A Vigenere autokey cipher is a polyalphabetic substitution cipher that uses a key word or phrase to encrypt a message. It was invented in the 16th century by Blaise de Vigenere and is considered to be one of the strongest ciphers of its time.
The Vigenere autokey cipher works by using a series of interwoven Caesar ciphers. The key word or phrase is used to determine the shift value for each letter in the message. The key is then appended to the beginning of the message, creating a new key for the subsequent letters. This process is repeated until the entire message is encrypted.
The Vigenere autokey cipher is considered to be secure because it uses a long key word or phrase, making it difficult for cryptanalysts to determine the exact key. Additionally, the interwoven Caesar ciphers make it difficult to break the cipher using frequency analysis.
The main limitation of the Vigenere autokey cipher is that it is vulnerable to attacks if the key word or phrase is too short or easily guessable. It is also possible to break the cipher using brute force methods if the key length is known.
The Vigenere autokey cipher is no longer commonly used for secure communication due to advancements in cryptography. However, it is still used in some applications, such as in puzzles and games, as well as for historical and educational purposes.