How did Julius Caesar’s shift cipher change letters in a message?

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A Vigenere cipher table displays the alphabet in both the top row and left column, with rows shifted to create a grid for encryption and decryption. Highlighted letters illustrate how to use the table. An explanation and example using the words cipher, key, and plaintext appears to the right. — cryptology In encrypting plaintext, the cipher letter is found at the intersection of the column headed by the plaintext letter and the row indexed by the key letter. To decrypt ciphertext, the plaintext letter is found at the head of the column determined by the intersection of the diagonal containing the cipher letter and the row containing the key letter.

The Caesar cipher is a substitution encryption technique where each letter in a message is replaced by a letter a fixed number of positions away in the alphabet.1

Specifically, Julius Caesar utilized a shift of three positions down the alphabet. This process was circular, meaning that if the shift reached the end of the alphabet, it would wrap back around to the beginning.1

In practice, this requires two lines: one containing the standard alphabet and a second containing the alphabet shifted by the chosen number of places. To encrypt a message, one replaces each letter of the original text with the corresponding letter found in the shifted line. Because each letter is consistently replaced by the same cipher letter, the method is susceptible to frequency analysis and can be easily broken using brute force, as there are only 25 possible shifts.1