page twenty-one

The Bureau

Somebody wired a dispatch this morning and somebody else took a copy of it off the line. It is in front of you, in a cipher, and nobody here knows the key. Today's number wrote the message, chose the system, cut the key and enciphered it — and then, before handing any of it over, broke the thing itself, so that the one promise this page makes is a promise it has already kept.

Click a letter in the intercept — or a tile below — and then type what you think it stands for. Backspace clears it. Nothing you type can be wrong for long.

System — Letters — Settled — Off the desk 0 Clock 0:00 Reads as English —

A to Z types a letter into whichever tile is lit, Backspace takes it out again, and the arrow keys walk along the strip. On a substitution day, two cipher letters pointed at the same plain letter turn red — that is never right, and it is usually the fastest way to find the mistake you made twenty minutes ago.

How to get in

  1. 01

    Count first, guess second

    The tiles are sorted by how often each cipher letter appears, commonest first, with a bar to show it. In telegram English the top of that list is almost always E, then T, A, O, N and the rest of the usual crowd. That is not a solution, it is a shortlist: it tells you which four or five letters are worth being wrong about, and stops you spending an hour on a letter that turns up twice.

  2. 02

    Use the crib

    Every dispatch on this circuit opens FROM … TO THE BUREAU and closes MESSAGE ENDS, and the sentences are divided by STOP. That is a lot of known plaintext. Lay MESSAGE ENDS against the last eleven letters of the intercept and you have seven or eight of the alphabet before you have done any thinking at all — and against a repeating key, the same trick hands you a run of key letters directly.

  3. 03

    Watch the gauge, not your hunch

    The bar marked reads as English scores every pair of letters your current sheet produces against how often that pair turns up in this circuit's traffic, and reports the result as a percentage of the way from gibberish to the real thing. It is the same number the machine climbs. If a change you were sure about drops it, you were not sure about it.

What is actually behind the tiles

A different system comes up depending on the day. A simple substitution is one fixed alphabet cut from a keyword: one cipher letter for each plain one, the same from beginning to end, which is why counting works on it. A Vigenere takes a short keyword and shifts each letter by the next letter of that word in turn, so a single plain letter comes out five or six different ways and a plain frequency count tells you almost nothing — you have to find the length of the key first, then solve each slice of the message as its own little Caesar shift. A Beaufort does the same thing backwards, subtracting the letter from the key rather than adding, which is the sort of detail that makes a good day's work out of an afternoon.

The strip under the intercept is the key itself, whichever system is up. For a substitution it is all twenty-six letters, sorted by how much of the message each one is holding. For the periodic systems it is one tile per letter of the key, and every tile you fill in lights up every fifth or seventh letter of the whole dispatch at once, the length of it, which is a very satisfying thing to watch happen.

Why it is certainly breakable

Handing you a cipher and wishing you luck would be cheap. So before this page renders a single tile it attacks the intercept itself, with no knowledge of the key: for a substitution it hill-climbs, starting from a frequency-matched alphabet and repeatedly swapping the two letters that most improve a bigram score built from a couple of thousand lines of this circuit's own traffic, restarting from random alphabets fifteen times to shake itself out of local optima. For a periodic key it counts coincidences at every period up to twelve to find the key length, then picks each key letter by fit. If the attack cannot get at least ninety-five per cent of the letters back, the message is thrown away and another one is written. Whatever you are looking at got through that. The exact figure and the time it took are in the brief above.

The same date integer that draws the flow field on the front page writes this dispatch and enciphers it, and it is doing twenty other jobs besides — it grows the Garden, writes the Chimes, deals the Lattice, files the Almanac, scatters the Observatory, raises the Atlas, breeds the Menagerie, lights the Orrery, invents the Codex, runs the Chronicle, builds the Folly, cuts the gears of the Engine, threads the Loom, opens the Exchange, invents the rules of the Gambit, locks the doors of the Warren, arranges the corridor in the Inquest, cuts the gorge under the Span and works the timetable at the Junction. Nothing here is saved between visits. Tomorrow there is a different message, from a different desk, in a different hand.