Single Alphabet (Monoalphabetic) Substitution Ciphers
A monoalphabetic substitution cipher replaces every letter with a single unique drawing, color, picture, number, symbol, or another type of letter. This means, if you have your first "E" encoded as an X, all of your other "E"s in the message will also be "X." Because of this pattern, the message is easy to encipher and decipher quickly for experienced agents. However, an analyst intercepting the message can use frequency analysis to help discover the meaning if they have enough message traffic.