# The Single Digit - a chess riddle

In front of you lies a safe. Within the safe, you are aware, lie the AI-discovered secrets to solving the game of chess - indeed, every one of its complexities, all of its unknowns, implausibly reduced from the mouth of Shannon to a form comprehensible to the average human. You owe it to the world to unlock its secrets.

To open the safe, you must enter the correct single digit, 0 thru 9, on the keypad. But you only get one chance. Enter an incorrect number, and the safe will be locked forever.

$$\hskip1.5in$$

On the side of the safe are written the following clues:

r1bk3r/p2p1pNp/n2B1n2/1p1NP2P/6P1/3P4/P1P1K3/q5b1
5n2/8/8/2R2Pk1/1P6/5K2/8/8
8/3kn3/5r2/1K6/4Q3/1B6/8/8
r1bqkb1r/pp1n1pp1/2p1Nn1p/8/3P4/3B1N2/PPP2PPP/R1BQK2R
5N1k/6p1/7p/4P3/pp2Q3/4q3/1P4PP/2b4K
Nary a logical inkling may overcome vacuousness; We mercilessly oust vain epistemology. 010010000110000101101001011011000100010001000010

With the keypad at your fingertips, what digit do you enter? __

• Spoiler alert: the most I can offer in the way of what's actually inside the safe is en.chessbase.com/post/… ;) – anodyne Mar 19 '20 at 7:12

Clues

The first five strings are written in chess FEN notation. The first and fifth positions are relatively straightforward mate in ones:
Board 1: Be7#

Board 5: Qh7#

The second and third boards are somewhat complicated to analyze. Fortunately, we can use the text clue to help.
Namely, taking first letters of the text message gives Nalimov; Wmove. Nalimov is the namesake of a particular endgame tablebase, which includes up to positions of at most 6 pieces. So we can plug in these positions into the tablebase. It turns out that these positions have a unique most efficient move. (The sequence afterwards is not unique, so that is unlikely to be relevant.)
Board 2: b5 (mate in 12)

Board 3: Qe5 (mate in 16)

The binary string is ASCII encoded for HailDB. This doesn't seem immediately useful, since HailDB doesn't appear to be chess-related (and is a defunct database project).
Board 4 appears to be an opening instead, with no mate in sight. The natural interpretation of this is that white just moved (or else Black really messed up in leaving their queen hanging). Note that if Black decides to capture the knight with the pawn, it opens the king to a fairly annoying Bg6, so it's not that bad of a sacrifice for White. But also as an opening it might be fruitful to search this on the internet, and sure enough there is in fact a recorded game that matches this opening, namely Deep Blue vs Kasparov (1997) game 6 (after White's move 8). This game ended up being the decisive match where Deep Blue beat Kasparov. (And perhaps DB in the ASCII clue can be reparsed as Deep Blue!)

Putting it all together

Boards 1, 2, 3, and 5 are mates 1, 12, 16, and 1 respectively. Board 4 was made 8 moves in. Alphanumerically this gives ALPHA. Connecting chess, alpha, and computers, one new chess program is AlphaZero, a neural network-based system recently developed by DeepMind. (This differs from most previous systems that were mainly alpha-beta pruners.) Thus, we should type in the single digit 0.

• Or maybe 8 because that's the odd one out? It seems a bit odd to clue a specific number by that word you found at the end. Anyway, great job! – Rand al'Thor Mar 19 '20 at 17:43
• Fantastic work! rot13(UnvyQO jnf vaqrrq zrnag gb pyhr Qrrc Oyhr'f tnzr 6 ivpgbel.) The last step is to find what digit would best link chess, the word you found, and the distinct flavor of everything heretofore. – anodyne Mar 19 '20 at 18:05
• I'd like to suggest this other number – Bass Mar 19 '20 at 18:24