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SIGABA (ECM)
Electric cipher machine (ECM) Mark II

SIGABA was a US cipher machine based on the electromechanical rotor principle. It was developed in the late 1930s as a joint effort of the US Army and Navy [1]. At the time it was considered a superior cryptomachine, intended to keep high-level communications absolutely secure. It was used throughout WWII and was so reliable that it was used well into the 1950s, when it was replaced by machines like the KL-7. As far as we know, SIGABA was never broken.
 
The image on the right shows one of the few SIGABA machines that have survived. It has the appearance of a rather bulky typewriter, featuring a full keyboard at the front. SIGABA is motor-driven and can print its text on a small strip of paper that runs across the front.

The machine has three banks of 5 wheels each, which is clearly visible in the patent below. The main bank (at the rear) contained 5 wheels with 26 contacts each. These were the main cipher wheels and worked similar to those of the German Enigma machine (more below).

The wheels were placed in a sub-assembly, sometimes called the rotor-basket, which could easily be removed from the machine by opening the top lid and releasing a few large bolts.
  
SIGABA CCM CSP-1700

Before, during and after WWII, the US Army and Navy each developed their own cryptographic systems. Neither service shared their knowledge with the other one, except for limited cooperation in the field of enemy signals intelligence. The first exception to this 'rule' was the development of SIGABA. It combined the knowledge of top cryptographers William F. Friedman and Frank B. Rowlett (Army) and Lieutenant Commander Laurence Safford (Navy).

The machine was called SIGABA by the US Army and ECM MARK II (Electric Cipher Machine) by the Navy. It is also known as Converter M-134 and CSP-888/889 (Navy). A modified Navy version is known as the CSP-2900. The Germans called it the American Big Machine.
 
SIGABA CCM CSP-1700 SIGABA front view after removing the hood Interior of the CCM SIGABA, showing the modified rotor basket. Printer assembly Right view of the CCM rotor basket Sigaba wheel without its wiring Each wheel has flat-faced contacts at either side Date code stamped onto the bottom plate

 
US Patent 6,175,625
According to a recently released NSA document, SIGABA was such a reliable machine that is was used throughout the 1950s. Some of its operating principles were not declassified until the year 2000 [1]. Shortly afterwards, in January 2001, US patent 6,175,625 was released [2].
 
The design of the SIGABA machine was filed for a patent on 15 December 1944, close towards the end of WWII. The patent was not published however, until 16 January 2001, shortly after the machine was declassified by the NSA. The patent lists Laurence F. Stafford and Donald W. Seiler as the inventors.

The patent covers 14 pages, including the 5 drawing sheets, and registers 21 claims. The drawing on the right is taken from the first page of the patent. It shows the machine after the removing the metal cover. The printing device (ENG-108) has also been removed in the drawing. It is connected to the two large plugs.
  
Click here to download the patent

The original SIGABA had three banks of 5 wheels each, the contacts of which are visible in the drawing above. The main bank (at the rear) contained 5 wheels with 26 contacts each. These were the main cipher wheels and worked similar to the cipher wheels of the Enigma machine.

The second bank (middle) also contained 5 wheels with 26 contacts each, but these were used to control the movement of the main cipher rotors. These were called the control wheels.

The third bank (front) contained 5 wheels that were smaller than the others. They only had 10 contacts each and would not move during encipherment. They are called the index wheels. Together with the control wheels, they controlled the movement of the cipher wheels.
 
Printer
The SIGABA can print its output directly onto a pre-gummed paper strip. The printer uses an ink ribbon and is mounted at the front of the machine, directly above the keyboard. The paper is fed to the printer from a reel that is mounted in a circular space at the right side of the machine.
 
The letters are printed onto the paper strip by a rotating print head with embossed characters around its circumference. The print head is driven mechanically from the rear right by the main motor shaft. A series of solenoids inside the printer select the point at which the print head is stopped when the hammer at the bottom pushes the paper strip against the head.

The printer can easily be removed from the machine by releasing four large bolts at the bottom. The image on the right shows the bare printer once it is removed from the machine.
  
Printer assembly

At the left of the printer are two large sockets for the connection between the electrical circuits at the bottom of the machine and the solenoids inside the printer. Two thick cables, each with a large connector at the end, are slotted into these sockets. A similar printer, with a rotating print head and solenoids, was used in the early 1950s with the later American KL-7 cipher machine.
 
SIGABA front view after removing the hood Printer assembly Front view of the printer assembly Rear view of the printer Rear view of the printer Left view of the printer, showing the two large connectors. SIGABA without the printer Empty printer space
Printer connectors fitted at the left side of the printer SIGABA keyboard Close-up of the printer Empty printer space Peper-release button Releasing the paper Right angle view of the SIGABA, showing the paper path.

 
Combined Cipher Machine (CCM/SIGABA)
SIGABA was developed in the late 1930s and was used by the US Army throughout WWII for secure communication at the highest level. Towards the end of WWII, there was an increasing need for secure communication with the British Armed Forces. It was decided that SIGABA would be modified, so that it would become interoperable with a modified British Typex machine.

The common machine was known as the Combined Cipher Machine (CCM), and was used from November 1943 onwards. The CCM/SIGABA modification was known as ASAM 5 by the Army and CSP-1600 by the Navy, but the complete machine (i.e. SIGABA + CCM adapter) was known by the Navy as CSP-1700 [10]. According to the machine's base plate, it was also built in 1943.
 
CCM was used for secure Allied communication during WWII and even served with the newly established NATO for a few years after the war.

The wheel assembly was removed and replaced by an assembly of just 5 coding wheels. Less secure than the original, but interoperable with a modified British Typex (called CCM/Typex).

The machine shown here is one of the very few CCM versions of SIGABA that have survived. Unfortunately, the cipher wheels and the motor assembly are missing from this one (see below).
  
Interior of the CCM SIGABA, showing the modified rotor basket.

Some publications about the SIGABA CCM suggest that the only modification to SIGABA was the substitution of the 15-rotor basket by a 5-rotor one. This was alledgedly done so that the machine could be converted back to its original state at any time. Judging from the CSP-1700 shown here however, this was not the case. The original machine has three entry/exit contact plates at either side of the rotor basket, whereas the CCM version only has one at either side.

Furthermore, the rotor basket is connected to the rightmost set of contacts by means of 6 wires that are screwed to 6 contact terminals at the bottom of the basket. Although the rotor basket can be removed relatively easy, a full SIGABA basket (with 3 x 5 wheels) can not be retrofitted.

 More about Typex
 
Right view of the CCM rotor basket Left view of the CCM rotor basket Top view of the CCM rotor basket Bottom view of the CCM rotor basket Switches and contact terminals inside the rotor basket Rear view of the CCM rotor basket (bottom up) Close-up if the (empty) wheel slots Another close-up of the (empty) wheel slots

 
Interior
Considering its age, SIGABA is a very compact, albeit heavy, unit. It has an almost cubical shape, with a keyboard sticking out at the front. The rest of the machine is divided into two sections: a large one at the top that contains all mechanical parts, the motor, the cipher wheels and the mode selector, and a small one at the bottom that contains the electrical parts and the wiring.
 
The bottom section can be accessed by removing the large base plate that is affixed at the four corners. The base plate contains four shock-mounts that should limit the amount of vibration when the machine is in operation.

The image on the right shows the interior of the bottom section, with the keyboard at the left. At the center are two arrays with switch contacts. These switches are operated directly by the keys of the keyboard. The rest of the bottom section is taken up by large bundles of cables and some additional electrical components.
  
Bottom view of the SIGABA showing the wiring and circuits

At the right in the image above, are the two large input and output connectors. The top section of the machine can be accessed by removing the large metal hood that is held in place by two bolts at the front and a hidden one inside the rear, behind the motor assembly (mode-switch set to O).
 
The image on the right shows the interior of the top section, seen from the front right. The empty space at the right is normally contains the motor and the main driving shaft. Unfortunately they are missing from our machine (see below).

The shiny construction at the front right of the machine, is the mode selector. It consists of a large vertical array of stacked switches with a knob at the top. It allows selection between cipher (C), decipher (D) and plain-text (P) mode. It can also be used to turn the machine off (O) when it is no longer used.
  
SIGABA interior

The printer unit is at the front right. It can be removed completely and is described above. Behind the printer, at the left rear, is the rotor basket. The one shown here is a CCM rotor basket and is described in more detail above. To the right of the rotor basket is a small panel with a square plug that can be used to select the operating voltage: 24V DC or 110V AC (no 220V setting!).
 
Bottom plate Close-up of a shock-mount Bottom view of the SIGABA showing the wiring and circuits Perspective view of the bottom section Close-up of the wiring Close-up of solenoid and contact Close-up of impulse contact Close-up of the switches, with the SPACE-bar depressed.
SIGABA interior The mode-selector at the front right Mode selector seen from the rear right Close-up of the mode selector Printer assembly Voltage selector set to 24V battery Selecting 105/125V operation Date code stamped onto the bottom plate

 
Restoration
The CCM/SIGABA shown on this page has probably been used with NATO in the years following WWII. Unfortunately, certain critical parts have been removed, so we suspect that this machine has been cannibalized for spare parts in order to repair other machines. We would very much like to get it running again. In the drawing below the missing parts have been coloured.


We have recently (May 2012) cleaned the machine and restored the existing parts. Below are some photographs of the restoration process. In case you have any parts for this machine or if you have additional information, please contact us. The following parts are currently missing:
 
  • Yellow: ZEROIZE switch with cable
  • Light blue: Motor with worm wheel
  • Red: Main shaft
  • Dark blue: Left bearing
  • Five large cipher wheels
It is quite possible that the ZEROIZE button was not used on the CCM version of the SIGABA at all. As we haven't found (part of) the cable assembly or any contact terminals in the bottom section, it is possible that the switch was never mounted on this version.
 
Front view Rear view. Note the INPUT and OUTPUT connectors at the bottom Rear view of the CCM Mark 2. Note the single entry wheels. View from the right The empty area where the motor and the main shaft should be
Right top view Left view Single entry wheels Partly disassembled machine Dismantled machine Filter below the printer

 
References
  1. NSA, Cryptologic Almanac 50th Anniversary Series, AFSAM-7
    Declassified by NSA on 12 June 2009.

  2. Laurance F. Stafford, Donald W. Seiler, U.S. Patent 6,175,625
    Filed 15 December 1944. Published 16 January 2001.

  3. JJG Savard & RS Pekelney, The ECM Mark II: Design, History and Cryptology
    Cryptologia, Vol. 23, No. 3, June 1999, pp. 211-228.

  4. R Pekelney, ECM MARK 2 and CCM MARK 1
    Operating instructions for ECM Mark 2 (CSP 888/889) and CCM Mark 1 (CSP 1600).

  5. Rich Pekelney, Electronic Cipher Machine (ECM) Mark II
    Detailed description of the ECM Mark II (SIGABA).

  6. Michael Lee, Cryptanalysis of the SIGABA
    Thesis for the degree Master of Science in Computer Science.
    University of California, Santa Barbara. June 2003.

  7. Wing On Chan, Cryptanalysis of SIGABA
    Project Report for the degree Master of Science.
    San Jose State University. May 2007.

  8. Heather Ellie Kwong, Cryptanalysis of the SIGABA Cipher
    Thesis for the degree of Master of Science.
    San Jose State University. December 2008.

  9. John Savard, The ECM Mark II, also known as SIGABA...
    Website, discussing the operation of SIGABA. Retrieved June 2012.

  10. Wikipedia, Combined Cipher Machine
    Retrieved November 2012.

  11. Jerry Proc's crypto pages, CCM Mk II (Combined Cipher Machine)
    Retrieved November 2012.

Further information

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