The Book of Air and Shadows

Crosetti stood in the doorway and stared at his mother, who coolly (rather excessive,
suspicious
, coolness, Crosetti thought) introduced the man as Radeslaw Klim. This person rose to a considerable height, perhaps six inches more than Crosetti’s, and shook hands with a stiff little bow. The man had an intelligent aquiline face, a foreign face, although Crosetti could not have pinned down why it was not an American one. Washed blue eyes looked out through round wire-rimmed glasses, under a great shock of stiff silver hair, which stuck up above his broad forehead like the crest on a centurion’s helmet. He was about the same age as Mary Peg, or a little older, and he was wearing a baggy rust-colored suit with a dark shirt under it, no tie, the suit a cheap one that hung badly on his long slender frame. Despite this, the man had a nearly military bearing, as if he had temporarily misplaced his beautifully tailored uniform.
    Crosetti sat in an armchair and his mother supplied him with a glass of iced vodka, a substance for which he found an unfamiliar but urgent need. After he’d drunk a slug he looked challengingly at Mary Peg, who said blandly, “Mr. Klim is Fanny’s friend. I asked him to come by and take a look at your cipher. Since you were stuck.”
    “Uh-huh,” said the son.
    “Yes,” said Klim. “I have looked, examined it somewhat. As you have guessed, it is a polyalphabetic substitution cipher and also is true that it is not a simple Vigenère. That is of course elementary.” He had a slight accent that reminded Crosetti of Fanny’s; his mien was gentle and scholarly enough to at least partially assuage Crosetti’s nascent resentment.
    “So what is it?” Crosetti asked sharply.
    “I believe it is a running key,” said Klim. “From a book of some kind. You understand how these work? The key is of very long extent compared to the plaintext, so the Kasiski-Kerckhoff Method is of no use.”
    “Like a book code?”
    “No, this is not the same thing. A book code is a code. The codetext is, let us say, 14, 7, 6, and that means you go to
World Almanac
or some such and look at page 14, line 7, word 6. Or you can use letters if you like, the fourth letter, the tenth letter. A running key uses a book, the same, but uses the book text as a continuous key. These are not so secure as people think, however.”
    “Why not? It’s similar to a onetime pad.”
    Klim shook his head. “Not so. Onetime pad has very high entropy, because the letters are randomly generated. That is, given one letter of your key you have no idea which of the other twenty-six will follow. Whereas, in a running key based on any English text, let us say, if you see Q, what is next letter for sure?”
    “U.”
    “Exactly. Low entropy, as I say. How we break these is we run probable plaintext alongside ciphertext until we see something intelligible.”
    “What do you mean by ‘probable plaintext’?”
    “Oh, words always appearing in English text.
The, and, this
, and so on and so forth. We run against ciphertext and suppose we find once that
the
gives us
ing
or
shi
when we work back through the tableau? We use such clues to discover more English words in key. Eventually we recognize actual source of running key, I mean, the book it comes from, in which case we have completely broken cipher. It is not very complex, but we would need a computer, or else large squads of intelligent ladies.” Here he smiled, showing small stained teeth, and his glasses glinted. Crosetti got the impression that Klim had at one time supervised such squads.
    “Would mine do?” asked Crosetti. “My PC, not my squads of ladies.”
    “Yes, if networked to others, which can be done. There are numbers of people in the world who like cracking ciphers for amusement and they will let one borrow computer cycles they are not using, late at night for example, and is always late at night somewheres. I can set this up if you like. Also, we are fortunate this is cipher from the year 1610.”
    “Why so?”
    “Because there are many less, many
fewer
, printed texts that could be used as running key source. In fact, taking what your mother has informed me of the character of these people, I would venture that the text is almost certainly the English Bible. So, shall we begin?”
    “Now?”
    “Yes. Is there objection?”
    “Well, it’s kind of late,” said Crosetti.
    “Does not matter. I sleep very little.”
    Mary Peg said, “I’ve offered Radeslaw