Author's Note: Hello! As an FYI there's a historical note waiting for you at the bottom of this fill :D Enjoy!

Disclaimer: The following characters belong to J.K. Rowling, and this story derives from her original works, storylines, and world. Please do not sue me, I can barely pay tuition. The poem you see here is by Emily Dickinson as well.

Hogwarts: Assignment #10, Women's History Task #2, Write a World War II!AU

Warnings: NA

This week's AU: WWII

An Exchange of Keys

Remus was not sure if his next meal would be breakfast or dinner and, quite frankly, he was good enough at ignoring the rumbling of his stomach not to care too much. Besides, when he was this deeply embedded in a project or task he was impossible to pry away. His fellow cryptanalysts—even Peter, James, and Sirius—had stepped out of the office for their breaks but had left him alone at his desk.

No matter how all-consuming his work was, he did notice a new presence in the office. There was a woman standing in the office's entrance, wearing a dark green uniform jacket. Her mousy brown hair was set in two reverse rolls on either side of her heart-shaped face before being tied back in a neat chignon at the nape of her neck. Otherwise her uniform was immaculate; the white collar she wore under her jacket was crisp and pressed, her tie neatly knotted, and the skirt that ended just below her knee perfectly pleated. One of her arms was cradled against her chest in a sling, the other held a teacup on a saucer. Her wide brown eyes were scanning the corkboard by the door, which was covered in quite sensitive information—maps of occupied France, announcements and messages from the higher-ups, promising drafts and algorithms that they were keeping from later…

"Excuse me," Remus spoke up. His free hand reached for the pistol in the top drawer of his desk, just in case. "Are you authorized to be here?"

"I am," she said. She reached in her jacket and flashed Remus a badge that he couldn't quite see from this far away. Still, she held it up confidently and long enough to give Remus time to examine it. She inspired far more confidence when she added, "Commander Moody grounded me here while I wait for safe passage back to London. I am not very useful with a broken arm."

"I see," Remus said.

"He thought that perhaps I would enjoy visiting the facility, as I am not very often here. Then again, he also said he would lock me in the basement if I disrupted hisoperations," she added—which was intensely reassuring to Remus since yes, that was exactly something that Moody would say. "So please, Don't mind me. As you were."

Remus hesitated but turned back to the code before him. He was fairly certain that he had found his point of entry—the tiny sliver of sense in the scrambled nonsense of a code that might give him an opportunity to understand and read the rest. In a simple code, for example, perhaps it was a frequently used letter or a frequently repeated syllable that indicated a commonly used term or letter. If he tugged at it properly, the entire code would unravel like a sweater. Or maybe it wouldn't. If he was truly on the right path, Remus had already unscrambled four levels of security in the nonsensical message he had started with. Then again, perhaps he was wrong and he was merely rearranging the alphabetical soup before him into a new equally soupy sequence.

The possibility bothered Remus; they had already missed the mark on four of the messages that had been smuggled or leaked to them. It frustrated him, that they kept losing the messages. They had either never managed to properly decode the chaotic gibberish they had been written in, or they had been too late in unraveling their meanings and relevance—and every time, the code changed ever so slightly as Vichy France's troops went on with their business.

Part of the work that went behind decoding was an acceptance that you could sink weeks or months into trying to understand a cypher, only for it to all be for nothing. Remus sometimes felt achingly close to putting order in the chaos before him only to have Commander Moody put him on another task as their unit's needs changed. But every letter before Remus seemed to take on more weight as the war went on.

When he looked up again, the woman had drifted across the room. She was now at the desk where James sat, putting his noble education to use by translating documents from French or German to English. Stacks of papers and folders piled one on top of the other littered the desk, and the woman's eyes were wandering again. She had put down the teacup's saucer so that she could drink with her unbroken arm.

"Excuse me," Remus asked again. She turned to look at him. He wasn't sure how to explain what he had to say next; he was still adjusting to the military's ways of speaking and being. He decided to simply be frank. "I… pardon me, but I'm having trouble placing you. The decoders and cryptanalysts, we don't usually wear uniforms."

"I'm not one of you," she said. Her fingertips brushed the paperwork on James' desk—equations and letters connected by lines in charcoal and ink. Then, she crossed the floor and held out her hand to shake his. "Lieutenant Nymphadora Black Tonks."

"Remus Lupin," he said. "I… I taught mathematics before being recruited to His Majesty's Services."

"Wonderful," she said. "I was always rubbish at mathematics."

"Another reason why you are definitely not a cryptanalyst, then," Remus said.

Lieutenant Black Tonks laughed.

"I'm an intelligence officer," she said. "And I'm quite good at that, which is a relief."

"It's curious that we haven't met, then," Remus said. He had been decoding since the war had started. He was not a healthy man—a particularly strong or fast or robust man. But he was clever and patient and persistent, and was happy that there was a way for those strengths to help as the world broke into war.

"We have, in a way," the Lieutenant said. She crossed her arms and smiled, nudging her chin towards the paperwork on his desk. "My codename is Giselle."

Remus looked down at the code he had been wrestling with and then back to the lieutenant, jaw slack.

"You… you're Giselle, then?" he asked. "The spy feeding us, well, most of our best information..."

"You'll have to confirm what that bit of paper says before we can be quite sure, but I've been posing undercover as a maid in a lovely little hotel that the Nazis have decided to set up shop in," she said.

"I've decoded most of the intelligence you bring back," Remus admitted. It came in at 6:30 a.m. sharp—Remus didn't know the mechanism of delivery; just knew that one of Commander Moody's clerks brought Remus the code every morning as his tea steeped.

"Have you?" she said, smiling. "Well then I'm glad we met before Moody sent me back home, then. I don't think I'll be posted here again; I got into quite a nasty bar fight and got fired from my supposed job to get that last message to you. If it's half as rich as what Moody expects it is, then it will all have been worth it"

"I'm still working on it," Remus admitted. He sighed. "We're hoping to extract a key from it, so that we can understand future messages better and more easily."

"I take it from your tone that it's not going well," she said.

"If it were easy to speak the language of Vichy France's intelligence, we wouldn't still be here fighting a war," Remus said.

"I suppose that's true," Nymphadora said. "And what boring lives you and I would lead then."

Remus smiled and eyed the code again.

"The trouble here is that I think you have brought back a message which does not use a symmetric key," Remus said.

She looked around the office—the chalkboards covered in chalk, the folders full of intercepted messages that still hadn't been looked at, baskets full of expired papers…

"Let's pretend that I am very good at getting into places I have no business and then behaving poorly—but that I don't know very much about codes, aside from how to obtain them," she said.

"Oh, yes, of course," Remus said. "Very sorry."

"No, not at all,. Dora said. "I admit I don't spend much time thinking about where the intelligence I recover goes once I pass it on. Come to think of it, I suppose it isn't quite intelligence until it's been through you lot and made useful. I would love to learn."

"A key is what we use to encrypt or decrypt messages," Remus said. "Some keys are simple, say—Morse code."

"Dits and dots in different sequences each correspond to a letter of the alphabet," she said.

"Exactly," Remus said. "Some are more complicated, for example… mmm… Let me show you how a Beale cipher works. Do you have a favourite line of poetry, perchance?"

She arched an eyebrow, but Remus passed her a pen and paper. She wrote it down. When she returned the pad to Remus, he immediately began numbering the letters in the poem, skipping repetitions. He didn't necessarily have to, but it was a preference of his and he thought it worked quite well as he surveyed the piece of poetry she had given him.

Had I not seen the Sun

I could have borne the shade

But Light a newer Wilderness

My Wilderness has made

"Right; let's number all the letters in the poem. H would be 1," Remus explained. "A 2, D 3, I 4, N 5, O 6, T 7, S 8, and so on, and so forth."

"What about the letters that aren't in the poem?"

"Well that would be for you and I to decide together, if we were corresponding," Remus said.

"How exciting," Nymphadora smiled. Remus blushed and tried to ignore the fact that corresponding with a spy… well, it most likely would be very exciting.

"Yes, well, perhaps we would pick a larger text, one where all the letters appeared. Perhaps we would round up all the letters that don't appear in your poem and assign them numbers. But for our purpose now, let's try to write something simple…"

He wrote down the name GISELLE and added a number above each letter.

1 12 12 9.

"Were I sending that message I would give you a different name, to avoid repeating letters and making the code any easier to crack than it must be," Remus said. "And of course, we could have further subcategorized the letters by word to add an extra layer of protection. This is a rather simple Beale cipher—the name of this kind of cipher, but you get the idea of it."

"Hmm," she said. "I… I did not expect to wake up this morning and write my name with a poem."

"Right," Remus nodded. "Well, in this case, your favourite poem would be our key for both encryption and decryption. You would look at the poem to write your name and I would look at the poem to turn the numbers into letters. I suspect that one key was used to encrypt this message, but we need another one to decrypt it."

She furrowed her brow.

"Imagine that I showed you my favourite poem, and we numbered all the letters in the same way," Remus said. "In your poem, 'd' was represented by the letter 3. In mine, it would be 1. You would scramble the letters of the message you wanted to read with my key, and then scramble those scramble letters according to your key. It's… it's complicated, and the codes that we're dealing with aren't Beale ciphers at all."

"I don't envy your job," she said.

"I promise it's easier when you hold multiple degrees in mathematics and dabbled in linguistics," Remus said. "Well, somewhat."

She laughed. She picked up the pad of paper and perched herself on the corner of his desk, awkwardly balancing the paper on her leg as she wrote with her good hand.

"What are you doing?" he asked.

"Writing you more poetry," she said. "I'm going to want to hear more from you."

She pulled the piece of paper from the top of the notepad and folded it in two, tucking it into the breast pocket of his button-down shirt. Then, she passed the paper back to him expectantly.

"Well," she finally said. "What was that poem you liked where D equaled 1?"

Remus smiled and took the pen and paper from her, scribbling out the words.

When he handed the paper back to her she smiled before folding it up and tucking it into her uniform blouse.

"A very good poem," she said. She slid off his desk. "I'll be expecting some very good letters then while I rot in London and wait for my next assignment."

Before Remus could answer, she slipped away just as his colleagues filed back in. It was as if Remus blinked and she was gone. James had brought Remus a cup of tea and dropped it on his desk.

"Who was that?" James asked.

"Her name is Giselle," Remus said. "She's an old friend."

Quick note because this recovering history major would rather jump out a window than write a historical AU without a note on context. First off, yes, The Imitation Game is one of my favourite movies. Secondly, I want to note the gendered dynamics of this fill; while women definitely worked as spies during the Second World War, there were also a disproportionate amount of women who worked as clerks, translators, and codebreakers to supply and navigate various intelligence reports. Many women faced everyday sexism while doing this kind of work, were barred from advancing and growing in their positions due to educational barriers, and the overflow of women in this sector came from a perception that this behind-the-scene work was dull, repetitive, and unheroic. Anyways, I'd encourage you to give "women codebreakers WWII" a quick Google! It's all very interesting. There's tons of resources about code breaking during the Second World War if you're interested, but let it be known that I did make up the Vichy France operation entirely.

Shipping Wars

Word count: 2177

Ship (Team): Remus Lupin/Nymphadora Tonks (Technicolour Moon)

List (Prompt): Spring Micro 1 (Hackers/Programmers AU)