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Rockex   BID/08/05 - BID/08/08
One-time Tape cipher machine - wanted item

Rockex was a valve-based online/offline one-time tape (OTT) cipher machine, developed in the USA from 1940 onwards by Canadian communications expert Benjamin deForest Bayly, at the request of British Intelligence. Rockex entered service in 1943 and was built from 1944 onwards by HMGCC at Hanslope Park (UK). Later versions of the machine were also known as BID/08/05, BID/08/06, BID/08/07 and BID/08/08 1 . The last machines were decommissioned in 1983 [1].

The design of Rockex was based on the earlier Telekrypton, a machine built in the US by the Western Union Telegraph Company as a proof of concept of the so-called Vernam Cipher, but which had not become a commercial success, because it was too large and had security issues.

Bayly turned Telekrypton into a One-Time Tape (OTT) cipher machine, which was then used by the British Security Coordination (BSC) on a link between Washington and New York [1]. He also modified it, so that only the 26 letters of the Latin alphabet would appear in the ciphertext.

Rockex first entered service in 1943 when it was used to pass messages between Camp-X in Whitby 2 (Ontario, Canada) and Great Britain. In 1944, production was moved to HMGCC at Hanslope Park (UK). In total only 12 Rockex I machines were built before it was succeeded by Rockex II. After the war, the machine was modified several times because of TEMPEST issues.

Rockex was used by the UK and Canada for TOP SECRET traffic. From July 1955 onwards, it was also approved for passing NATO messages of all classifications including COSMIC. Nevertheless the machine was not used by other NATO countries, due to lack of sufficient machines [5].

The later Rockex Mk. III, IV and V versions were built at Borehamwood (UK) where also the secret keystream tapes were manufactured. In 1962, Rockex was succeeded by the compatible Noreen (BID/590) cipher machine, but remained in service for military and diplomatic traffic alongside Noreen in the UK and Canada until 1983. The machine was also used in the Commonwealth countries Australia and New Zealand, but only for secure diplomatic traffic with the UK [6].

  1. The lower BID/08 designators (e.g. BID/08/02) were used for Typex.
  2. During WWII, Camp X was the unofficial name of a paramilitary, clandestine and commando training installation of the BSC, located in Ontario (Canada) between Whitby and Oshawa. It is known today as Intrepid Park.
  3. The image above shows a Rockex cipher machine with a compatible Noreen BID/590/2 on top. The photograph was taken in September 2013 in David White's Diplomatic Wireless Service exhibition in Hut 1 at Bletchley Park, on its final opening day.

Rockex was a fairly large and heavy system mounted in a metal frame consisting of four bended legs. At the top is the so-called 700 Unit which contains all mechanical parts and the relays. At the centre, just below the wooden 'table top' is the 804 Unit (sometimes called the 800 Unit) which contains the electronic valve-based circuits. At the bottom is the Keyer (no designator).

Two paper tape readers are mounted in front of the 700 Unit at the top. One is a 5-level tape reader, which is used to read the plaintext (when used offline). The other one is a 6-level reader which is used to read the keystream tape. The signals of both readers are 'mixed' in the 804 unit by means of modulo-2 addition (XOR). They keystream tape is fed to the 6-level tape reader from a circular paper tape holder that is placed horizontally on the table top at the right.

Rockex overview and controls. Image based on a Public Domain photograph by Crypto Matt (via Wikipedia) [2].

Inside the 700 Unit is an electromotor which drives the main shaft and eight GPO type 3000 telephone relays. The unit is normally powered directly from the 220V/50Hz mains, but it was also possible to power it from the 110V/60Hz mains. In such cases a step-up transformer was mounted at the bottom, aside the Keyer, and some mechanical gears were swapped to cope with the different mains frequency (50/60Hz). In most situations Rockex was used in offline mode.

  • Rockex Mk I
    First version, developed in the US and (partly) built in the UK. Only 12 units made.

  • Rockex MK II - BID/08/05
    Introduced late 1944 and built at Hanslope Park (UK).

  • Rockex Mk III - BID/08/06
    Post-war variant which suffered from TEMPEST issues.

  • Rockex Mk IV - BID/08/07
    First TEMPEST complient version.

  • Rockex Mk V - BID/08/08
The name Rockex
By convention, the 'ex' was added at the end of the name of all British cipher machines at the time. It is believed that the name Rockex was derived from a performance of the Radio City Music Hall Rockets dancers. Another possible explanation is that it was named after the Rockefeller Center in New York where the BSC had its headquarters in room 3603.

In some (historical) publications and on the internet, it is suggested that Rockex and Telekrypton were one and the same machine, but this is not the case. Telekrypton was the predecessor of Rockex and the two machines were not interoperable. However, as Rockex replaced Telekrypton on a number of important wartime communication links, users sometimes kept calling the new machine 'Telekrypton'.

The two tape readers. The 6-level key tape reader is at the left. Photograph by Jerry Proc [1].

Keystream tape
Rockex has two tape readers at the top left. One of these is used for the so-called keystream tape: a punched paper tape that is filled with random characters A-Z. As the international teleprinter alphabet (ITA2) is based on 5-bit data, one would expect a 5-level tape reader in this position, but this is not the case. The keytape reader of the Rockex expects a 7-bit wide tape, with 6-bit data. This means that an uncommon tape width was used for the keystream tape.

Noreen key tape compared to 5-level teleprinter tape

In the image above, the two tape formats are compared. At the right is a common 5-level teleprinter tape (blue). The one on the left is the Rockex/Noreen key tape (red) which has an unusual width and was probably custom made. It is seen from the front/top of the machine. Note that the extra tape channel (6) was added before channel 1 and that a margin was added at the side of channel 5, probably to prevent the tape from being inserted the wrong way around.

The keystream tape contains a random A-Z character data stream in the usual 5-level teleprinter format. This 5-bit information is mixed with the data from the other tape reader. Whenever the 6th hole is present in the key tape, the data is not enciphered but sent directly to the output. This way a space can be inserted in the printed text after each 5th character, while a double space was inserted after each 5th group. The 6th keytape channel was also used to automatically insert the (unencrypted) 5-letter message indicator that marked the beginning of each 49-group section.

The key tapes are compatible with those of the later Noreen cipher machine, and were created by means of a pseudo-random generator that was codenamed DONALD DUCK (probably because it produced gibberish) followed by a so-called paragrapher device. This device punched random data into the paper tape in blocks of 50 groups of 5 letters each. The first 5-letter group was the message indicator which was not encrypted. This was done by punching the 6th hole in the key tape. The indicator group was followed by 49 encoded 5-letter groups. Each group was separated by a single space (again, using the 6th hole) with a double space after each 5th group [1].

Example of a Rockex key tape

The image above shows an example of a keytape as it is guided through the tape reader. This means that channel 5 of the tape is at the bottom (at the edge with the wide margin) and that the extra channel (6) is added before channel 1 and is now at the top. Note that the random data on the key tape consists only of the letters A-Z. This was done to ensure – in combination with the so-called discriminator – that only the letters A-Z would appear in the encrypted output.

Example of a printed cipertext generated by Rockex. Thanks to Richard Girling for providing the ciphertext [7]

The example above shows what the printed output of the Rockex ciphertext may have looked like. The text is formatted into 5-letter groups. Each block of 5 groups is separated by a double space. CR and LF are automatically inserted after each 10th group and a double LF is inserted after each 50-group segment. In the example, the first group of each segment is highlighted to show that this is the message indicator which is incremented in alphabetical order. In reality the indicator was printed in black too. Thanks to Richard Girling for providing the ciphertext [7].

Separate tapes were used for encoding and for decoding. They were idendified by the colour of the tape and by the colour of their spool core. In the UK, the encoding tape was generally green and was wound on a blue marked core. The decoding tape was red and was wound on an orange marked core. In Canada the tapes were coloured yellow and red respectively. The core of the decoding spool was slightly wider than the one that was used for encoding. A sensing switch in the keytape supply mechanism sensed this and informed the machine of which tape was present. The key tapes were manufactured at a secret government facility in Borehamwood (UK) [6].

For proper security it is important that the keystream tape contains a sequence of evenly spread truely random characters. Producing such a random keystream was a major challenge during WWII. In the early days of the Telekrypton cipher machine, such tapes were produced manually.

When the need for keystream tapes increased during the course of war, the manual production was replaced by electromechanical methods. The machine that was used for the production of Rockex key tapes was codenamed DONALD DUCK, possibly because it speaks gibberish [3]. It wasn't before the application of a noise source however, that truely random key streams were produced. In the UK, a noise generator with five flip-flops was developed at GCHQ just after WWII by former GPO-engineer Don Horwood, who had also worked on Colossus at Bletchley Park.

Block diagram
The simplified block diagrams below show how Rockex works. At the left are the two punched paper tape readers. The upper one is a 6-level reader which is used for the keystream tape. The lower one is a normal 5-level reader which is used for the plaintext tape. The signals from both tape readers are combined (added) in a digital valve-based XOR circuit (actually one XOR for each bit). This operation is generally known as the Vernam Cipher and the machines are called 'mixers'.

Principle of the Vernam Cipher

The data from each of the tape readers consists of 5 data-bits, but the keystream tape reader has 6 channels. The extra channel controls the operation of a bypass circuit. Whenever the 6th hole is present, the remaining 5 data bits are sent directly to the output. This was used for inserting spaces automatically between the 5-letter groups, and for non-printable characters (such as LF and CR). It was also used for inserting the Message Indicator at the beginning of each section.

Block diagram of Rockex in enciphering mode

The keytape only contains the (random) letters A-Z, plus formatting data. In order to ensure that the (printed) output also consists of only letters (A-Z) and formatting data (SPACE, LF, CR), a discriminator is added to check the output of the XOR for unwanted characters (the so-called stunt 1 characters). If, during encoding, a stunt character is encountered, the input is halted and the key character is output instead. Next, the key tape is advanced and the next key character is tried, and so on, until the desired output (A-Z) is yielded 2 . This is then sent to the output.

Block diagram of Rockex in decipher mode

At the receiving end, the bypassed character is mixed with the same character from the key tape, which produces a NULL code. As a NULL code has no effect in the 5-bit ITA2 code, this code will be ignored and has no effect on the decoded text. Furthermore, this method has no negative effect on the security of the cipher. Although it causes some overhead, it has the advantage that all characters present in the ciphertext (including numbers and stunt 1 characters) can be sent.

When deciphering as described above, the formatting data from the key tape has to be ignored and any stunt characters on the ciphertext tape will also have to be suppressed. For this reason it is necessary to switch between enciphering (EN) and deciohering (DE). In deciphering mode, the machine works as shown in the block diagram above.

  1. In telegraph terminology, 'stunt characters' is a common expression for the control codes: LF, CR, LTRS, FIGS and SPACE.
  2. In May 2015, the exact operation of the Rockex/Noreen key tape has been the subject of a discussion on the Crypto Collectors News Group (now: ICCH), in which the method described here was coined by Frode Weierud [3]. This has since been confirmed by Crypto Museum on a restored Noreen cipher machine which uses identical key tapes [8], and is corroborated by documents on Rockex that were released by NSA.

The history of Rockex starts around 1940 when, during WWII, the British Security Coordination (BSC) was looking for a way to pass secure messages between the BSC offices in New York and Washington. The BSC was established by British Prime Minister Winston Churchill to protect the British interests in the Americas, investigate enemy activities and mobilise pro-British opinion in pre-war America. By some it is seen as a propaganda instrument to bring America into the war.

Although Great Britain already had good cipher machines at their disposal, such as the Typex, they wanted the BSC traffic to remain secret indefinite. Canadian communications expert Benjamin deForest Bayly was hired to find a solution. He found two Telekrypton machines lying around in the warehouse of the Western Union Telegraph Company in the United States. The Telekrypton was based on the so-called Vernam Cipher: a fast method of mixing a plaintext data stream with a secret key stream, invented by Gilbert Sandford Vernam back in 1918. Western Union had built some machines based on this principle, but they were not very successful.

Telekrypton had two major drawbacks: it was way too large and difficult to maintain, and the keystream tape was looped around after approx. 7 feet. Nevertheless, Bayly bought the two remaining Telekrypton machines and made them suitable for the British cause. He made the machine much smaller and increased the length of the keystream tape to at least the length of the message. Furthermore, he allowed only two identical tapes and instructed them to be both destroyed immediately after use. This way the machine became a real One-Time Pad system.

Telekrypton was first used on the link between the BSC offices in New York and Washington in January 1942, soon followed by links to Ottawa (Canada) and Camp X (Whitby, Ontario, Canada). In the following year, Bayly fixed a number of weaknesses in the Telekrypton design and modified the combining logic so that only the 26 letters of the Latin alphabet were produced. All control codes (space, LF, CR, etc.) were rejected and replaced by the key character. This way it became possible to nicely arrange the output in groups of five letters each, separated by spaces.

The modified machine became known as Rockex and was introduced in 1943. Although the machines were initially built in the US, production was moved to HMGCC at Hanslope Park (UK) in 1944. In total only 12 Rockex I units were ever built before it was succeeded by Rockex Mk. II. An early example of a Rockex II machine was supplied to Bletchley Park towards the end of WWII [1].

After the war, Rockex Mk. II was followed by the Mk. III but all three versions appeared to have security issues (TEMPEST). These were fixed in the later Mk. IV and Mk. V versions. In July 1955, Rockex was even approved for passing NATO COSMIC and NATO SECRET traffic [5]. In 1962, a smaller offline variant called Noreen was introduced, but most Rockex machines remained in service until the late 1960's. The last Rockex machine was decommissioned as late as 1983 [1].

Known locations
  1. The exhibition in Hut 1 was closed down in September 2013. Current whereabouts of the Rockex machine are unknown.

  1. Jerry Proc and contributors, Rockex
    Website. Retrieved January 2015.

  2. Wikipedia, Rockex
    Retrieved January 2015.

  3. Frode Weierud, Discussion on the operation of the XOR circuit
    Crypto Collectors Newsgroup (now: ICCH). May 2015.

  4. Rufus L. Taylor, Disclosure of Details of ROCKEX to the French
    Ref ID: A61029. USCIB: 39.2/10. 20 September 1954. SECRET.
    Declassified by the NSA on 21 April 2014 (EO 13526).

  5. C.H. Sampson, Use of Cryptographic machine ROCKEX for All Classifications of Traffic
    NATO, SGM-488-55. 25 July 1955. SECRET.
    Declassified by NATO on 24 November 1999 (IMSM-431-99).

  6. David White, Personal correspondence
    January 2015.

  7. Richard Girling (G4FCD), Example of a possible Rockex ciphertext output
    Received May 2015.

  8. Crypto Museum, Restoration of a Noreen cipher machine
    Netherlands, June 2015.
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Crypto Museum. Created: Tuesday 13 January 2015. Last changed: Saturday, 30 April 2022 - 08:46 CET.
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