Which of the following was a multi-alphabet cipher widely used from the 16th century (1553) to the early 20th century (1900s)?

The Vigenère cipher is recognized as a multi-alphabet cipher that gained prominence from its introduction in the 16th century, specifically in 1553, until its decline in the early 20th century. This cipher employs a method of encrypting alphabetic text through the use of a simple form of polyalphabetic substitution. In contrast to monoalphabetic ciphers, which substitute each letter of plaintext with a fixed letter in ciphertext, the Vigenère cipher maintains several alphabets. Each letter of the keyword, which is repeated or extended to match the length of the plaintext, determines the shifting of the corresponding letter in the plaintext.

A primary advantage of the Vigenère cipher is its ability to defend against frequency analysis—an analysis used to crack monoalphabetic ciphers—because it obscures the relationship between the plaintext letters and the ciphertext letters. This characteristic contributed significantly to its widespread use throughout its period.

While Atbash is a monoalphabetic cipher with a single alphabet substitution, and the Beaufort cipher is similar to the Vigenère but operates in reverse, both do not exhibit the multi-alphabetic properties of the Vigenère. Transposition ciphers rearrange the letters of the plaintext without altering them, which does