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A '''substitution cipher''' is a very simple (and easily broken) cipher (type of [[encryption]] in which every letter of the alphabet is replaced by a different letter. For example, each letter might be replaced by the letter below it:

A '''substitution cipher''' is a very simple (and easily broken) cipher in which every letter of the [[alphabet]] is replaced by a different letter. For example, each letter might be replaced by the letter below it:

  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

  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

  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

  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

Then "CONSERVAPEDIA" would encrypt into "GSRWIVZETIHME."

Then "[[CONSERVAPEDIA]]" would encrypt into "GSRWIVZETIHME."

In the above example, each letter is replaced with the letter that comes four places after it in the alphabet. This kind of cipher has the advantage that the encryption system can be memorized: all that needs to be remembered is that A encrypts into E and the rest follows automatically. However, this kind of substitution cipher (known as a ''Caesar cipher'') is extremely weak. There are only twenty-five possible Caesar ciphers, so if the cryptanalyst knows or suspects that the message has been encrypted with a Caesar cipher, the message can be decrypted simply by trying each of the twenty-five possibilities.

In the above example, each letter is replaced with the letter that comes four places after it in the alphabet. This kind of cipher has the advantage that the encryption system can be memorized: all that needs to be remembered is that A encrypts into E and the rest follows automatically. However, this kind of substitution cipher (known as a ''Caesar cipher'') is extremely weak. There are only twenty-five possible Caesar ciphers, so if the cryptanalyst knows or suspects that the message has been encrypted with a Caesar cipher, the message can be decrypted simply by trying each of the twenty-five possibilities.