Vigenère Cipher

The Vigenère cipher is a Caesar cipher whose shift changes from letter to letter. A keyword sets the shift: it repeats along the entire length of the message, so the same plaintext letter gets encrypted differently in different places. Because of that, simple frequency counting stops working.

For three centuries the cipher was considered unbreakable and was called "the indecipherable cipher". In 1863 Friedrich Kasiski published a method: repeated sequences in the ciphertext reveal the length of the key, and then the text splits into several Caesar ciphers, each of which falls to frequency analysis.

A key shorter than the text is the weak point. If you make it the same length as the text, random, and use it only once, you get the Vernam cipher, the only theoretically unbreakable one. In practice the problem moves to storing and transmitting such a key.