Australian mathematician reveals how he cracked the Zodiac Killer's coded message after 50 years

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A Melbourne code-breaker bored by the COVID-19 lockdown has cracked a 50-year-old riddle left by the infamous Zodiac Killer.

The infamous serial killer murdered at least five people in northern California, U.S, during 1968 and 1969.

The Zodiac was never caught despite sending a series of taunting letters and word puzzles to San Francisco newspapers up until 1974.

Some were accompanied by bloodied fragments of clothing as proof.

One of the four ciphers was solved in 1969, in which the Zodiac said he was killing people to collect as slaves for the afterlife.

The remaining three cryptograms have captivated amateur sleuths for more than 50 years in the hope that solving them may lead to the identity of the killer.

Melbourne mathematician Samuel Blake along with two other code breakers from the U.S and Belgium have now cracked the 340 Cipher, a 340-character puzzle sent by the serial killer to the San Francisco Chronicle on November 8, 1969.

The shocking fragment that led to them cracking the code were the chilling words 'GAS CHAMBER', he said.

Dr Blake got interested in the Zodiac's riddles after seeing online videos from U.S. cryptologist David Oranchak who has been trying to crack the code for 15 years.

'It was sort of a way to get through the Melbourne covid lockdown, was to play around with this in my spare time,' Dr Blake told ABC News on Saturday.

Dr Blake, a visiting fellow at the University of Melbourne, reached out to Mr Oranchak in March then worked on decoding the message using the university's supercomputer Spartan.

The team tried 650,000 different ways to solve the cypher.

'Just by chance we happened to stumble upon a fragment of how it could be solved,' he said.

The macabre fragment that popped up in the sea of noise was 'GAS CHAMBER'.

'Using that fragment we were able to reverse engineer the entire solution, and got the entire message out from the Zodiac,' he said.

The Zodiac's riddle was a homophonic substitution cipher, where large areas of text was replaced with symbols.

Dr Blake said the Zodiac's trick that made it so hard to solve was that it was not written left-to-right and top-to-bottom as English text is normally read.