Spreading (cryptology)

from Wikipedia, the free encyclopedia

Spreading ( English straddling ) is in the cryptology in the encryption of texts used method. Here are clear text character by ciphertext characters of different lengths replaced .

method

Spreading is used in particular with hand keys , i.e. encryption procedures carried out manually (with pencil and paper), especially with the so-called "espionage ciphers", such as the VIC cipher . To do this, the 26 capital letters of the Latin alphabet are written in three rows and ten columns of a table . Mostly you start with a password that contains the most common letters , such as "ERNSTL", and fill the rest of the table with the remaining letters, in the simplest case in alphabetical order. It is important to leave two fields empty in the first line of the table (not necessarily the last two as here). In the last line, after all letters have been entered, there are still two fields that can be filled with special characters (here “.” And “/”). If necessary, these can be assigned special functions, such as letter-digit switching, but they can also just be blenders .

  0 1 2 3 4th 5 6th 7th 8th 9
  E. R. N S. T L. A. B.
8th C. D. F. G H I. J K M. O
9 P Q U V W. X Y Z . /

The above table ( English straddling checkerboard ; German literally: "spread chessboard") now allows the monoalphabetic substitution of letters by numbers, whereby the letters in the first line are replaced by single-digit numbers and the letters in the other two lines by two-digit numbers. This special feature, namely that the plaintext characters are replaced by ciphertext characters of different lengths, is known as "spreading". The resulting cipher is called “spread”.

example

As an application example, the plain text “We think Wikipedia is good” should be encoded with the password “ERNSTL”. The individual letters of the plain text are substituted by the following numbers according to the table. For the sake of clarity, a space is inserted after the cipher of each individual letter . 

 W  I  K  I  P  E  D  I  A  F  I  N  D  E  N  W  I  R  G  U  T
94 85 87 85 90  0 81 85  6 82 85  2 82  0  2 94 85  1 83 92  4

For the transmission of the spread ciphertext one would of course remove the spaces and arrange the text in groups of five, for example, to camouflage the spreading: 

94858 78590 08185 68285 28202 94851 83924

The authorized recipient, who, like the sender, is in possession of the password that serves as the key (here ERNSTL), can generate the same table and use it to convert the numbers back into the original letters. This gives him the original plain text.

In practice, further procedural steps , such as transpositions, are often applied to the spread text in order to increase the cryptographic security of the procedure and to make unauthorized decipherment more difficult (see also: VIC cipher ).

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 .
  • Fred B. Wrixon: Codes, Ciphers & Other Secret Languages ​​- From Egyptian Hieroglyphics to Computer Cryptology . Könemann, Cologne 2000, p. 280. ISBN 3-8290-3888-7

Web links

Individual evidence

  1. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 57.
  2. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 58.
  3. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, pp. 35 and 57.