Vigenère's autokey: a short primer followed by the plaintext itself as the key, so the key never repeats.
Procedure
How it works
Vigenère himself proposed this stronger form in 1586. The key starts with a short primer and then continues with the message: with primer QUEENLY, the key for ATTACKATDAWN is QUEENLYATTAC, and adding the letters modulo 26 gives QNXEPVYTWTWP.
To decrypt, the receiver uses the primer for the first letters; every plain letter found becomes a key letter a little later. Since the key never repeats, the Kasiski method and the index of coincidence have no period to find.
It is still weak: the key is the plaintext itself, so it has the statistics of English. Guessing a probable word somewhere in the message reveals plain letters a primer length further on, and the whole text unravels from there.
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