56

You look over at Robin, and you see him kneeling over a large sheet of paper that he's stretched out on the floor. Squinting at it, you can just make out an absolute mess of scrawled notes and tiny diagrams, all in your grandmother's handwriting. You can't make any sense of it—but a few times, you can see spots where your grandma has written out various words and translated them into different coded forms. More than anything, she seems to have been interested in codes that involve replacing letters with numbers.

"I don't know all of these codes," Robin says, eyes keenly fixed on the paper, "but I think that one's called a Polybius square."

He points at a small, square grid, each space filled up with different letters. Around the outside, the rows and columns of the grid are labeled with numbers. You notice a few similar grids dotted about the page, each with different combinations of letters.

"I remember reading about this; I think Selene Corvina used it sometimes to write stuff in secret," Robin tells you. "It's a pretty easy code to use, but it would be difficult to break unless you knew how. You start by making a five-by-five grid with the letters of the alphabet—well, you have to leave one out—but the trick is that you start with a secret word that nobody else knows. Like there—"

He taps on one of the grids. Quickly, you try to take it in:

□————1————2————3————4————5

1————S————I————L————V————E

2————R————A————B————C————D

3————F————G————H————J————K

4————M————N————O————P————Q

5————T————U————W————X————Y

"Obviously the keyword here is 'silver,'" Robin says, finger tracing over the word at the beginning of the grid. "So instead of starting with 'A, B, C,' you put the secret word in first, and then fill in the rest of the alphabet afterwards with the letters of 'silver' left out. Then each of the letters gets, like, its own coordinate in the grid, and those coordinates replace the letters. So since 'S' is the first letter in the grid, its coordinate is '1,1,' or just '11'—and 'I' is '1,2,' or '12,' and so on. Then you'd just write whatever you want to, but using numbers instead of letters. Look, your grandma did it there—"

Sure enough, when you glance below the grid, you see that your grandma has written out a seemingly incomprehensible string of numbers: "314321151151."

Now that you know how the code works, however, it doesn't take long to work out that the word it represents is actually "forest."

Just as Robin said, the code is pretty simple once you understand it. The part you struggle most to get your head around is the fact that your grandma was so interested in codes to begin with. What would she have wanted to keep so secret that she resorted to using code?

"Wow. Your grandma must be pretty cool," Robin laughs. "And look at all these words she was putting into code—'Leo,' 'moon,' 'earth.' Is she into astronomy too? I wonder what she was using it for—wait—that book you're holding—"

Suddenly, Robin's wide eyes are fixed on the book in your hand. You'd practically forgotten you were holding anything at all—but when you look down and focus on it properly, you spot it right away.

Numbers instead of letters.

"That must be where she wrote it down," Robin murmurs, huddling close to you to get a better look at the tiny writing. "I wonder what she was trying to—oh—I probably shouldn't be reading your grandma's private stuff, should I?"

He gives a nervous chuckle and backs away again, face flushing slightly.

"Sorry, just got distracted. I'll just see what else is in here."

He busies himself with the chest once more, leaving you with your grandma's diary.

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