# Playing with a chess cipher

Alice and Bob both love encryption, and Alice loves to play chess. Unfortunately, Bob isn't very good at chess, so he doesn't think it's very fair to have to play the game for real. So instead of playing chess, Alice and Bob decided to turn chess into their very own chess cipher. Once they had agreed to a scheme Alice sent the following test message to Bob.

1.  Nf3  d6
2.  Nc3  b5
3.  Ne4  h6
4.  Nxd6 Kd7
5.  Nxf7 a6
6.  Nd6  h5
7.  Nxb5 axb5
8.  g4   e5
9.  Nxe5 Ke8
10. gxh5 b4
11. c3   bxc3
12. b4   Nd7
13. b5   Nxe5
14. d3   Nc4
15. dxc4 c6
16. Bg5  cxb5
17. Be7  Qxe7


So what message did Alice send?

Hint 1

There are three steps needed to reach the final solution. Figure out which cipher they are using; determine the constraints, if any; determine how the chess moves fit into the chosen cipher to create a plain text answer.

Hint 2

The content of the framing story offers a hint to the specific cipher that they use. Once you know the cipher, some of the chess moves will provide the rest of the information you need.

Hint 3 (a bit of big one)

Chess is a bit like a boxing match, the fighters always start in one of the corners. From there, a lot of the moves are just posturing, but it's the hits that make contact that people come to see.

• One can view the game described here. Aug 29, 2020 at 1:36
• I thought bob was an expert in logic Aug 29, 2020 at 2:12
• I'm so sad this isn't a 8x8 Playfair cipher using ASCII Range 64-127 xD Sep 1, 2020 at 12:52
• @LukasRotter Chess is more of a traditional kind of game Sep 1, 2020 at 13:23
• @LukasRotter Not related to music, but most of the game is fluff. Or rather, setting it up for the parts of the game that are important to the cipher. Sep 1, 2020 at 14:19

The message is

HELLO WORLD

because

Map the following 5x5 grid onto the board:

The first move by white tells us the bottom right corner of the grid.

Then for each capture, take the underlying letter. One capture is outside the square, which represents a space

As hinted in the story, the 5x5 grid layout used is the same as in the Playfair cipher, even though we don't actually en/decrypt using playfair.

Notes / feedback:

Since the concept is fairly simple, it may be surprising this was only solved now with a load of hints in the comments. I can only tell you the reasons I couldn't solve it sooner:

- I picked up on the Playfair hint right aways, but assumed for the whole time until a comment that we actually had to decrypt using playfair. So I spent most of the time trying to figure out what's the key and what's the plaintext, and what custom variations could've been used (possibly more than one 5x5 grid, or a 8x8 ASCII grid). Maybe a hint only pointing towards a 5x5 grid would've been clearer.
- After the boxing hints, I actually thought about mapping a 5x5 grid onto it and only counting captures, but since at least one move was out of bounds of any 5x5 grid, I ditched it without even trying. That may have been stupidity on my part, since using an out-of-bounds capture as a space is not a bad idea.
- The thing that did it for me was the comment that the first move tells you the corner of a 5x5 grid and that one of the moves is out-of-bounds by design.

• Yup, and thanks for providing the notes. It's hard to judge difficulty and I'm still trying to refine my puzzle making abilities, so those notes really help a lot. I do realize now the problem with the cipher, I actually didn't realize how it was more typically used Sep 12, 2020 at 17:06