A group of codebreakers have managed to crack the infamous 340 cipher used by the Zodiac Killer over 50 years ago.

In the late 1960s, heading into the early 1970s, a serial killer going by the pseudonym "Zodiac" murdered at least five people in California. During his spree, the killer sent taunting messages to the press written through a cipher, where letters are substituted for different letters or numbers (or in the case of the Zodiac killer, a series of symbols). 

The first letters were sent to three papers in the Bay Area, each containing a different part of the cipher. The papers printed the ciphers (or cryptograms) as Zodiac threatened to kill again if his instructions weren't followed.

The cipher, which was simpler than later efforts, was solved a week later by hand on August 8, 1969, by Donald Gene and Bettye June Harden. It read:

"I LIKE KILLING PEOPLE BECAUSE IT IS SO MUCH FUN IT IS MORE FUN THAN KILLING WILD GAME IN THE FORREST BECAUSE MAN IS THE MOST DANGEROUS ANIMAL OF ALL TO KILL SOMETHING GIVES ME THE MOST THRILLING EXPERENCE IT IS EVEN BETTER THAN GETTING YOUR ROCKS OFF WITH A GIRL THE BEST PART OF IT IS THAT WHEN I DIE I WILL BE REBORN IN PARADICE AND ALL THE I HAVE KILLED WILL BECOME MY SLAVES I WILL NOT GIVE YOU MY NAME BECAUSE YOU WILL TRY TO SLOW DOWN OR STOP MY COLLECTING OF SLAVES FOR MY AFTERLIFE EBEORIETEMETHHPITI".

Based on this, it's pretty incredible that he was able to come up with a cypher complicated enough to fool the FBI for years but couldn't put the effort in to give his spelling a quick double-check, or maybe ask a friend to: "look, try not to read too much into the content but can you have a quick check that my grammar's ok".

The letter contained no real clues, other than that the killer had some fairly odd delusions going on, needed to work on his spelling, and may have been inspired by The Most Dangerous Game, a movie about a man who is hunted by an aristocrat for kicks.

After the first message, the killer's ciphers became more complicated. One, which contains 340 characters, has managed to elude decryption for 51 years.

A team of codebreakers decided to try and crack the code specifically because they knew it would be a challenge. 

David Oranchak, Sam Blake, and Jarl Van Eycke used software to help them break the cipher, first by finding the many possible reading directions that could be used if the cipher was transpositional. By sheer luck, Oranchak found that one solution for how the cipher could be transposed revealed fragments of messages, including "hope you are," "trying to catch me" and "or the gas chamber".

This gave them clues that the message wasn't transcribed in one big block as it was presented, but instead was broken into three smaller blocks of text made up of nine lines, followed by nine lines, followed by a final two.

By starting in the top left hand corner, then moving down one line and across two spaces to get the next letter, a key which could be translated into letters and then words emerged. The letter "B" for instance, was represented by "?7", "c" by a simple "9" and "A" by a whole load of symbols unavailable on a keyboard. You can see these neatly shown in the video released by the team below. 

Through use of this method, and some slight adjustments by ignoring a few words that stood out before transposing the text, a message was revealed:

I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME
THAT WASNT ME ON THE TV SHOW
WHICH BRINGS UP A POINT ABOUT ME
I AM NOT AFRAID OF THE GAS CHAMBER
BECAUSE IT WILL SEND ME TO PARADICE ALL THE SOONER
BECAUSE I NOW HAVE ENOUGH SLAVES TO WORK FOR ME
WHERE EVERYONE ELSE HAS NOTHING WHEN THEY REACH PARADICE
SO THEY ARE AFRAID OF DEATH
I AM NOT AFRAID BECAUSE I KNOW THAT MY NEW LIFE IS
LIFE WILL BE AN EASY ONE IN PARADICE DEATH 

"Of all the things that stood out was the line 'that wasn't me on the TV show'," Oranchak explained in the video. "At this point I jumped out of my chair because I knew the cipher was received on November 8, 1969, which is about two weeks after someone calling themselves Zodiac called into a TV talk show hosted by Jim Dunbar. While the caller was on the air, he said 'I need help, I'm sick, I don't want to go to the gas chamber.'"

This for Oranchak made the solution seem real, as it fit with the events around the time it was received. The rest of the message also seemed quite in character for the Zodiac Killer. 

The team have submitted it to the FBI, who have acknowledged it publicly, though haven't released further information on it just yet.

The team behind the decryption add that they're disappointed that the cipher didn't contain any information (that they've found so far) that could be used to identify the killer, just "more of the attention-seeking garbage" from Zodiac.

"The very end doesn't flow quite right yet," Oranchak adds. "But maybe someone can figure out how it's supposed to read."