The Running Key cipher has the same internal workings as the Vigenère cipher. The difference lies in how the key is chosen; the Vigenère cipher uses a short key that repeats, whereas the running key cipher uses a long key such as an excerpt from a book. This means the key does not repeat, making cryptanalysis more difficult. The cipher can still be broken though, as there are statistical patterns in both the key and the plaintext which can be exploited.

If the key for the running key cipher comes from a statistically random source, then it becomes a 'one time pad' cipher. One time pads are theoretically unbreakable ciphers, because every possible decryption is equally likely.

The 'key' for a running key cipher is a long piece of text, e.g. an excerpt from a book. The running key cipher uses the following tableau (the 'tabula recta') to encipher the plaintext: