It takes Carl two full weeks to set up the prank on Mother, sweating under Crease's raised eyebrow the whole time. It's an interesting challenge, and one Carl relishes; besides that, though, Crease has promised a great reward ("get him so good he shuts up for a full day, and I'll make sure you have reservation for two at Citronelle") and Mary's birthday is coming up next month. A little undercover work is a small price to pay.
He starts by joining Mother's favorite BBS and spending a couple of nights checking out his messages and posting history. It's time consuming, not least because he keeps getting distracted reading discussions about crop circles in the shape of the Stay Puft Marshmallow Man and secret FBI meetings with purple aliens living inside the Earth. Eventually Carl gets a fair idea of which news groups would be the best places to plant the seeds of mischief, and the names of a few group members he can probably convince to help, if he needs them.
Writing the encrypted text proves more difficult, especially given Crease's specific instructions about the hidden message. They're not on a job right now, though (good for the prank but less so for his bank account), and he crams down enough reference material to make the thing plausible, at least on the surface - history, classical cryptography, even music theory. It's actually more interesting than he'd thought it would be, and Carl gives at least two seconds worth of thought to going back to school. Then he remembers why he left and goes back to drinking coffee and sitting with his feet on the desk.
He almost gives the game away twice - once when he's making notes about the encryption and forgets about the research he's actually paid to be doing (part of their shtick in attracting business involves being up on the latest tech developments), and once when he doesn't realize Mother's in the room at dinner and starts talking to Liz about eighteenth century painters (Marty rolls his eyes at this, though, and Carl knows he's successfully played it off as infatuation).
When he's done, he double and triple checks the transcription and posts the document anonymously on three different news groups. Then he logs back off and on again under his second name to call his first name an idiot, and waits for the reaction. If he can keep from laughing until Mother hits the punchline, it'll be a miracle, but he's tried to make the encryption both sophisticated enough to be plausible and simple enough that it won't take the conspiracy nuts more than a few hours to crack it.
"You're never gonna believe this, guys," says Mother ten minutes later, after he's dialed in to check his messages. "Some guy in Latvia just found a booklet containing the initiation rituals of the Bavarian Illuminati." No one bothers to respond, since they've heard this stuff from Mother before, and he turns from the screen briefly to give them all shifty looks. "You know, the secret society that's acting as the power behind the modern governments of Europe? Once we decode this thing, we'll finally be able to prove that Reagan was a cyborg."
Crease bites his lip but doesn't respond, and Carl can see that this is what tips Marty off. He meets Marty's eyes across the room and shrugs. Marty grins and nods back - letting Carl know he'll let this play out. Whistler's in on it, too - he leans back in his chair and smiles the way he always does, when he knows something the others don't.
"Oh?" he says to Mother.
"'Course," says Mother, absentmindedly, his hands already working determinedly on the keys. "I mean, we've know that for years, but with proof..."
"Mmm?" Whistler hums, questioningly, just to keep him going.
"And the thing is, I've been saying for ages that Buck's algebraic cryptography was used by the Illuminati. Keyless cryptographic puzzles, solvable only by the elite because of their training and education. What better for a group who called themselves the Perfectibilists?" Mother snorts. Carl can tell he's hit the jackpot with this subject; Mother never liked the elite, especially after his eighteen months in jail.
"It's time consuming to decode by hand, but for a document like this that they wouldn't need every day, it's an extra layer of security. Having our computing power means it won't take all day; once the government declassifies the formulas carved inside the pyramids, I'll be able to do this even faster." He taps the keys, frowns, then deletes a line furiously and types something else in (Mother, for someone so good with his hands, is a surprisingly terrible typist).
At this point Carl has to leave the room or he's going to burst, so he offers to run down to the corner bakery and is immediately innundated with requests.
"That's three black coffees, two cheese danishes, and a powdered sugar donut - anything for you, Mother?"
"No, uh... no."
Crease offers up cash for the food silently, seeing Carl's growing nervousness. Carl gives him a terse nod and almost runs to the corner. When he comes back, though, he hesitates outside the office, unable to decide whether he'd rather be in the room when it all comes out or not.
Eventually he goes back in, only to find Marty leaning over Mother's shoulder peering at the screen. Carl gulps, but Marty's only asking a technical question. Whistler's got his head cocked, too, paying close attention to the answer.
"So you're running this through a modified Babbage engine?"
"Yeah, I built it last year when we were looking at that Bank on Q street. Thought we might need it to do frequency analysis before we could get into their system. Didn't need it, though - they really ought to work on password security. Anyway, I built one of these things out of Tinker Toys when I was a kid. Dead easy."
Marty gives him an incredulous look. "Right," he says.
"I did," Mother insists. "Well, not one big enough for this kind of work. Same principle, though. Ah," he says. "Got something." Everyone crowds around except for Whistler, coffee and donuts forgotten. "Let me just output the plaintext..."
The message comes up on the screen, and seeing it there in stark black and white makes Carl suddenly a little afraid for his life. Despite Mother's weird beliefs, it wouldn't surprise Carl to find out that his coworker knows twenty seven ways to kill him in his sleep.
"All right, very funny," says Mother. His mouth twists to one side and he shakes his head ruefully.
"What's it say?" asks Whistler.
Marty clears his throat. "It says, uh. It says, 'your Mother wears army boots.'" Whistler lets out a laugh that turns quickly into a cough.
Behind Mother's head, Crease gives Carl a nod, then pulls out a piece of paper and passes it over - it's got a date on it, and a confirmation number.
"Your reservation," he says, smirking. Carl takes it.
"Of course," says Mother, brightening, "this means I'll have something new to hold over Greeney, my guy in London. He's not even awake yet. Maybe I can get him to trade the info about this for those pictures of the grassy knoll he's been promising me."
Crease tries to snatch the paper back, but Carl tightens his fist around it.
"Not even a full minute!" says Crease. "Much less a full day."
"Too late," says Carl, smiling with the satisfaction of a job well done. He jams the paper in his pocket and turns back to the table to pick up his coffee. "Donut?"