Why is Vigenère Autokey self-synchronizing? According to the Wikipedia, and to several of our school study materials, Vigenère autokey ciphers are considered to be self-synchronizing stream ciphers.
If I understand it correctly, the main idea of self-synchronizing ciphers is that they should re-sync in an event of missing characters in the ciphertext because the key only depends on previous N characters.
But, as far as I understand, in Vigenère autokey, every character depends on all the previous characters being in the right order, isn't that so? So, in an event of a character loss, the rest of the ciphertext after the lost character will never be deciphered correctly.
So, what am I missing? Do I misunderstand self-synchronizing ciphers, or am I missing something about the Vigenère autokey cipher?
&quot;Autokey&quot; refers to an encryption method that uses the plaintext itself to generate the ciphertext. &quot;Self-synchronization&quot; of a stream cipher is not an encryption method. It is a modern (from 1946) process for a stream cipher to update its state based on the previous ciphertext.
Is Vigenère's Autokey Cipher an example of a self-synchronizing stream cipher?  No, but it could be coded, one supposes, to be self-synchronizing for certain methods of transmission and reception.
Background
Girolamo Cardano (1501-1576), a prolific writer and mathematician from Milan, came up with the first autokey cipher in 1550, but he designed it very poorly. As David Kahn explains in The Codebreakers (Macmillan, 1967), &quot;He employed the plaintext as a key to encipher itself, starting the key over from the beginning with each new plaintext word.&quot; (Kahn, p.144).
Blaise de Vigenère improved on Cardano's work when he &quot;did not recommence his key with each plaintext word, which was his [Cardano's] weakness, but kept it running continuously.&quot; (Kahn, p. 147). This innovation was &quot;entirely forgotten&quot; until the latter part of the nineteenth century. Vigenere's autokey is stronger than the famous &quot;Vigenère Cipher&quot; that was later mistakenly attributed to him, but it is a relic of the past.  Picture pen and paper, or even better, quill pen and parchment.
This idea of an autokey cipher that does not repeat the keyword has been used in some more modern cryptographic methods--rotor machines, for example--and the principle is still relevant. So, autokey is a symmetric encryption method with a curious history and many variations (also see Bellaso's method from 1564; Kahn, p. 144). On the other hand, self-synchronization of a stream cipher means maintaining its correct state.