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Portable telephone encryptor - this page is a stub

BRAHMS was the codename of a highly secret portable speech encryption device, developed in 1980 at the Government Communications Headquarters (GCHQ) 1 in Cheltenham (UK). Carried inside an unobtrusive black briefcase, the device was used for secure communications between high ranking government officials, inluding (then) British Prime Minister Margret Thatcher at the time of the Falklands War [1]. The system was approved for high-grade traffic (TOP SECRET).

The image on the right shows the device as it was on public display as part of the exhibition Top Secret at the Science Museum in London UK between July 2019 and March 2020. It is housed in a 'Custom' briefcase and consists of a simplex voice encryptor, a paper tape reader (for loading the daily key) and a handset with an integrated push-to-talk (PTT) 2 button at the rear.

The device has two cables: one for connection to the mains, and one for connection to a regular (analogue) telephone line, or PSTN (POTS). It was connected in parallel to an existing telephone.
BRAHMS telephone encryptor as shown in the exhibition 'Top Secret' at the Science Museum in London in 2019. Image copyright Crypto Museum [1]

This was necessary because the device has no controls for dialling a number. Before placing a call, the cryptographic key had to be loaded by means of a punched paper tape that was fed into the tape reader at the right. Once this was done, the call was initiated 'en clair' with the existing (insecure) telephone set. As soon as the call was established, it was taken over by BRAHMS.

BRAHMS converts speech to a digital signal using a vocoder 3 and then sends it over the analogue thephone line by means of audible tones 4 at a speed of 2400 baud (bits per second), which was the maximum speed that could be obtained anywhere in the country at the time. Only one party could speak whilst the other one had to listen. For this, the speaker had to press the PTT button on the handset. 2 According to the developer, 1 speech quality was rather poor at 2400 baud, in particular with female speakers due to the higher pitch of the voice [2]. Nevertheless, it was used heavily in 1982 by (then) Prime Minister Margret Thatcher at the time of the Falklands War [3].

It is currently unknown which encryption algorithm was used, but given the fact that the key had to be loaded by means of an 8-level puched paper tape, it is possible – if not likely – that it was the secret SAVILLE algorithm. SAVILLE had been developed at GCHQ in the late 1960s, together with the US National Security Agency (NSA), and was also used in early voice encryption systems like STU-I, STU-2, KY-57 and Spendex 40. It eventually became the defacto standard for NATO.

  1. According to the BBC, BRAHMS was developed by a GCHQ technician named 'Mike' [2].
  2. In the Operating Instuctions described as PRESSEL [5].
  3. Although it hasn't been confirmed yet, it is likely that Linear Predictive Coding (LPC) was used for this.
  4. Also known as Audio Frequency Shift Keying (AFSK).

BRAHMS on public display at the Science Museum in London in 2019. Photograph by Paul Reuvers (Crypto Museum) [1]
Photograph of BRAHMS published by GCHQ on 8 June 2022 on Twitter [4]
Abridged Operating Instructions (low resolution) [5]
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BRAHMS on public display at the Science Museum in London in 2019. Photograph by Paul Reuvers (Crypto Museum) [1]
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Photograph of BRAHMS published by GCHQ on 8 June 2022 on Twitter [4]
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Abridged Operating Instructions (low resolution) [5]

The diagram below shows how BRAHMS was connected to the subscriber line – in parallel to the existing (insecure) telephone set with dial – and explains roughly how the device works, based on the limited information that is currently available in the public domain. At the right is the hand­set, which consists of a microphone, a speaker and a PTT button. The microphone and speaker are connected to a CODER and DECODER respectively — together known as CODEC or VOCODER.

The CODEC converts speech to digital data and vice versa. Although it hasn't been confirmed yet, it is likely that the CODEC was an LPC-10 Vocoder, as that was the state of technology that was also used on contemporary devices from sister organisations, such as the American STU-I and STU-II, and the Dutch Spendex 40. At the time, it was probably the smallest device in its class.

When the user presses the PTT, the analogue signal from the microphone is digitised (coder) and passed to the crypto unit, where it is encrypted under control of the externally loaded key. From there, it is passed to the MODEM, which converts the digital data stream into audible tones, that are then sent over the (analogue) telephone line using Audio Frequency Shift Keying (AFSK). 1

When the user releases the PTT, the audible tones from the telephone line are converted by the MODEM into a digital data stream that is subsequently decrypted by the crypto unit and passed to the Decoder. The Decoder converts the decrypted data stream into intelligible speech – using a complex scheme of mathematical calculations – which is passed to the handset's loudspeaker.

Due to the nature of the LPC-10 encoding and the low baudrate, the system cannot be used for authentication. In other words, it is impossible to recognise the voice of the person at the other end of the line. This is one of the reasons why a call always had to be initiated in the clear.

  1. It is also possible that Phase-Shift Keying (PSK) was used for this.

Similar contemporary equipment
Secure Telehone Unit STU-I (KY-70)
Secure Telehone Unit (ITT, Northern Telecom)
Philips Spendex-40 secure telephone for voice, fax and computer
 More about the SAVILLE encryption algorithm

Churchill's scrambler phone
During World War II (WWII), (then) Prime Minister Winston Churchill used a similar system known as the Scrambler Phone (later: Secraphone), but that system was far less secure than BRAHMS.

Secraphone could easily be broken and was only intended as protection against an unintentional eavesdropper, such as the exchange operator or an engineer working on the telephone lines.

 More information

Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal

The operating principle of the BRAHMS is very similar (but not identical) to a wartime machine known as SIGSALY, that was developed in 1941 at Bell Labs and built by Western Electric.

SIGSALY was suitable for realtime full-duplex voice conversations, but was so big that the British unit had to be installed in the basement of department store Selfridges in London. The image on the right shows one half of the system. The other half is behind the camera.

 More information

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  1. Science Museum, exhibition Top Secret, July 2019 - March 2020
    London, 29 November 2019.

  2. Gordon Corera, How Margret Thatcher's secret Brahms phone was invented
    BBC News, Security correspondent, 8 June 2022.

  3. Wikipedia, Falklands War
    Visited 9 June 2022.

  4. GCHQ, Image of a BRAHMS telephone encryptor
    Twitter, 8 June 2022.

  5. GCHQ, Abridged Operating Instructions
    Retrieved from lo-res image [4].
Further information
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Crypto Museum. Created: Thursday 09 June 2022. Last changed: Tuesday, 14 June 2022 - 05:54 CET.
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