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Using a keyword is an excellent way to manage the transmission of a cipher to the person you wish to send your coded message to. The most difficult part in sending an encoded message is ensuring that your target is the only one who has the key to decode it. This gets especially difficult when dealing with a polyalphabetic substitution cipher, since an error in managing the multiple alphabets used can make your message just as incomprehensible to your target as the people you are trying to keep from intercepting it. It is very easy to send, tell, or previously agree to a keyword so that you and your target are on the same sheet of music.

There are several methods in using a keyword to create a cipher.

With a single keyword(let's use George Washington), there are two basic methods, with variations.

One way is to simply start with the keyword and write the rest of the alphabet afterwards, dropping any repeated letters - georwashintbcdfjklmpquvxyz.

A reverse-keyword cipher is used as above, but with the alphabet reversed and the keyword starting from the back - zyqxvuqpmlkjfdcbtnihsawroeg.

There are several other ways you can use a single keyword, like keeping the alphabet in order but using the keyword at the back, etc. The more attentive of you will also notice that I actually used two words as my keyword, but in context, they are one unit.

For a polyalphabetic substitution cipher, you can use a phrase, with the number of words designating the number of alphabets to use. Each alphabet would be organized as the single-keyword examples above. By using a keyword sentence to generate the key for polyalphabetic substitution, you can make it harder for anyone trying to break your cipher by changing the number of words (and therefore sentences) every time you change your keyword.

You can let this get as complex as you wish. You can send a message where the first (or last, or third, etc) sentence is a polyalphabetic substitution keyword sentence encoded with a simpler cipher, allowing you to send a new key with every message based upon a previously agreed upon master keyword.

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