The Red machine: Procedures

• Introduction
• Working
• History
• Genuine messages
• Procedures (This page).
• My Simulator
• Cryptanalysis
• SIS's Cryptanalysis
• Challenges

Introduction – Five-Digit Traffic

The RED machine was used by Japan to transmit encrypted diplomatic messages between the two world wars. Transmissions were made by radio, but also by telegraphy on public networks.

RED messages were identified by the first group, which consisted of five digits. This identification was used by both official correspondents (Japanese agencies) and foreign cryptanalysts (US, etc.) to distinguish this traffic from other types of traffic.

Two Encryption Networks

There were two RED networks:

• One, located in the Far East, connected the main headquarters of Japanese forces in Asia to Tokyo.
• The other connected the Ministry of Foreign Affairs in Tokyo to the main Japanese embassies: Washington, London, Rome, Paris, Berlin, Moscow, Warsaw, and Ankara.

• The numeric indicator had two forms: either composed of the numbers 0 to 4 (e.g., 02143) or composed of the numbers 5 to 9. Perhaps these two forms identified the two networks?
• Messages encrypted with the Purple machine also began with a group of five digits, but these were composed of either even digits (02468) or odd digits (1,3,5,7,9).

Keys Used

The encryption keys had to be shared between the correspondents. A RED key is divided into two parts:

Structure of a RED key

• The Plugboard Configuration
• The Rest of the Key:
  • The Removed PINS
  • The Starting Position of the Wheels (Breakwheel, Half-Rotor of the Sixes, Half-Rotor of the Twenties)
  • The Encryption or Decryption Mode
  • The direct or Reverse Mode (Direction of Wheel Movement)

The 5-digits indicator

The key used was primarily specified by the value of the numeric indicator, except for the plugboard configuration.

Thus, in the case of the message of January 8, 1937, exchanged between Tokyo and Berlin (see page …), the indicator 02143 corresponded to the following key:

• Pins removed: 5, 20, 31, 34 (then the period is 43)
• Ciphering Mode: ciphering
• Direction Mode: direct
• Start position of the wheels: 39 (breakwheel), 1 (Sixes), 11 (Twenties)

The keying depends on the digital indicator. In fact, the document DOCID 4001123 (see References) shows us a more complex view. Using values dependent on the date, subtracted from the digital indicator, provides the different elements of the key.

An indicator was chosen from a list of 240 values. This list was never changed.

Plugboard setting

The Plugboard value was identical for all messages sent during a 10-day period. A month was divided into three periods:

• 1st period: from the 1st to the 10th of the month.
• 2nd period: from the 11th to the 20th of the month.
• 3rd period: from the 21st to the end of the month (28th to 31st).

The message's timestamp specified the time period and therefore the plugboard to be used.

Note: At the end of the RED machine's use, the period of use for a plugboard configuration was reduced to a single day.

At first glance, one might think that the plugboard configuration came from a key table. In fact, Rowlett (see SIS's Cryptanalysis) discovered that this configuration followed an algorithm. Thus, the Japanese code-room clerks could predict this configuration in advance.

Note: If you don't need a key table, this simplifies the procedures. Indeed, embassies were spread all over the world. Distributing key tables to them is not easy. Conversely, security is reduced: if the enemy discovers the algorithm for generating the plugboard configurations (which was the case), they no longer need to discover them through cryptanalysis!

Message Length, Traffic

The message of January 8, 1937, was found to be approximately 1,300 characters long (see Genuine messages). This message was divided into five parts but used a single key. I believe this division into several parts was intended to reduce the risk of having to retransmit the entire message in the event of errors.

We also know that Rowlett & Kullback analyzed an extremely long message. It dated December 21, 1933, and was over 2,000 characters long.

During the Manchurian events of 1936, traffic increased, between 10 and 15 messages per day (Far East network).

Drafting

The RED machine only allowed the transmission of the 26 letters of the alphabet. Thus, the highlighted Roma-ji vowels were doubled. For example, Yōkoso is written YOOKOSO.

Here are some transformations:

• LA: punctuation.
• QE: semicolon
• CA ... CI: highlighting.

The drafting observed in 1941 for the Purple machine does not apply here in 1937 (see F. Weierund). Thus, the bigrams AI and EI are not transformed.

Over the years, the drafting rules and standard phrases evolved. In 1939 and later, these rules remained the same for both the RED and PURPLE systems. This was a great help in the cryptanalysis of the PURPLE machine.

References

Books and articles

• The Story of Magic, by Frank B. Rowlett, Aegean Park Press, 1998.

Internet

• NSA - Red and Purple, A Story Retold (U) [declassified] (1982?) (link)