Polyphonic encryption
In cryptology , i.e. the science that deals with the encryption and decryption of texts, methods in which different plaintext characters are assigned to one and the same ciphertext character are referred to as polyphonic encryption (also: polyphonic encryption ) or polyphony for short . They thus serve to reduce the required ciphertext characters.
Conversely - formulated from the perspective of the ciphertext - polyphony means that the individual ciphertext characters can have different plaintext meanings. From this point of view, the term “polyphonic”, derived from ancient Greek , is explained for these methods, which consists of the two words πολύ polý “much, more” and φωνή phonḗ “voice”, meaning “polyphonic”. In a figurative sense, one imagines that the individual ciphertext characters would speak with several (many) “voices”, that is, they would have different plaintext meanings.
A contradicting method is homophonic encryption , in which individual plaintext characters are assigned several ciphertext characters.
Polyphonic encryption was used centuries ago and was mainly used by amateurs. But they were still used even in World War II . For example, the German Navy used polyphonic encryption for its weather key to indicate temperatures. Furthermore, in the years 1944/45, the short-signal procedure couriers used polyphones to encode the air pressure .
example
In order to protect the sending German submarines from being aligned by Allied warships or land stations, the Navy set the goal of keeping the radio messages as short as possible. As part of a typical short weather signal , the air pressure should be encrypted with just a single capital letter of the Latin alphabet . At the same time, an air pressure range from 948 mbar (and less) to 1046 mbar (and more) had to be covered with a sufficiently fine resolution in steps of 2 mbar. This is 50 levels. Since our alphabet only offers 26 capital letters, the solution was found to polyphonically assign the 50 air pressure levels (plain text) to the 26 letters (ciphertext).
This means that the available alphabet was used polyphonically (here, more precisely: twice), once for the range from 948 to 996 mbar and secondly for 998 to 1046 mbar. The following scheme was used:
1046 mb = A 1020 mb = N 996 mb = A 970 mb = N 1044 = B 1018 = O 994 = B 968 = O 1042 = C 1016 = P 992 = C 966 = P 1040 = D 1014 = Q 990 = D 964 = Q 1038 = E 1012 = R 988 = E 962 = R 1036 = F 1010 = S 986 = F 960 = S 1034 = G 1008 = T 984 = G 958 = T 1032 = H 1006 = U 982 = H 956 = U 1030 = I 1004 = V 980 = I 954 = V 1028 = J 1002 = W 978 = J 952 = W 1026 = K 1000 = Y 976 = K 950 = Y 1024 = L 998 = Z 974 = L 948 = Z 1022 = M 972 = M
The assignment to the relevant of the two air pressure ranges was the responsibility of the recipient of the message, who, knowing the approximate weather situation, concluded the correct of the two possible values.
Like all messages sent by the German submarines, such polyphonically encrypted short weather signals were not sent directly, but encrypted in advance. The ENIGMA-M4 key machine was used for this .
literature
- Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, ISBN 3-540-67931-6 .
supporting documents
- ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 38.
- ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 75.
- ↑ Dirk Rijmenants: Kurzsignale . (English, accessed August 25, 2008)