Reflector (cryptology)
Under a reflector ( English reflector ), also known as reverse roll (UKW) designated is understood in the cryptology , especially in connection with rotor cipher , a component , which returns a signal in the direction again ( " reflected ") come out of it is.
application
Reflectors were used in some key machines that work according to the rotor principle for encryption that emerged in the early 20th century (see also: Edward Hebern ). Prominent examples are the British Typex , the Japanese San-shiki Kaejiki , the Italian OMI machine , the Swiss Nema and above all the Enigma machine used by the German Wehrmacht in World War II . In the case of the latter in particular, the reflector is also referred to as a "reverse roller" (VHF). The VHF was invented in the 1920s by the German engineer Willi Korn , an employee of the Enigma inventor Arthur Scherbius at Chiffriermaschinen-Aktiengesellschaft (ChiMaAG) in Berlin , where the Enigma was further developed and manufactured at the time. The UKW was patented on March 21, 1926.
advantages
With the reverse cylinder, Willi Korn achieved that the key procedure becomes involutive , i.e. if a U is encoded into an X at a certain point in the text, then an X would also be encoded into a U at this point. He simplified the operation and construction of the machine, because you no longer have to differentiate between encryption and decryption . In addition, he hoped for an increase in safety, because the current now flows through the set of rollers twice. Korn explains the (supposed) advantages of his reversing roller in the patent specification:
“This decline in the current through the cipher roller set causes further scrambling. As a result of this arrangement, it is possible to get by with relatively few encryption rollers and still maintain a high level of encryption security. "
disadvantage
However, this was a fallacy with far-reaching consequences. On the one hand, the VHF means that no more letters can be encoded in itself, because the current cannot in any case take the exact route back through the set of rollers that it came from. It is always directed back on a different path than when it flowed to the reversing roller. Mathematically, one speaks here of fixed-point-free permutations . This restriction may seem like an insignificant minor matter, but it actually means a drastic reduction in the alphabets available for encryption and, moreover, a new vulnerability of the ciphertext . By avoiding fixed points, statements about the text are possible, which are a very important aid in deciphering . The British codebreakers from Bletchley Park coined the motto "Nothing is ever itself." If the attacker knows that a letter is never the encryption of itself, then this knowledge opens up abbreviations for him and he no longer has to laboriously work through each individual case ( see also: Cryptographic weaknesses and deciphering the Enigma ).
Successful cipher machines - which in practice prove to be " unbreakable " - such as the American Sigaba , avoided the use of a reflector.
literature
- Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin 2000, ISBN 3-540-67931-6 .
- Friedrich L. Bauer: Historical Notes on Computer Science . Springer, Berlin 2009, ISBN 3-540-85789-3 .
Individual evidence
- ↑ Louis Kruh, Cipher Deavours: The commercial Enigma - Beginnings of machine cryptography . Cryptologia, Rose-Hulman Institute of Technology, Taylor & Francis, Philadelphia PA 26.2002,1 (January), p. 2, ISSN 0161-1194 , PDF; 0.8 MB , accessed February 22, 2019.
- ↑ Patent specification Electrical device for ciphering and deciphering DRP No. 452 194, p. 1. cdvandt.org (PDF; 0.5 MB), accessed on February 22, 2019.
- ↑ Patent specification Electrical device for ciphering and deciphering DRP No. 452 194, p. 1. cdvandt.org (PDF; 0.5 MB), accessed on February 22, 2019.
- ↑ Robert Harris: Enigma . Novel. Weltbild, Augsburg 2005, p. 71. ISBN 3-89897-119-8 .