Dress code: How a Winnipeg codebreaker cracked one of the 'world's top unsolved messages'

"Bismark Omit leafage buck bank."

That seemingly random string of words appears in something called the Silk Dress cryptogram, 23 handwritten lines on two sheets of crinkled paper that were discovered in a hidden pocket of a Victorian-era dress bought in Maine in 2013. 

The lines seemed like an encoded message from the late 1800s, with references to North American cities, including Calgary and Winnipeg.

"There were lots of theories [about its meaning], from the American Civil War to simply instructions for dress-making," said Wayne Chan, a University of Manitoba computer research analyst.

For about a decade, it stumped the global cryptanalytic community and was even listed as one of the "World's top 50 unsolved encrypted messages" on the cryptology blog Cipherbrain.

Chan says he was drawn to the mystery. "Why did this woman have a bunch of secret codes in this pocket in her dress?"

He cracked it in February, and his conclusions were published in the cryptology journal Cryptologia.

The mystery of the Silk Dress cryptogram starts in 2013 in Searsport, Maine, when dress collector Sara Rivers-Cofield decided to buy a long, flowing 1880s-era silk bustle dress she had been eyeing at an antique mall.

"I knew it had been there for a while," she told CBC. "I brought it home with my mom and it was sort of the newest big-deal acquisition for my collection."

When they examined the dress to figure out how it was put together, Rivers-Cofield and her mother found two pieces of crumpled paper concealed in a secret pocket under the overskirt.

"It's a bit of a private spot — it almost seems like it was protected," she explained. "It said 'Bismark Omit leafage buck bank.' It was just nonsense. So we were like, what's going on?"

She put it on a dress collecting blog she ran at the time.

"It's certainly the most popular blog post I ever did, because the code-cracking community picked up on it pretty quickly," she said. "Somebody said, 'That's a telegraph code.'"

Still, it would take nearly a decade for someone to solve it.