Each cipher alphabet is named by the first letter in it. The cipher alphabet on the second row uses B for A and C for B etc. If 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. 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. 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 This is what a Vigenère square looks like:Ī B C D E F G H I J K L M N O P Q R S T U V W X Y Zī C D E F G H I J K L M N O P Q R S T U V W X Y Z AĬ D E F G H I J K L M N O P Q R S T U V W X Y Z A Bĭ E F G H I J K L M N O P Q R S T U V W X Y Z A B CĮ F G H I J K L M N O P Q R S T U V W X Y Z A B C Dį 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 Each cipher alphabet is just another rightward Caesar shift of the original alphabet. With the square, there are 26 different cipher alphabets that are used to encrypt text. This makes the message much harder to decode because the codebreaker would have to figure out both cipher alphabets.Īnother example of a polyalphabetic substitution cipher that is much more difficult to decode is the Vigenère square, an innovative encoding method. The encoder would make up two or more cipher alphabets using whatever techniques he or she chooses, and then encode their message, alternating what cipher alphabet is used with every letter or word. It is also possible to have a polyalphabetic substitution cipher, where multiple cipher alphabets are used. The previous examples were all examples of monoalphabetic substitution ciphers, where just one cipher alphabet is used. For example, if the keyword is CIPHER, the cipher alphabet would look like this:Ī b c d e f g h i j k l m n o p q r s t u v w x y zĬ 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 end of the cipher alphabet is the rest of the alphabet in order without repeating the letters in the 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. Caesar rotated the alphabet by three letters, but any number works.Īnother method of substitution cipher is based on a keyword. So, for example, "WIKIPEDIA" encrypts as "ZLNLSHGLD". Finally, X, Y and Z are replaced by A, B and C respectively. Hence, A is replaced by D, B by E, C by F, etc. To encrypt a message with the Caesar cipher, each letter of message is replaced by the letter three positions later in the alphabet. In a substitution cipher, letters (or groups of letters) are systematically replaced throughout the message for other letters (or groups of letters).Ī well-known example of a substitution cipher is the Caesar cipher. Classical ciphers are often divided into transposition ciphers and substitution ciphers.
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