ago and I hope I have got it right,
but my question is "do you have any information about it?
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After replying that he might be looking for the 5-UCO (BID/30),
we received the following response:
It is indeed the 5-UCO (BID 30} machine. I have looked at Richard P.'s
web page [3] and noticed the BID 30 Start sequence, which is what I was after
in terms of using the morse key.
However, I am a bit unsure of the QRV part shown in 2, 3, 4 and 5 on both
Station A and Station B, as I thought I used to send Mark in
morse (-- .- .-. -.-) which, if you run that together, would be QRK.
Otherwise everything else is correct.
The Royal Navy Base I was at when using the BID/30 in 1962 was
HMS Juffair in Bahrain.
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Device One-Time Tape cipher machine (mixer) Purpose Secure military and diplomatic communications Model HST, 5-UCO, 7-UCO Designator BID/30, BID/30/1 Developers Col. G. ff Bellairs, Dr. G. Timms, Mr D.C. Harwood Manufacturer ? Year 1943 ~ Contry UK Users UK, USA, Canada, Australia, New Zealand Successor KW-26 TFS yes Alphabet ITA-2 Tempest Insecure Dimensions (HWD) 1830 × 485 × 485 mm Weight 205 kg Quantity ? Price USD 12,000 [7] Cost GBP 5,000/y
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BID/30 1943 Initial version BID/30/1 1953 Improved span class
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BID/30 (HST) is known by the following names:
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HST High-Speed Telegraphy 5-UCO 5 Unit Code 7-UCO 7 Unit Code BID/30 British Interdepartmental 30 BID/30/1 Same, but Revision 1
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It is likely that the official name of the machine is HST, which was
the name it was known by in the user community.
The name 5-UCO, also written as 5 UCO or 5UCO, refers to the
number of encrypted bits (5 Unit Code), and was probably used by
technicians [5][8].
Technicians also used the name 7-UCO, since that is the
format of a serial data word 1
(1 start bit, 5 data bits and 1 stop bit).
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Actually the word length of a teleprinter signal is 7½ bits, but the
HST converts it from 7½ to 7 bits before encryption, and then back
to 7½ bits again before transmission.
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1A Crystal oscillator 2A Clock divider chain 3A Phase comparator 4A Relay section 5A Controls and readouts 6A Main motor and tape readers 7A Alarm bell 8A Fuses and power distribution 9A 0/80/160V DC PSU
1B ? 2B ? 3B ? 4B ? 5B Commutators & DC motor 6B ? 7B ?
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- GCHQ, Photograph of complete BID/30/1 (5-UCO) machine
Photographs kindly supplied by GCHQ and
reproduced here with kind permission from Director GCHQ.
3 December 2012. Crown Copyright.
- Jerry Proc, BID 30/5-UCO (5-Unit Controlled)
Accessed March 2015.
- Richard P. (anonymous contributor), BID 30, (5UCO) Mechanical Detail
Jerry Proc's crypto pages (website) [2]. Retrieved March 2015.
- The National Archives,
Awards to civilian personnel in respect of cypher machine development
AVIA 65-977. Original reference: JY/808/01. 1955-1960.
- Richard P. (anonymous), My recollections of BID/30 (HST, 5-UCO)
Personal correspondence, 8 April 2015.
- NATO/ACCA, On-Line Cipher Equipments
SGM-279-54. 29 March 1954. NATO SECRET.
Declassified by NATO in 2006 (IMSM-0001-2006).
- NATO, Automatic Crypto-Equipment Requirements for the Allied Command Atlantic
SGM-560-55. 15 August 1955. NATO SECRET.
Declassified by NATO on 24 November 1999 (IMSM-0431-99).
- Anthony (Tony) Bellchambers, Looking for a machine called HST
Personal correspondence, 22 June 2014.
- John Youde (G0GUF), RNARS 32271, Looking for a large machine called 'racks'
Personal corresondence, 2-5 January 2017.
- Anonymous, Recollections of the HST
Personal correspondence, 22 January 2022 - 21 February 2022.
- NSA Catalog of US and UK cipher equipment - 1953
Cifax, Offline Literal Crypto Systems, Teletype Online Crypto Systems and Ciphony.
LCS(53)/N/R. NSA, 1953.
p. 59 (PDF p. 64)
Friedman Archive. Declassified by NSA 2014-06-11 (E.O. 13526).
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© Crypto Museum. Created: Friday 13 March 2015. Last changed: Thursday, 20 August 2026 - 21:37 CET.
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![Image provided by GCHQ and reproduced here with kind permission from Director GCHQ [1]. Crown Copyright.](img/5uco_full.jpg)
![Image provided by GCHQ and reproduced here with kind permission from Director GCHQ [1]. Crown Copyright.](img/5uco_thumb.jpg "image # 5uco_full.jpg")
![Simplified mechanical operation. Based on [3].](svg/5uco_mech.svg)
![Simplified TX flow. Based on [3].](svg/5uco_tx.svg)
![Simplified RX flow. Based on [3].](svg/5uco_rx.svg)
