The ciphers in this substitution section replace each letter with another letter according to the cipher alphabet. Ciphers in which the cipher alphabet remains unchanged throughout the message are called Monoalphabetic Substitution Ciphers.

If we permit the cipher alphabet to be any rearrangement of the plain alphabet, then we can generate an enormous number of distinct modes of encryption. There are over 400,000,000,000,000,000,000,000,000 such rearrangements, which gives rise to an equivalent number of distinct cipher alphabets. Each cipher alphabet is known as a key. If our message is intercepted by the enemy, who correctly assumes that we have used a monoalphabetic substitution cipher, they are still faced with the impossible challenge of checking all possible keys. If an enemy agent could check one of these possible keys every second, it would take roughly one billion times the lifetime of the universe to check all of them and find the correct one.This simple brute force approach clearly will not work.