SIGABA

The Sigaba is a rotor cipher machine that was used in World War II in the communications of the U.S. military. The Sigaba, also briefly ECM ( for Electric Cipher Machine, German: "Electrical key machine" ) or more specifically, ECM, was called Mark II by the U.S. Army as a Converter M -134 -C and the U.S. Navy as a CSP -889 and - referred to as CSP -2900 - in a modified form. Unlike the based on similar principles German cipher machine ENIGMA could with the Sigaba encrypted radio messages undeciphered ( "cracked" ) are.

History

The key machine Sigaba based on principles of the use of rotors ( rolling) for the purpose of encryption of text, as the American Edward Hugh lifters ( 1869-1952 ) invented in 1917 and filed for a patent. These principles were refined in particular under the leadership of the American cryptologist William Frederick Friedman in terms of an irregular stepping of the rotors and thus significantly enhances cryptographically. His colleague Frank Rowlett had the idea of ​​using more rollers for irregular stepping of Chiffrierwalzen. So finally in 1936 the M -134 -C Sigaba emerged in quick succession as forerunners in the year 1933, the M -134, the M -134 -A, called SIGMYC, and.

Function

Similar to the German cipher machine ENIGMA also Sigaba used several rolls, which are permuted several times the letter of the plaintext to be encrypted, and finally result in the ciphertext. Unlike the ENIGMA, but used only three or at most four rolls, arrived at the Sigaba fifteen rollers. Furthermore, the Sigaba avoided the cryptographic main weakness of the ENIGMA, namely the reverse roller. For optional setting mode encryption or decryption, a toggle button, located to the right of the reels is used.

The fifteen rolls of Sigaba are divided into three sets of rollers to five rotatably mounted rollers. Only one set of rollers, the cipher rotor Bank ( German: " Chiffrierwalzensatz " ), consisting of the five Chiffrierwalzen, causes the actual encryption of the text, while the other two sets of rollers that control rotor Bank ( German: "tax roll set ") and the index rotor bank ( German: "Index roller set ") only serves to control and produce an irregular possible stepping the Chiffrierwalzen.

• The five Chiffrierwalzen (English: cipher rotors ) have both sides of the 26 capital letters of the Latin alphabet to 26 electrical contacts that are connected by insulated wires 26 inside the roller on the (then) secret way together in pairs. Leaving the entering from one side of a contact plate in the roller power back on the other side via another contact plate. Thus, the desired character to encrypt a permutation ( Zeichenvertauschung ) is reached.

• The five control rollers (german: control rotor ), also have on both sides at 26 electrical contacts which are also referred to by the 26 letters of the alphabet. However, unlike the control rollers Chiffrierwalzen obtained four signals which are split by passage of the control roll set into ten groups of one to six wires.

• The third and final set of rolls has five rolls index (german: index rotor ) which are significantly smaller than the Chiffrierwalzen and control rollers and having only ten contacts on each side. The contacts of the index rolls are marked with letters, but carry numbers, starting from 10 to 19 of the first index drum up to 50 to 59 for the fifth index roll. In contrast to the other ten rollers, the rollers do not move index during encryption. Their output signals control the switching operation of the Chiffrierwalzen an irregular manner.

Due to the irregular rolling stepping and neither involution nor fixed point free permutations, which causes the roll set, the Sigaba is cryptographically much stronger than the German cipher machine ENIGMA and could never be broken, contrary to this.