Codebook

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Cryptographic codebook from the American Civil War that was used by the Union Army to encrypt telegraphic messages

A code book is a directory in which, on the one hand, letters , digits , syllables , numbers , words or whole sentences are listed and, on the other hand, certain combinations of characters are assigned to these "text fragments" ( phrases ) .

Code books like in the picture served not only to maintain confidentiality of messages, but they were in the Telegraph also be used to locate the text length and thus the telegram - fees lower.

example

Page from a Prussian telegraph code book, 1832

A simple example of a code book and how to use it would be:

AAA   Komme heute
BBB   Komme morgen
CCC   Komme übermorgen
DDD   Komme nächste Woche
EEE   Komme überhaupt nicht
FFF   Bleibe kurz
GGG   Bleibe lang

If the sender wants to send the message “Come next week, stay a while”, he can use the above code book to transmit “DDDFFF”. In reality, of course, code books are much more extensive than in this example. It is not uncommon for books to be hundreds of pages long, similar to telephone books or encyclopedias .

use cases

A distinction is made between code books, which are used for the purpose of simple or uniform data transmission , code books, which are used for data compression (reducing the amount of data), and code books, the purpose of which is to keep the information to be transmitted secret . The following are examples of the respective applications:

Mixed forms are also conceivable. The weather short key not only served to keep the weather reports secret, but also made it possible to shorten the length of the radio messages significantly and thus reduce the risk of finding the position of the submarines.

If it is a question of secrecy and some terms or sentences occur much more frequently than others, it is advisable to use several secret equivalents for frequently occurring phrases, for example, if the visitor messages from above should remain secret, also the letter combinations " Allow QQQ ”,“ DEF ”or“ XYZ ”as equivalents for a visit that will not take place until the next week. In the cryptographic sense, one then speaks of homophones , that is to say “ unified sounds ”, and means that a plain text , namely “Coming next week”, is assigned several ciphertext equivalents. The purpose of this is to make unauthorized decipherment more difficult and to prevent the potential attacker from discovering the meaning of the code using a frequency analysis.

Code books in history

Deciphered terms from the Zimmermann dispatch

While code books for Morse Code or ASCII are of course publicly known and used in a general and uniform manner, cryptographic code books must be kept secret. If they fall into the hands of the enemy, the communication is exposed and no longer secret. This can have fatal consequences, especially in times of war, especially if the code's compromise goes unnoticed.

An example of the compromise happened in the case of the signal book of the Imperial Navy (SKM) in the First World War . It was a secret code book that was carried on the German warships. A few days after the beginning of the First World War, the German cruiser SMS Magdeburg ran aground in the Baltic Sea . The ship had to be abandoned and two copies of the signal book on board were dumped overboard in accordance with regulations. Fatally for the Germans, however, they were recovered by Russian divers a short time later. One copy was immediately forwarded to the British Admiralty, and it was already in front of the British Navy Minister Winston Churchill in October 1914 , without the German side being aware of this.

Another example, also from the First World War, is the Zimmermann dispatch , the code of which could also be cracked, which led to the entry of the USA into the First World War.

Practical aspects

This
identification group booklet captured from the German submarine U 505 was printed on pink blotting paper with water-soluble red ink so that it could be destroyed quickly in the event of danger.

A distinction is made between one-part and two-part code books. In one piece codebooks of both the plaintext and the ciphertext are equally alphabetical ( lexicographic ) order . This has the advantage that only a single book is required for encryption and decryption , but the decisive disadvantage that if parts of the code book are known, it is much easier for an attacker to draw conclusions due to the alphabetical order.

This disadvantage is avoided in the two-part code books, in which only the plain text is listed alphabetically in the first part, but the ciphertext consists of unordered strings. For decryption, of course, you then need an "inverse" code book, i.e. a second part in which the ciphertext is arranged alphabetically.

In practice, codes are often " over-encrypted " (example: over-encryption with the help of a shift cipher or monoalphabetic substitution ) in order to make unauthorized decipherment more difficult.

Another practical aspect of codes is the ability to "de-mutilate" them. Due to poor transmission conditions, especially with radio messages, it can happen that individual characters in the ciphertext are mutilated due to interference . Instead of the sent "DDD", the addressee suddenly receives, for example, "DXD". However, if the code is designed redundantly , for example in such a way that three identical letters (or three consecutive letters in the alphabet) must appear, the recipient can reconstruct "DDD" from "DXD" ("de-mutilate") and so despite the disturbed reception the decrypt correct message.

Codebooks must be kept secret, especially when used in the military, and must not "fall into enemy hands" intact. Therefore, they were often specially prepared in order to be able to destroy them easily and quickly in an emergency. Paper and ink were chosen in such a way that the writing ran off and became illegible on contact with water. Particularly thin paper was also used to save weight, but also to enable destruction by lighting or, if necessary, by eating.

See also

literature

Web links