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5-UCO   BID/30
One-time tape cipher machine - not in collection

5-UCO 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 1 , the US and NATO. The machine was suitable for fully synchronous secure telegraphic data over the HF radio bands. 5-UCO is also known as BID/30. 2

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 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.

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 [1].

5-UCO was developed during WWII by Colonel G. ff Bellairs, Dr. G. Timms and Mr. D.C. Harwood, around 1943 [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 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 [2]. In the mid-1960s, the 5-UCO (BID/30) was replaced by the NSA-developed TSEC/KW-26 stream cipher (codenamed Romulus and Orion).

  1. 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.
  2. BID means British Inter Departmental. Systems with a BID designator are generally used by more than one single governmental agency or department. More...

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.

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 [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, 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 [5].

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$ 3000 - US$ 6000 for the Ecolex and just US$ 1200 for the ETCRRM [7].

Vernam Cipher
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

 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:

Simplified mechanical operation. Based on [3].

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.

Simplified TX flow. Based on [3].

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.

Simplified RX flow. Based on [3].

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.

  • 1A
    Crystal oscillator
  • 2A
    Divider chain
  • 3A
    Phase comparator
  • 4A
    Relay section
  • 5A
    Controls and readouts
  • 6A
    Main motor and two Creed 6S punched paper tape readers
  • 7A
    Alarm bell
  • 8A
    Fuses and power distribution
  • 9A
    0/80/160V DC Power Supply Unit (PSU)
  1. GCHQ, Photograph of complete BID/30/1 (5-UCO) machine
    Photographs kindly supplied by GCHQ. 3 December 2012. Crown Copyright.

  2. Jerry Proc, BID 30/5-UCO (5-Unit Controlled)
    Retrieved March 2015.

  3. Richard P. (anonymous contributor), BID 30, (5UCO) Mechanical Detail
    Jerry Proc's crypto pages (website) [2]. Retrieved March 2015.

  4. The National Archives,
    Awards to civilian personnel in respect of cypher machine development
    AVIA 65-977. Original reference: JY/808/01. 1955-1960.

  5. Anonymous contributor Richard P., Personal correspondence
    Received 8 April 2015.

  6. NATO/ACCA, On-Line Cipher Equipments
    SGM-279-54. 29 March 1954. NATO SECRET.
    Declassified by NATO in 2006 (IMSM-0001-2006).

  7. 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).
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Crypto Museum. Created: Friday 13 March 2015. Last changed: Wednesday, 21 April 2021 - 21:49 CET.
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