After posting this week’s cipher challenge, we received a few questions, namely: what’s a rail fence cipher? We asked author James Lilliefors for a quick explanation and he provided this:

Several of the characters in Viral use various ciphers to mask their communications. These range from the relatively simple rail fence and split rail ciphers to the much more complex ADFGVX cipher—the seemingly unbreakable code used by the German military during World War I—along with some home-grown varieties.

With the rail fence cipher, the plaintext is written downwards and diagonally on “rails” of a three-level imaginary fence. So the words “rail fence” would look like this:

And the code (reading left to right, across the three levels) would be: rfealecin

Hope this helps!