A team of codebreakers has managed to decrypt mysterious 170-year-old messages, published as adverts in UK newspaper The Times. Amazingly, they turned out to be related to a rescue mission in the Arctic, sent to try and recover the infamous Franklin expedition, in which Lord John Franklin and all his 129 crew members died.

Elonka Dunin – a codebreaker so well known that a character in Dan Brown's The Lost Symbol is named after her – and her fellow cryptographers Klaus Schmeh and A.J. Jacobs set their minds to the task of decoding a series of messages sent between March 1850 and March 1855. The messages – which usually started with "S lmpi" and ended with "J de W" – went unbroken by everyone but whoever they were intended for, for well over 100 years.

No. 16th.-S.lkqo. C. hgo & Tatty. F. kmn at npkl F. qgli Ingk S mhn F. olhi E qkpn. S. niql s mnhq F. qgli. Austin S pgqn C. kioq 6th F. iqhl. born. 13th F. kipo a F khg. hmip. to E. mlhg by D oi. S. pkqg C omgk B. hkq. qkng F. ioph. to hnio. S. ompi C. mkop F. oiph to Mr. C. nhmg & F. mpkh. nmkq E. lhpq. J. de w.

A message printed on October 1, 1851

Then, in 1980, The Times published the coded message once more, asking their readers to attempt to crack it. Again, nobody managed it, but several people did point out something intriguing: "a possible connection between a plaintext latitude and longitude and expeditions to find the North West Passage", according to a paper published in the journal Cryptologia in 1992.

Here's where the infamous Franklin Expedition comes in. On the morning of May 19, 1845, the Franklin Expedition set sail in an attempt to navigate their way through the Northwest Passage, a route connecting the Atlantic and the Pacific through the Arctic Ocean. When they became trapped in the ice, many of them were able to survive for a whole two years on supplies they had brought with them, but ultimately all crew members died. 

Five years later, naval officer Admiral Sir Richard Collinson led an expedition to find the lost ship, looking for Franklin and his doomed crew in the Canadian Arctic. The dates for this failed mission – 18050-1855 – entrants to the Times' 1980 competition noticed, lined up nicely with the publishing of encrypted ads. The theory, which eventually won first prize for a retired judge who entered the competition, was that people on Collinson's expedition were using it to communicate updates with their financial backers.

However, when codebreaker John Rabson attempted to decode the message in 1992, what he was able to decode did not line up with reporting to financial backers.

"Your wife and family were all well when I left Bernard and Tatty both at home," one read, while another informed the reader, once they had cracked the code, "Lady Peel husband was killed last month by fall from his horse".

Another decade after this, Elonka Dunin and her team picked up the trail. During a talk at the Hackers on Planet Earth conference – seen by Motherboard – she explained that looking at the groups of letters they noticed similarities to the Marryat Signal Code used by ships to send encrypted messages to each other using flags. The flags represent different numbers. By raising the flags in sequence, a message can be spelled out using a cipher, which can then be decoded using the same cipher on the receiving ship. Dunin applied numbers to the letters in the encrypted ads, and sure enough, this was the system they were using.

The messages that had carefully been encrypted and then placed in a newspaper were still more of the same.

"I wish to try if you can read this and am most anxious to hear that and, when you return, and how long you remain," one message read. "Do write a few lines darling, please. I have been very far from happy since you went away."

Dunin's investigation confirmed that the messages were used to convey updates about family life, from people at home to people on board the Collinson expedition.

"Prior to [Collinson's] multi-year journey, there had been debate about how his family was to stay in touch with him, especially because he was always on the move," Dunin explained on Facebook. "They came up with something quite creative. They knew that pretty much anywhere he went, he would be able to obtain copies of the newspaper The (London) Times."

"So his family would put encrypted classified ads in the paper, and then whenever he was in port, he could find a copy of the newspaper, and find out how people were doing."

The system meant that Collinson could communicate with home as long as he could read the Times, wherever he landed. Encryption meant that nobody else could see the contents. Many of the messages remain un-broken, but Dunin – and others – are looking into it.