Advanced Placement (AP) Computer Science 2025 – 400 Free Practice Questions to Pass the Exam

The Vigenère square, also known as the Vigenère table, is primarily used to align the encryption alphabet with the plaintext in a particular way that allows for more complex encryption than simple substitution ciphers. In this cipher, a keyword is used to determine which alphabet to use for each letter in the plaintext.

By referencing the Vigenère square, each letter of the plaintext is matched with a letter of the keyword. The intersection of the corresponding row (determined by the plaintext letter) and column (determined by the keyword letter) gives the encrypted letter. This method makes the encryption more secure because each letter of the plaintext can be encrypted with different letters from the keyword, thereby preventing simple frequency analysis from being effective.

The use of the Vigenère square does not involve randomizing the message, counting letter frequency, or generating random keys; instead, it methodically aligns the keyword with the plaintext to produce the ciphertext according to the rules of the Vigenère cipher.