Editing Classical cipher (section)

===Substitution ciphers===
{{Main article|Substitution cipher}}

In a substitution cipher, letters, or groups of letters, are systematically replaced throughout the message for other letters, groups of letters, or symbols.

A well-known example of a substitution cipher is the [[Caesar cipher]]. To encrypt a message with the Caesar cipher, each letter of message is replaced by the letter three positions later in the alphabet. Hence, A is replaced by D, B by E, C by F, etc. Finally, X, Y and Z are replaced by A, B and C respectively. So, for example, "WIKIPEDIA" encrypts as "ZLNLSHGLD". Caesar rotated the alphabet by three letters, but any number works.

Another method of substitution cipher is based on a keyword. All spaces and repeated letters are removed from a word or phrase, which the encoder then uses as the start of the cipher alphabet. The end of the cipher alphabet is the rest of the alphabet in order without repeating the letters in the keyword. For example, if the keyword is CIPHER, the cipher alphabet would look like this:

{| class="wikitable"
! normal alphabet
| {{mono|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}}
|-
! cipher alphabet
| {{mono|c i p h e r a b d f g j k l m n o q s t u v w x y z}}
|}
The previous examples were all examples of monoalphabetic substitution ciphers, where just one cipher alphabet is used. It is also possible to have a [[polyalphabetic cipher|polyalphabetic substitution cipher]], where multiple cipher alphabets are used. The encoder would make up two or more cipher alphabets using whatever techniques they choose, and then encode their message, alternating what cipher alphabet is used with every letter or word. This makes the message much harder to decode because the codebreaker would have to figure out both cipher alphabets.

Another example of a polyalphabetic substitution cipher that is much more difficult to decode is the [[Vigenere square|Vigenère square]], an innovative encoding method. With the square, there are 26 different cipher alphabets that are used to encrypt text. Each cipher alphabet is just another rightward Caesar shift of the original alphabet. This is what a Vigenère square looks like:

                 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 R S 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 M N 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 H I 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 B C 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 W X 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 R S 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
                 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
                 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
                 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 B C 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 W X 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 Q R
                 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 L M 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 G H 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 B C 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 V W X Y

To use the Vigenère square to encrypt a message, a coder first chooses a keyword to use and then repeats it until it is the same length as the message to be encoded. If {{mono|LEMON}} is the keyword, each letter of the repeated keyword will tell what cipher (what row) to use for each letter of the message to be coded. The cipher alphabet on the second row uses B for A and C for B etc. That is cipher alphabet 'B'. Each cipher alphabet is named by the first letter in it. For example, if the keyword is {{mono|LEMON}} and the message to encode is {{mono|ATTACKATDAWN}}, then the encoding is:

{| class="wikitable"
! Plaintext 
| {{mono|ATTACKATDAWN}}
|-
! Key 
| {{mono|LEMONLEMONLE}}
|-
! Ciphertext 
| {{mono|LXFOPVEFRNHR}}
|}

Some substitution ciphers involve using numbers instead of letters. An example of this is the [[Great Cipher]], where numbers were used to represent syllables. There is also another number substitution cipher{{which|date=April 2022}} that involves having four different number pair options for a letter based
on a keyword.

Instead of numbers, symbols can also be used to replace letters or syllables. One example of this is [[Zodiac]] alphabet, where signs of the zodiac were used to represent different letters, for example, the symbols for the sun stood for A, Jupiter stood for B, and Saturn stood for C. Dots, lines, or dashes could also be used, one example of this being [[Morse Code]], which is not a cipher, but uses dots and dashes as letters nonetheless. The [[pigpen cipher]] uses a grid system or lines and dots to establish symbols for letters. There are various other methods that involve substituting letters of the alphabet with symbols or dots and dashes.