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Although monoalphabetic systems are quite simple and generally produce the same security for messages, some systems go to elaborate efforts to create the appearance of more security. However, despite the extra amount of effort that goes into encoding and decoding these messages, they are no more secure than the simple system you're about to learn.
One of the most reliable monoalphabetic systems uses a keyword to provide security. This keyword, which should be easy to remember, must be known by both parties. To encode the message, the encoder writes out the normal alphabet in the standard order, and then writes the keyword beneath the alphabet, excluding letters if they were already written. When the end of the keyword is reached, the encoder continues to write down the remaining letters of the alphabet in order. By writing the rest of the letters in order, the only thing the parties need to know is the keyword. The rest of the cipher alphabet is simply the unused letters. The example below illustrates a ciphertext alphabet created with a keyword. And after creating the plaintext/ciphertext alphabets, we can just them to encode a plaintext message.
One final step that should be taken of course is to split the message into regular word groupings of 4 or 5 letters so that the word divisions are not apparent, as shown below (the four nulls are bolded.)
As you've probably noticed, monoalphabetic substitutions are named because only one cipher alphabet is used for encoding the plaintext. Since only one alphabet is used, frequency ananlysis will most often very easily led to the correct plaintext.
So, you know the simple systems and the keyword system, now want to know how simple they are to crack?