Shadow of the Hegemon, the - Book 2 (Ender)

then it had to be in some kind of code.
    For the next few hours Bean wrote programs to help him manipulate the data contained in those lines. He tried mathematical schemes and graphic reinterpretations, but in truth he knew all along that it wouldn't be anything that complex. Because whoever created it would have had to do it without the aid of a computer. It had to be something relatively simple, designed only to keep a cursory examination from revealing what it was.
    And so he kept coming back to ways of reinterpreting the binary code as text. Soon enough he came upon a scheme that seemed promising. Two-byte text code, but shifted right by one position for each character, except when the right shift would make it correspond with two actual bytes in memory, in which case double shift. That way a real character would never show up if someone looked at the file with an ordinary view program.
    When he used that method on the one line, it came up as text characters only, which was not likely to happen by chance. But the other line came up randomseeming garbage.
    So he left-shifted the other line, and it, too, became nothing but text characters.
    "I'm in," he said. "And it is a message."
    "What does it say?"
    "I haven't the faintest idea."
    Carlotta got up and came to look over his shoulder. "It's not even language. It doesn't divide into words."
    "That's deliberate," said Bean. "If it divided into words it would look like a message and invite decoding. The easy way that any amateur can decode language is by checking word lengths and the frequency of appearance of certain letter patterns. In Common, you look for letter groupings that could be 'a' and 'the' and 'and,' that sort of thing.
    "And you don't even know what language it's in."
    "No, but it's bound to be Common, because they know they're sending it to somebody who doesn't have a key. So it has to be decodable, and that means Common."
    "So they're making it easy and hard at the same time?"
    "Yes. Easy for me, hard for everyone else."
    "Oh, come now. You think this was written to you?"
    "Ender. Dragon. I was in DragonArmy, unlike most of them. And whom else would they be writing to? I'm outside, they're in. They know that everyone is there but me. And I'm the only person that they'd know they could reach without tipping their hand to everybody else."
    "What, did you have some private code?"
    "Not really, but what we have is common experience, the slang of Battle School, things like that. You'll see. When I crack it, it'll be because I recognize a word that nobody else would recognize."

    "If it's from them."
    "It is," said Bean. "It's what I'd do. Get word out. This picture is like a virus. It goes everywhere and gets its code into a million places, but nobody knows it's a code because it looks like something that most people think they already understand. It's a fad, not a message. Except to me."
    "Almost thou persuadest me," said Carlotta.
    "I'll crack it before I go to bed."
    "You're too little to drink that much coffee. It'll give you an aneurysm."
    She went back to her own mail.
    Since the words weren't separated, Bean had to look for other patterns that might give things away. There were no obvious repeated two-letter or three-letter patterns that didn't lead to obvious dead ends. That didn't surprise him. If he had been composing such a message, he would have dropped out all the articles and conjunctions and prepositions and pronouns that he possibly could. Not only that, but most of the words were probably deliberately misspelled to avoid repetitive patterns. But some words would be spelled correctly, and they would be designed to be unrecognizable to most people who weren't from the Battle School culture.
    There were only two places where the same character was apparently doubled, one in each line. That might just be the result of one word ending with the same letter that began another, but Bean doubted it. Nothing would be left to chance in this message. So he wrote a little program that would take the doubled letters in one word and, beginning with "aa," show him what the surrounding letters might be to see if anything looked plausible to him. And he started with the doubled letters in the shorter line, because that pair was surrounded by another pair, in a 1221 pattern.
    The obvious failures, like "xddx" and "pffp," took no time, but he had to investigate all the variants on "abba" and "adda" and "deed" and "effe" to see what they did to the message.