# Alphabet (cryptology)

In cryptology , unlike in common parlance , an alphabet is understood to be any arrangement of symbols that represent the basis for a plain text or the ciphertext obtained from it with the aid of an encryption process and a key . In the simplest case, the 26 capital letters of our usual Latin alphabet are shifted from their natural place by one. In the general case, they are permuted (rearranged) as randomly as possible , so that a “scrambled” alphabet is created. It is also often referred to as the key alphabet or the secret alphabet .

A secret alphabet can of course also consist of other characters than the usual capital letters. In classical cryptography , unusual characters were often created for secret alphabets , especially in connection with simple monoalphabetic substitution ciphers (see also web links ). For further meanings of the term alphabet, see also Alphabet (Computer Science) .

## Examples

These scrambled involutorial alphabets were proposed by the Italian cryptologist Giovan Battista Bellaso in 1555 .

The starting point is the standard alphabet , i.e. the 26 capital letters of the Latin alphabet in their usual order:

• a B C D E F G H I J K L M O P Q R S T U V w x y z

With a Caesar shift by one, the standard alphabet becomes the following:

• BCDEFGHIJKLMNOPQRSTUV WXYZA

With a Caesar shift by three, the standard alphabet now becomes:

• DEFGHIJKLMNOPQRSTUVWX YZABC

Cryptographically much stronger than shifted alphabets are if possible “scrambled”. Completely random ones that are difficult for a person to remember would be ideal , such as:

• EKMFLGDQVZNTOWYHXUSPA IBRCJ

That's why people like to use key words that are easy to keep in mind and derive an alphabet that is as scrambled as possible. For example, "WIKIPEDIA" is used as the key and all letters of the key word are first placed at the beginning of the alphabet to be generated, whereby letters that appear multiple times, such as the second and third "I" in WIK I PED I A, are not taken into account. WIKIPEDIA becomes WIKPEDA. Then the remaining letters of the Latin alphabet are filled in and you get:

• WIKPEDABCFGHJLMNOQRST UVXYZ

Instead of the “normal” padding used here, which has the disadvantage with regard to cryptographic security that the key alphabet usually (as here) ends in “… XYZ”, other padding methods are also conceivable and common. A "reverted" padding, in which the remaining alphabet is filled in in reverse (reverted) order, results in the following secret alphabet:

• WIKPEDAZYXVUTSRQONMLJ HGFCB

If you omit the key word here, you simply get a reverted alphabet that is used in Atbasch , one of the simplest encryption methods.

• ZYXWVUTSRQPONMLKJIHGF EDCBA

This alphabet is at the same time involutorial , i.e. self-inverse. This means that the double application leads back to the original text. This has the practical advantage that the user does not have to differentiate between encryption and decryption . Both process steps are identical. However, involutive alphabets also represent a considerable cryptographic weakness. Probably the most famous example of this is provided by the German Enigma machine from World War II .

An alternative to the reverted is the "progressive" padding, ie starting with the last letter of the password, here "A" followed by the rest of the unused alphabet. For the password chosen in this example, however, the padding order is identical to normal padding.

It is also possible to choose an arbitrarily scrambled sequence of letters. Although this has the practical disadvantage that it usually cannot be kept in mind, it offers the advantage of improved cryptographic security.

## terminology

In general terms, cryptography is about converting plain text into ciphertext through encryption with a key . Depending on how the plain text is split up, how many "alphabets" are used for encryption and how many characters these alphabets are made up of, and finally how the ciphertext is composed, the different cryptographic methods and the alphabets used be systematically categorized. The following nomenclature is used for this purpose:

### Plain text decomposition

• The plain text is broken down into individual characters: monographic (also: simple )
• The plain text is broken down into pairs of characters: bigraphic
• The plain text is broken down into symbol triples : trigraphic
• The plain text is broken down into quadruples of characters: tetragraphically
• The plain text is broken down into character quintuples : pentagraphic
• The plain text is broken down into character octuples : octographic

### Alphabet structure

• The alphabet consists of two characters: binary encryption , e.g. B. binary code
• The alphabet consists of three characters: ternary encryption
• The alphabet consists of four characters: quaternary encryption
• The alphabet consists of five characters: quinary encryption , e.g. B. ADFGX
• The alphabet consists of six characters: senior encryption , e.g. B. ADFGVX
• The alphabet consists of ten characters: denary encryption , e.g. B. the ten digits
• The alphabet consists of 26 characters, for example our usual Latin alphabet
• The alphabet consists of 32 characters (5  bits ), for example with the Baudot code
• The alphabet consists of 128 characters (7 bits), for example in ASCII
• The alphabet consists of 256 characters (8 bit), for example in the extended ASCII
• The alphabet consists of 2 64 characters (64 bit), for example in the DES method in ECB mode

### Ciphertext generation

• The ciphertext is generated from single characters: monopartit
• The ciphertext is generated from pairs of characters: bipartite
• The ciphertext is generated from triples of characters: tripartite
• The ciphertext is generated from character octets : oktopartit , e.g. B. Bytes