Which method uses at least two different shifts, changing the shift with different letters in the plaintext?

The method that utilizes at least two different shifts, changing the shift with different letters in the plaintext, is known as multi-alphabet encryption. This technique is a key component of classical cipher systems, such as the Vigenère cipher, where multiple substitution alphabets are employed based on a repeating keyword. Each letter of the plaintext is shifted according to the corresponding letter of the keyword, effectively creating a dynamic shifting mechanism that increases the complexity and security of the cipher.

This approach enhances security compared to single-alphabet encryption, where a single fixed alphabet is used for all letters. By employing multiple shifts, multi-alphabet encryption makes frequency analysis more difficult. Each letter in the plaintext can be transformed in unique ways based on its position and the corresponding letter from the key, creating a more complex ciphertext compared to simpler substitution methods that might rely on a one-to-one relationship.

In summary, multi-alphabet encryption increases the variability of shifts applied to the plaintext, providing greater resistance against cryptanalysis techniques.