Autokey cipher

This cipher was invented in 1586 by Blaise de Vigenère with a reciprocal table of ten alphabets.

Instead of a single letter, a short agreed-upon keyword is used, and the key is generated by writing down the primer and then the rest of the message, as in Vigenère's version.

The autokey cipher, as used by members of the American Cryptogram Association, starts with a relatively-short keyword, the primer, and appends the message to it.

With Vigenère's autokey cipher, a single mistake in encryption renders the rest of the message unintelligible.

Therefore, methods like the Kasiski examination or index of coincidence analysis will not work on the ciphertext, unlike for similar ciphers that use a single repeated key.

and by attempting the decryption of the message by moving that word through the key until potentially-readable text appears.

The gaps can quickly be filled in: The ease of cryptanalysis is caused by the feedback from the relationship between plaintext and key.