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KG-13 was a universal digital solid-state (transistorised) full-duplex key generator, developed around 1963 by, or on behalf of, the US National Security Agency (NSA). The device was intended for encryption and decryption of externally generated data, such as digitised voice and facsimile data. The KG-13 key generator was commonly used in combination with the HY-2 vocoder.

The KG-13 is an extremely complex, large and heavy piece of equipment. It measures 89 x 59 x 48 cm, weights more than 110 kg and consists of 3 identical key generators, 2 synchronizers, an alarm assembly and 2 power supply units (PSUs). It is based on the earlier development of the KG-3 (transmitter) and KG-12 (receiver).

The key is entered into the key generators by means of Remmington Rand computer punch cards 1 every 24 hours. These cards are used to set the initial state of the KOKEN shift register stages and the tetrahedral key combining logic.

Despite its high weight, it was a huge improvent over earlier systems, such as the KW-26, not least because of the transistor-based circuits, which were constructed with FLYBALL modules.

The KG-13 provided Traffic Flow Security (TFS), which means that it produced a continuous data stream, even if no message was being sent. The advantage of TFS is that an eavesdropper will not be able to detect the beginning and end of a message, and that the initial key is never repeated. The data that was sent between two messages was fully random and was generated by a noisy diode, similar to the SEND SYNCHRONIZER in the KW-7 (which did not have traffic flow security).

The KG-13 entered service around 1963 and was heavily used during the Vietnam War, often for secure voice transmission in combination with the HY-2 vocoder. The machine was succeeded in the 1970s and 80s by modern smaller encyption devices, such as the KY-57 and later the KY-99. Nevertheless, the last heavy and bulky KG-13 units were taken out of service as late as 1990.

  1. Remington Rand computer punch cards have the same physical size as IBM cards, but have circular punched holes, rather than IBM's rectangular ones.

The diagram below shows and example of how the KG-13 was used with the HY-2 vocoder. At the left is the audio input/output, which could either be a handset or an external telephone or audio line. The HY-2 converts the analogue voice into digital information which is then passed on to the KG-13 for encryption. The encrypted data is then passed to an external 2400 bps modem.

At the receiving end, the data received by the MODEM is first decrypted in the KG-13 and then passed onto the HY-2, which then reconstructs or synthesizes the human speech, based on the data in the 16 vocoder channels. This results in a typical 'Donald Duck' style voice.

Cryptographic algorithm
Although the cryptographic algorithm of the KG-13 has not yet been published, some hints to its operating principles can be found in publicly available and declassified NSA material. It is known, for example, that the crypto logic of the KG-13 is comprised from several KOKEN shift register stages and a tetrahedral key combining logic. The principle of the KOKEN registers is shown here:

A KOKEN register consists of several cascaded D-type flip/flops that are clocked only when the output of the preceding state is high. When the device is first switched on, all registers are reset so that their outputs (Q) are set to zero. Next some of the flip/flops are preset (Q=1), controlled by the key card. The inverted outputs of each stage are then combined in a secret manner into a single output bit, that is also used as the clock signal for the first stage. The key combining logic consists of several logic AND, OR and XOR circuits, and is configured by the current key card.

The KG-13 was one of the first fully solid-state or transistorised devices developed by the NSA in the early 1960s. Like the KW-7, it is entirely constructed from small pre-built logic circuits, known as FLYBALL modules, that were developed especially for the NSA by Sylvania in Florida (US).

Each module had a distinct colour to identify its (logic) function. The image on the right shows some of the circuit boards of the KG-13 (one of which is missing), each of which holds 9 pink-coloured FLYBALL modules with the actual shift registers, or Koken registers. The yellow module contains a single 2N404 emitter follower which is used as a clock driver for the Koken registers.

 More about FLYBALL modules


Between October 1982 and January 1983, whilst working at the US Air Force, electronics engineer James Atkinson discovered a series of serious flaws in the KOKEN stages of the KG-13's internal pseudo random stream generator [4]. Atkinson had been memorizing all current and historical circuit diagrams of the KG-13, the KY-3 and all of their FLYBALL modules, as a mental exercise. When going over the circuit diagram in his mind, he began to doubt its mathematical strength.

More than 20 years after the introduction of the KG-13, he was able to prove that most of the KOKEN stages were mathematically flawed, rendering the system compromised, and possibly leaking highly classified information to the Russians. Although there is no direct proof that the system had meanwhile been broken by the Soviet Union, it has to be assumed that it was.

Embarrased as they were, the NSA fixed it by modifying the KOKEN circuits and issuing a Modification Work Order (MWO) for all operational and spare machines in the field, an operation on which tens of millions of dollars were spent [4].

  1. TSEC/KG-13 Datasheet
    CSP 6620A. Department of the Navy, September 1962. Obtained via [2].
  1. Jerry Proc and contributors, KG-13
    Retrieved May 2016.

  2. Nick England, KG-13 datasheet
    Retrieved May 2016. Reproduced here by kind permission.

  3. Bill Rhoads, Photograph of KG-13 system
    Obtained May 2016 via [1].

  4. Linked-in, James Atkinson
    Retrieved May 2016.
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Crypto Museum. Created: Thursday 12 May 2016. Last changed: Thursday, 26 November 2020 - 17:43 CET.
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