Mark Clowes (38M 🇬🇧)

Index - 2013 Xmas Cipher Challenge Part 1 Solution

Nice easy one to get started. The challenge invites you to download a file.

redacted.ppm

This is an unusual format I wasn't familiar with. GIMP is able to open it. It is basically a bitmap file stored using ASCII.

'Herodotus' is also a clue; research time: "The first recorded uses of steganography can be traced back to 440 BC when Herodotus mentions two examples in his Histories. Demaratus sent a warning about a forthcoming attack to Greece by writing it directly on the wooden backing of a wax tablet before applying its beeswax surface. Wax tablets were in common use then as reusable writing surfaces, sometimes used for shorthand."

So the message is hidden somewhere inside the file. My first thought was that it was stored as a comment of some sort, but this was a dead end. Let's investigate the part of the image that has been redacted. We expect 0 0 0 RGB values.

What we get looks like:

00  000  000  00  000  000  00  00  000  00 

So they are all black, but some black pixels are defined with 00 and some with 000 making them distinguishable. So a quick

%s/ 000 / 255 /g

and the solution is then revealed: