5

Linden Hills, Minneapolis

With shaking hands, Salem smoothed the note from her mother on the bathroom counter of the third-floor apartment commandeered for police use. Bel stood watch at the door. Since they’d arrived at Grace’s, Bel’s gaze had grown hollow, her skin ashen, but she still held herself like a rod.

Bel blinked toward the note. “What’s it mean?”

Salem shot her a weak smile. “Not anything worth bothering the FBI about. I bet it’s a note Mom wrote years ago.”

Yet, she didn’t quite believe that, or they wouldn’t be here now.

She reached into her purse and tugged out a pen and pad of paper before grabbing the ancient spectacles. Their thin, rusting wire was wrapped around misshapen glass lenses. The ear bands were little more than metal sticks crafted of a copper so old it had turned green. Salem held them toward the light and squinted. The lenses were all scratched up. She set them to the side and pinned her attention on the note.

Bits: bwsmttmijwcbzmdmvombpmvowpwumnwttwebpmbziqtbzcabvwwvm

She was most comfortable solving computer problems. All the clean 1s and 0s could be perfectly lined up to crack a code as crisply as a key slid into a new lock. Her thesis research had taught her that Charles Babbage’s Difference Engine was conceived in 1822 and the computer program written for it by Ada Lovelace in 1842, but it was Turing’s Enigma cracker, first envisioned in 1936 and built in 1940, that demonstrated the code-breaking power of computing machines.

Alan Turing developed the apparatus for the British government during World War II. Turing’s machine built off of an earlier model to crack even the most advanced German code, effectively shaving at least two years off of WWII and creating the first working model of a general computer. Salem wished for a handheld model now to help her crack the code her mother had left.

“Agent Stone asked me if we knew where Grace or Vida were.” Bel’s words startled Salem. She glanced up from the note as Bel continued. “When we were both in my mom’s bedroom. I told him we didn’t know anything. We don’t, right?”

“Not yet we don’t.” Salem returned her attention to the message, clicking her focus back into processor mode. When she’d initially shown an interest in computers, she’d been excited to discover how many females had been involved in their development. Jean Jennings Bartik was one of six women who created programs for ENIAC, the first electronic general computer, in the 1940s. A decade later, Grace Hopper led the creation of COBOL, the original widely-used computer programming language. Computer science had been built on the work of women, who in the early years entered through the field of mathematics.

With computers, Salem felt like she was home.

But this code from her mom was old school, which was unsurprising given Vida’s general avoidance of computers. With all the ws and zs and js, it was unlikely to be a transposition cipher, where letters were jumbled to create an anagram. It was more probable that her mom had written her a substitution cipher, either a simple Caesar or a Vigenère.

“I don’t think the FBI knows anything either.” Bel pressed her ear to the door. “And it looked like the beginnings of a task force out there. Plus, the ME would have come and gone by now if this was a standard homicide. I don’t like any of this.”

Salem didn’t either. She tapped the pencil eraser on the paper while she pondered. In a Caesar cipher, each letter in the alphabet was replaced by a letter a fixed number of positions down the alphabet. So if the Caesar cipher had a right shift of four, every a in the code became a d, every b an e, every c an f, and so on. It was a fairly easy code to break by using frequency analysis, starting with short words. In English, for example, a single-letter word was only going to be I or a, and a three-letter word was most likely to be the or and. Once those letters were established, the cryptanalyst worked out from there, making educated guesses until the puzzle was solved.

Salem began chewing on the end of the pencil. She held up the note so she could examine it from different angles. A Vigenère cipher was a Caesar cipher on steroids. If one didn’t know the keyword, the code was uncrackable. At least it was until Babbage discovered that modular arithmetic and a dash of intuition could break le chiffre indéchiffrable.

Salem scribbled Vida’s note on her pad, trying the Vigenère cipher first, using Bits as the keytext. When that didn’t work, she switched to the simple Caesar cipher, testing every possibility in chronological order: right shift of one, right shift of two, right shift of three. It would have been easier if Vida had included spaces between words so Salem could run a frequency analysis, but she worked with what she had. Right shift of four, right shift of five …

When Salem arrived at right shift of eight, her heartbeat picked up. She felt the familiar buzz of a puzzle coming together.

Bel shifted at the door. “Any luck?”

Salem nodded. She was close. Letters were turning into words, words into messages.

“Okay.” Bel leaned her ear back against the door. “But can you hurry? We’ve been gone too long.”

“Yes.” Salem knew that word was the appropriate response, though she hadn’t processed what Bel said. She was almost inside the mystery. She could taste it. She scribbled furiously, decoding the substitution nearly as efficiently as a computer.

“Ah!” The solution flooded through all at once. Bits had thrown her off. Vida hadn’t meant it as her name. It was part of the code, which a plus-eight Caesar cipher revealed to be:

Talk: to Keller about revenge then go home follow the trail trust no one

The cold tongue of fear licked her spine. This was a fresh note, and her mom had intended for them to find it at Gracie’s. That meant that whatever tragedy had befallen Grace had also happened to Vida.

And she had known it was coming.

For the first time, Salem wondered why the FBI had been called in.

“Bel,” Salem whispered, “I don’t think our moms were randomly kidnapped.”

A heavy knock landed on the bathroom door.

Salem, wound tight, squealed. Bel snapped into a fighting stance, her expression steely.

“Ms. Odegaard and Ms. Wiley? Agent Stone would like to meet with the two of you in the lobby, if you don’t mind.”