Keyword Cipher

Instead of using a Caesar shift cipher or randomly rearranging the plain alphabet to achieve the cipher alphabet, the sender chooses a keyword or key phrase. For example I will choose the key phrase PHOTO DARK ROOM. The person encrypting the message begins by removing any repeated letters and any spaces (PHOTDARKM), and then uses this as the beginning of the jumbled cipher alphabet. The remainder of the cipher alphabet is merely the remaining letters of the alphabet, in their correct order, starting where the keyword ends. Hence, the cipher alphabet would be as follows.

If I wrote the ciphertext, "KDSSW," the person receiving the message would know I mean "HELLO."

The advantage of building a cipher alphabet in this way is that it is easy to memorize the keyword or keyphrase, and hence the cipher alphabet. The cipher alphabet is also a bit more random than simply using a Caesar shift cipher (Singh 1999).