So what kind of ciphers are actually used? Ciphers for authentication purposes include the password mentioned earlier. On the other hand, ciphers for the purpose of hiding information include the famous Caesar cipher and the cryptogram mentioned earlier. The details will be discussed later.

This Cryptogram, can you see that the unit to be encrypted is the “sentence”? In other words, the “sentence” that you want to convey secretly is replaced by another “sentence” that the parties involved have decided to deal with secretly in advance. For example, if you want to say, “Come at two o’clock!” is to be replaced with a completely unimaginable sentence such as “I made ramen!”. Therefore, in order to anticipate various situations, it is necessary to decide in advance how the ciphertext should respond to a larger number of sentences.

However, this has its limitations. If an unanticipated situation arises, and we want to send a sentence for which we have not agreed on the corresponding ciphertext in advance, we cannot encrypt it. How can we make it possible to send more flexible sentences? For now, how about reducing the encryption unit from “sentence” to “word”?