This is a Java applet that implements the best known attack on the Jail Cell RC4 cipher.
The top blue section (with boxes) shows the correct key. In reality, the cryptanalyst would not know this; he is trying to guess this.
The top red section (with boxes) shows the current guess at the key. The generator using the correct key tracks forward and backwards with the generator using the guessed key so that they can be directly compared. At any time, if boxes in vertical alignment contain the same number, then that piece of the guessed key is correct.
The three lines of blue, unboxed numbers show the keystream generated by the correct key. This is the information that the cryptanalyst would know for a known-plaintext attack, and the cryptanalyst tries to guess a key that will generate the same keystream.
The red numbers that appear below the blue keystream show the keystream generated by the current guessed key up to the point where the guessed key outputs an incorrect value and contradicts the known keystream. If these red numbers ever extend across all three lines of blue numbers, then a solution has been found (the solution might be a key which produces a full series of collisions with the correct key without actually being correct, but it has a very high probability of being correct).
Permutations Eliminated: this is the number of complete permutations which are known not to produce the same keystream as the real key. When this number reaches half the total number of permutations, chances are about 50/50 that a solution will have been found.