# Forms

of the text coding mark Polyalphabeti substitution Polyalphabeti replacement ciphers in cryptography, with which another letter/ indication is assigned to a letter/an indication in each case. Contrary to the monoalphabetical substitution for the indications of the plain language several secret text alphabets are used.

## to Caesar coding with progressive index

this simple coding method works the Caesar procedure similarly with the difference that the current plain language character depending upon its position in the plain language strand inAlphabet is shifted, whereby one begins if necessary again at the beginning. , As this procedure is, also the Geheimtext can so simple be decoded fast, by shifting the indications depending upon its position in the other direction in the alphabet.

### example

`plain language: IN T E R N E T positions: 1 2 3 4 5 6 7 8 Geheimtext: J P W I W T L B`

## Vigenère coding

the Vigenère coding (after Blaise de Vigenère) was considered for a long time as safe coding algorithm. A keyword determines, how many Alphabets to be used. The alphabets are derived from the Caesar substitution .

The decoding of a Vigenère cipher succeeded to the British mathematician Charles Babbage for the first time around the year 1854. This discovery however at that time publicly does not admit made. The Prussian officer Friedrich Kasiski published its in the year 1863Solution and entered thereby history.

### examples

the keyword is „AKEY “, the text „SECRET “. Four Caesar substitutions code the text. The first substitution is a Caesar coding with the key „A “. „A “is the first letter in the alphabet. It shifts firstLetter of the text which can be coded, „the G “, around 0 places, „G “remains. The second letter of the key, „the K “, is the elfte letter in the alphabet, it shifts the second indication of the text, „the E “ten indications. Out „E “becomes „an O “(see table).The third indication of the key („E “) shifts four, „Y “24 places. The shift of the next letter of the text begins again with „A “, the first letter of the key:

```Text:       SECRET key:  AKEYAKEYA Cyphertext: GOLCIWRGS
```
 text K e y A B C D E F G HI J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W XY Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z B C D E F G H I J K L M N O P Q RS T U V W X Y Z A C D E F G H I J K L M N O P Q R S T U V W X Y Z A B D E F G H I J K L MN O P Q R S T U V W X Y Z A B C E F G H I J K L M N O P Q R S T U V W X Y Z A B C D F G HI J K L M N O P Q R S T U V W X Y Z A B C D E G H I J K L M N O P Q R S T U V W X Y Z A BC D E F H I J K L M N O P Q R S T U V W X Y Z A B C D E F G I J K L M N O P Q R S T U V WX Y Z A B C D E F G H J K L M N O P Q R S T U V W X Y Z A B C D E F G H I K L M N O P Q RS T U V W X Y Z A B C D E F G H I J L M N O P Q R S T U V W X Y Z A B C D E F G H I J K MN O P Q R S T U V W X Y Z A B C D E F G H I J K L N O P Q R S T U V W X Y Z A B C D E F GH I J K L M O P Q R S T U V W X Y Z A B C D E F G H I J K L M N P Q R S T U V W X Y Z A BC D E F G H I J K L M N O Q R S T U V W X Y Z A B C D E F G H I J K L M N O P R S T U V WX Y Z A B C D E F G H I J K L M N O P Q S T U V W X Y Z A B C D E F G H I J K L M N O P QR T U V W X Y Z A B C D E F G H I J K L M N O P Q R S U V W X Y Z A B C D E F G H I J K LM N O P Q R S T V W X Y Z A B C D E F G H I J K L M N O P Q R S T U W X Y Z A B C D E F GH I J K L M N O P Q R S T U V X Y Z A B C D E F G H I J K L M N O P Q R S T U V W Y Z A BC D E F G H I J K L M N O P Q R S T U V W X Z A B C D E F G H I J K L M N O P Q R S T U VW X Y

### cryptanalysis

of keywords, which are relative to the text short, do not offer security. A correlation of the text with itself, in each case shifted a place, supplies the length of the key. Is the key length (period) n admits of the Caesar coding, reduces the cryptanalysis of the Vigenère coding to those: all first, second,…, nth letters to one period belong in each case to the same Caesar coding and a frequency analysis betrays the letter allocation.

With a text, which consists only of the repetition of an indication, showsitself the period directly in the Geheimtext. A normal text exhibits sufficiently redundancies, so that starting from a certain length of the text in the comparison to the key the period can be derived also here (Kasiski - test, Friedman test). Only a text out statistically equal distributedLetters would be not so easily accessible to an Ciphertext only attack.

```Text:          EEEEEEEEEEEEE keys:     AKEYAKEYAKEYA Cyphertext:    EOICEOICEOICE correlation 1:  EOICEOICEOICE      No correlation with Geheimtext Correlation 2:   EOICEOICEOICE     No correlation with Geheimtext Correlation 3:    EOICEOICEOICE    No correlation with Cyphertext Correlation 4:     EOICEOICEOICE   Correlation with Cyphertext!
```

In this way getsone quite fast the key length of the coded text out. Now the Geheimtext must be only divided in columns. The columns, which were coded with the same letter, are summarized. One solves the appropriate alphabet shift of the individual partial texts now by means of frequency analysis.

### autokey coding

those Autokey - Vigenère coding avoids the periodicity of the keyword. Instead to the key the plain language is attached:

```Text:          SECRET keyword: AKEY key:     AKEYGEHEI Cyphertext:    GOLCOQUMA
```

## Vernam

the special case that the key is just as long as the text which can be coded, is called to coding, howthe key XOR is linked with the plain language. Actsit with the key around a coincidental consequence of letters, one speaks oneself coding also of a One TimePAD -.

the keyword the number and selection of the coding alphabets determines rotor machines with the Vigenère coding. Same carry outRollers or wheels, on which the letters of the alphabet are engraved. Correctly to each other oriented, one reads off the coded text directly from them.

If one agrees to change with each letter the position of the rollers to each other the number of the alphabets the available leaves itselfby a multiple increase (see Enigma).