It took 51 years to crack, but one of the taunting messages written in code and attributed to the Zodiac Killer has been solved, according to the F.B.I.
The mysterious 340-character cipher, which was mailed to The San Francisco Chronicle in November 1969, does not reveal the killer’s identity. But it does build on his image as an attention-seeking killer who reveled in terrorizing the Bay Area in the late 1960s.
“I hope you are having lots of fun in trying to catch me” and “I am not afraid of the gas chamber” are two of the dark boasts in the message, according to David Oranchak, a software developer in Virginia who said he had decrypted the cipher with the help of Sam Blake, an applied mathematician in Melbourne, Australia, and Jarl Van Eycke, a warehouse operator and computer programmer in Belgium.
Mr. Oranchak, who runs a website and YouTube series about the Zodiac Killer’s ciphers, said he was excited to have solved the code after 14 years of trying to break it. But he said he was also worried about the effect it might have on victims’ families.
“The message in that cipher — I don’t see it as being helpful to them,” he said. “It’s more of the same junk that the killer liked to write about. It’s just intended to hurt people and make them afraid.”
The F.B.I., which employs a team of code-crackers in its Cryptanalysis and Racketeering Records Unit, said it had verified Mr. Oranchak’s claim of having broken the code, known as the 340 cipher. The agency said the cipher was one of four attributed to the killer, and was first submitted to an F.B.I. lab on Nov. 13, 1969.
The bureau said it had received the solution on Dec. 5 from a cryptologic researcher.
“Over the past 51 years, C.R.R.U. has reviewed numerous proposed solutions from the public — none of which had merit,” the F.B.I. said in a statement. “The cipher was recently solved by a team of three private citizens.”
The F.B.I.’s San Francisco field office also released a statement on the breakthrough, which was reported by The San Francisco Chronicle on Friday. The statement said the field office was aware that a cipher attributed to the Zodiac Killer “was recently solved by private citizens.”
“The Zodiac Killer case remains an ongoing investigation for the F.B.I. San Francisco division and our local law enforcement partners,” the office said. “The Zodiac Killer terrorized multiple communities across Northern California and even though decades have gone by, we continue to seek justice for the victims of these brutal crimes.”
The San Francisco office said it would not comment further because of “the ongoing nature of the investigation, and out of respect for the victims and their families.”
The code had long baffled cryptographers, law enforcement agents and armchair sleuths obsessed with the shadowy killer, who was blamed for five murders in the late 1960s. Only one previous cipher attributed to the Zodiac had been solved, and it was decoded by a California couple not long after it was sent in the 1960s.
This one was considered much more complex, suggesting the killer was frustrated that the first one had been deciphered so easily, Mr. Oranchak said.
The team that cracked it came together earlier this year, Mr. Blake said, after he reached out to Mr. Oranchak with some ideas on how to unravel the patchwork of symbols and characters.
“It’s considered one of the holy grails of cryptography,” Mr. Blake said. “At the time, the cipher had resisted attacks for 50 years, so any attempts to find a solution was truly a moonshot.”
For months, Mr. Blake said, he and Mr. Oranchak tested, by trial and error, around 650,000 possible solutions, running them through a code-breaking program written by Mr. Van Eycke.
But the program turned up nothing until it suddenly yielded a surprising combination of words on Dec. 3, including “gas chamber” and “trying to catch me.”
“That’s what caught our attention,” said Mr. Oranchak, who explains the team’s code-cracking process in a YouTube video. “That was the key.”
Mr. Oranchak said he was astounded when the decrypted message revealed the phrase “that wasn’t me on the TV show,” because the letter had been sent about two weeks after a man claiming to be the Zodiac had called into a Bay Area television show and had spoken to the lawyer Melvin Belli. That meant the solution the team found fit the timeline from that era, Mr. Oranchak said.
Mr. Blake said the code had been cracked “with a massive search through many candidates using sophisticated software which can efficiently solve homophonic substitution ciphers.”
“Not only were we lucky enough to find the needle in the haystack,” he said, “but we were lucky enough to pick the right haystack in order to start searching for the needle.”