BID 30/5-UCO (5-Unit Controlled)

The BID 30/5-UCO was a one time tape (OTT) Vernam cipher encryption system developed by the U.K. during World War II for use on teletype circuits. It was also used by U.S. intelligence agencies after World War II. The British enjoyed great success with this system because, being fully synchronous, it could be electrically regenerated on tandem high frequency (HF) radio links (i.e. one link connected to the next). It also provided traffic-flow security (TFS) and operated directly with commercial circuits. Another inherent capability of the 5-UCO was that the operator at the receive end could maintain crypto-synchronization if the path delay suddenly changed by "walking up and down" the key tape (one character at a time or one bit at a time). This procedure avoided the operationally cumbersome task of a restart.

Like predecessor U.S. OTT equipment (SIGTOT), it used "mountains" of key tape to operate on a 24/7 basis. The sheer magnitude of generating, certifying, and destroying key tape and the cost of distribution and accountability limited 5-UCO use to the most sensitive traffic. The Army Security Agency sought to develop a replacement, an effort later taken over by the newly-formed National Security Agency and resulting in the fielding of the KW-26 ROMULUS system.

In 1960, supplying the key tape for a single 5-UCO in continuous use cost £5000 per year (AVIA 65/977).

Colonel G. ff Bellairs was awarded sums totalling £2250 for the development of 5-UCO, £750 in 1943, and £1,500 in 1960. Dr G. Timms and Mr D. C. Harwood were also awarded £1,000 for work on random tape for 5-UCO (AVIA 65/977).

William R Hadley , who worked on the 5UCO, offers his solution to one of the machine's minor deficiencies. "Those machines were made by the British and used pot metal for the clamps holding the wiper brushes on the distributor.  If you tightened the screws too much ,the threads in the clamp would  strip thus resulting in a clamp replacement.  If left too loose, it would cause the distributor to get out of alignment (and sync) with other distributors in other machines.  I cheated and had a bunch of clamps made out of steel which allowed me to really tighten the clamps and rarely had a machine go down for repair while at stationed in Okinawa".

5uco_as.jpg 5UCO/BID30-1 setup. Click on image to enlarge. (Copyright HM Government and released courtesy of the Director of GCHQ)

The equipment was rack mounted and about 6 ft high. At the centre , there was a sealed gearbox driven by a 160 VDC 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 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 2359 Hrs the clatter from a row of BID’s 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.

Richard P.

BID 30 Rack.pdf
BID30 Tx Distributor.pdf 
BID 30 Mechanical.pdf
All drawings by Richard P. Drawings are not to scale.

BID30 start sequence. (Provided by Richard P.)

The 5-UCO in colour  According to GCHQ, 5UCO was so secretive that all examples are believed to have been destroyed. 5-UCO was developed to handle the most secretive messages during the Second World War which included sending Bletchley Park’s decrypted Enigma messages  (ULTRA) to the British military commanders in the field. It was still in use in Bahrain at HMS Jufair in 1965. (Photo via E & T web page) 
5UCO//BID 30 closeup of distributor assembly. 
A 5-UCO was seen as late as 1970 in the MOD main Building but it cant be confirmed if it was still in use.  Perhaps it was there for occasional contact with some unit that did not have BID 610 or KW-26 ?? If used properly, 5-UCO was an unbreakable cipher.

At HMS Jufair it is reported that there were four 5-UCO machines running 24 hours a day. These would be in contact with UK  (RAF Stanbridge), Cyprus (12SU Episkopi),  RN Mauritius, RAF Gan (Maldive Island staging post). Another was used for the occasional conference call. Collecting large quantities of tapes from RAF Muharraq was not a duty to look forward to !

Credits and References:

1) Arnold G. Reinhold
3) William R Hadley <wmhadley(at)>
4) Richard P.
5) John Alexander <jalexuk(at)>
6  5UCO colour photo  E & T web page  (unreachabble as of Jan 2023)

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Jul 15/19