Editing SIGABA (section)

==Operation==
[[File:SIGABA-ECM (Army M-134-C, Navy CSP-888) at NCM top view.agr.jpg|thumb|Top view of SIGABA showing rotor assembly and Controller switch]]
Because SIGABA did not have a reflector, a 26+ pole switch was needed  to change the signal paths through the alphabet maze between the encryption and decryption modes. The long “controller” switch was mounted vertically, with its knob on the top of the housing. See image. It had five positions, O, P, R, E and D. Besides encrypt (E) and decrypt (D), it had a plain text position (P) that printed whatever was typed on the output tape, and a reset position (R) that was used to set the rotors and to zeroize the machine. The O position turned the machine off. The P setting was used to print the indicators and date/time groups on the output tape. It was the only mode that printed numbers. No printing took place in the R setting, but digit keys were active to increment rotors.

During encryption, the Z key was connected to the X key and the space bar produced a Z input to the alphabet maze. A Z was printed as a space on decryption. The reader was expected to understand that a word like “xebra” in a decrypted message was actually “zebra.” The printer automatically added a space between each group of five characters during encryption.

The SIGABA was zeroized when all the index rotors read zero in their low order digit and all the alphabet and code rotors were set to the letter O. Each rotor had a cam that caused the rotor to stop in the proper position during the zeroize process.

SIGABA's rotors were all housed in a removable frame held in place by four thumb screws. This allowed the most sensitive elements of the machine to be stored in more secure safes and to be quickly thrown overboard or otherwise destroyed if capture was threatened. It also allowed a machine to  quickly switch between networks that used different rotor orders. Messages had two 5- character indicators, an exterior indicator that specified the system being used and the security classification and an interior indicator that determined the initial settings of the code and alphabet rotors. The key list included separate index rotor settings for each security classification. This prevented lower classification messages from being used as cribs to attack higher classification messages.

The Navy and Army had different procedures for the interior indicator. Both started by zeroizing the machine and having the operator select a random 5-character string for each new message. This was then encrypted to produce the interior indicator. Army key lists included an initial setting for the rotors that was used to encrypt the random string. The Navy operators used the keyboard to increment the code rotors until they matched the random character string. The alphabet rotor would move during this process and their final position was the internal indicator. In case of joint operations, the Army procedures were followed.

The key lists included a “26-30” check string. After the rotors were reordered according to the current key, the operator would zeroize the machine, encrypt 25 characters and then encrypt “AAAAA”. The ciphertext resulting from the five A's had to match the check string. The manual warned that typographical errors were possible in key lists and that a four character match should be accepted.

The manual also gave suggestions on how to generate random strings for creating indicators.  These included using playing cards and poker chips, to selecting characters from cipher texts and using the SIGABA itself as a random character generator.
<ref>[https://maritime.org/doc/crypto/ecm/index.htm CSP-1100 (C) Operating Instructions for ECM Mark 2 and CCM Mark 1], U.S.Department of the Navy, 1944</ref>