He showed me a sheet of paper he'd prepared, with two lines of letters running parallel to each other.

Here's a very basic cipher alphabet, he said. The top row is what you call the plaintext, the bottom row is the ciphertext. Notice how the ciphertext begins with our keyword, cornuta? After that, it's just a regular alphabet, with the letters from cornuta removed so they don't get duplicated.

How does it work?

Paul picked up a pencil from his desk and began circling letters. Say you wanted to write 'hello' using this cornuta cipher. You would start with the plaintext alphabet on top and find 'H', then look at its equivalent in ciphertext below it. In this case, 'H' corresponds to 'B.' You do that with the rest of the letters, and 'hello' becomes 'buggj.'

That's how Colonna used cornuta?

No. By the fifteenth and sixteenth centuries, Italian courts had much more sophisticated systems. Alberti, who wrote the architecture treatise I showed you last week, also invented polyalphabetic cryptography. The cipher alphabet changes every few letters. It's much harder.

I point to his sheet of paper. But Colonna couldn't have used anything like that. It just makes gibberish. The whole book would be full of words like 'buggj.'

Paul's eyes lit up. Exactly. Complex encipherment methods don't produce readable text. But the Hypnerotomachia is different. Its ciphertext still reads like a book.

So Colonna used the riddles instead of a cipher.

He nodded. It's called steganography. Like writing a message in invisible ink: the idea is that nobody knows it's there. Francesco combined cryptography and steganography. He hid riddles inside a normal-sounding story, where they wouldn't stand out. Then he used the riddles to create deciphering techniques, to make it harder to understand his message. In this case, all you have to do is count the number of letters in cornuta, which is seven, then string together every seventh letter in the text. It's not that different from using the first word of every chapter. Just a matter of knowing the right intervals.

That worked? Every seventh letter in the book?

Paul shook his head. Not the whole book. Just a part. And no, it didn't work at first. I kept coming up with nonsense. The problem is figuring out where to start. If you choose every seventh letter beginning with the first one, you get a completely different result than if you choose every seventh letter beginning with the second one. That's where the riddle's answer plays a role again.

He pulled another page from his pile, this one a photocopy of an original page from the Hypnerotomachia.

Right here, in the middle of this chapter, is the word cornuta, spelled out in the text of the book itself. If you begin with the *C* in cornuta, and write down every seventh letter for the next three chapters, that's how you find Francesco's plaintext. The original was in Latin, but I translated it. He handed me another sheet. Look.

Good reader, this past year has been the most trying I have endured. Separated from my family, I have had only the good of mankind to comfort me, and while traveling the waters I have seen how flawed that good can be. If it is true, what Pico said, that man is pregnant with all possibilities, that he is a great miracle, as Hermes Trismegistus claimed, then where is the proof? I am surrounded on one side by the greedy and the ignorant, who hope to profit by following me, and on the other side by the jealous and the falsely pious, who hope to profit by my destruction.

But you, reader, are faithful to what I believe, or you could not have found what I have hidden here. You are not among those who destroy in God's name, for my text is their foe, and they are my enemy. I have traveled broadly in search of a vessel for my secret, a way to preserve it against time. A Roman by birth, I was raised in a city built for all ages. The walls and bridges of the emperors stand after a thousand years, and the words of my ancient countrymen have multiplied, reprinted today by Aldus and his colleagues at their presses. Inspired by these creators of the old world, I have chosen the same vessels: a book and a great work of stones. Together they house what I will give to you, reader, if you can understand my meaning.

To learn what I wish to tell, you must know the world as we have known it, -who studied it most of any men in our time. You must prove yourself a lover of wisdom, and of man s potential, so that I will know you are no enemy. For there is an evil abroad, and even we, the princes of our day, do fear it.

Carry on, then, reader. Strive wisely for my meaning. Poliphilo's journey grows harder, as does mine, but I have much more to tell.

I turned the sheet over, looking for more. Where's the rest of it? That's it, he said. We have to solve more to get more. I looked at the page, then up at him in amazement. In the back of my mind, from a corner of unsettled thoughts, came a tapping noise, the sound my father always made when he was excited. His fingers would drum the rhythm of Corelli's Christmas Concerto, twice as fast as any allegro movement, on whatever surface he could find,

What are you going to do now? I asked, trying to stay afloat in the present.

But the thought occurred to me anyway, putting the discovery in perspective: Arcangelo Corelli finished his concerto in the early days of classical music, more than one hundred years before Beethoven's Ninth Symphony. Even in Corelli's day, though, Colonna's message had been awaiting its first reader for more than two centuries.

The same thing you are, Paul said. We're going to find Francesco's next riddle.

Every hallway in Dod is empty as Gil and I return to the room, numbed by the long walk north from the parking lot. An airy silence prevails throughout the building. Between the Nude Olympics and the Easter festivities, every soul is accounted for.

I turn on the television for word of what's happened. The local networks carry the Nude Olympics on the late news, after there's been time to edit the footage, and the runners in Holder Courtyard float across the screen in a blur of whites, blinking under the glass like fireflies trapped in a jar.

At last the female news anchor returns to the screen.

We have breaking news on our top story.

Gil emerges from his bedroom to listen.

Earlier tonight we reported to you about what may be a related incident at Princeton University. At this hour the accident at Dickinson Hall, which some witnesses describe as a fraternity stunt gone wrong, has taken a tragic turn. Officials at Princeton Medical Center confirm that the man, reportedly a university student, has died. In a prepared statement, Borough Police Chief Daniel Stout repeated that investigators would continue to examine the possibility that, quote, nonaccidental factors had played a role. In the meantime, university administrators are asking students to remain in their rooms, or to travel in groups if they need to be outside tonight.

In the studio, the anchor turns to her cohost. Clearly a difficult situation, given what we saw earlier at Holder Hall. Returning to the camera, she adds, We'll be coming back to this story later in the hour.

He died? Gil repeats, unable to believe it. But I thought Charlie… He lets the thought trail off.

A university student, I say.

Gil looks up at me after a long silence. Don't think like that, Tom. Charlie would've called.

Against the far wall, the framed picture I bought for Katie sits at an uncomfortable cant. I dial Taft's office, just as Gil returns from his bedroom and hands me a bottle of wine.