In a world where amateurs and the FBI finally cracked the Zodiac's famous 340-cipher in 2020, his two shortest messages — one claiming to spell his name — remain unbroken to this day. The Zodiac killer, who murdered at least five people in Northern California in the late 1960s, taunted police and newspapers with a stream of letters and four ciphers. For more than half a century, two of those four resisted every cryptanalyst, professional and amateur, who threw themselves at them. The smaller two still do.

First, the part that is solved, because it proves the rest is not hopeless hype. The Zodiac's first cipher, the 408 sent in three pieces to Bay Area papers in 1969, was broken within a week by a married couple of schoolteachers, Donald and Bethye Harden. The harder one — the 340-character cipher mailed to the San Francisco Chronicle in November 1969 — held out for 51 years. Then in December 2020, a three-man team announced a solution: American software developer David Oranchak, Australian mathematician Sam Blake, and Belgian programmer Jarl Van Eycke. Blake generated roughly 650,000 reading-order variations to attack the cipher's unusual diagonal transposition; Oranchak ran them through Van Eycke's AZdecrypt engine until coherent English fell out. The plaintext opened: "I HOPE YOU ARE HAVING LOTS OF FUN IN TRYING TO CATCH ME." The FBI's San Francisco field office publicly confirmed the solution, and the team's full peer-style write-up was later published describing exactly how the encryption worked. This is verifiable, reproducible cryptanalysis, not a guess.

Which is what makes the two survivors so maddening. The 13-character cipher, the Z13, arrived in an April 1970 letter that began "This is the Zodiac speaking" and declared: "By the way have you cracked the last cipher I sent you?" Above thirteen symbols he wrote the line "My name is —." If he was telling the truth, his identity has been sitting in plain sight, encrypted, for over half a century. The 32-character cipher, the Z32, came in a June 1970 letter accompanied by a hand-drawn map of the Bay Area and a claim that the cipher revealed where he had planted a bomb. Neither has ever been convincingly solved.

Now the hard, deflating evidence — the kind Inverted World refuses to skip. The reason Z13 and Z32 remain open is not necessarily that they're cleverer than the 340. It may be that they are mathematically too short to solve at all. The 340 yielded because 340 characters of homophonic substitution carry enough statistical structure for a machine to lock onto. Thirteen symbols do not. With a cipher that short, an enormous number of different plaintexts can fit equally well, and there is no rigorous way to prove which — if any — is the one the author intended. Cryptographers describe Z13 as effectively underdetermined: even a correct solution might be impossible to verify as correct. That is why the field, including the trio who cracked the 340, has openly cautioned that Z13 may be permanently unsolvable in any provable sense.

There's a sharper possibility too: that the short ciphers contain nothing. The Zodiac lied constantly. He inflated his body count, claimed bombs he never planted, and threatened school buses to manufacture terror. "My name is" followed by gibberish would be entirely in character — a thirteen-symbol trap designed so that armchair detectives would chase their own reflections forever. Decades of "solutions" naming this or that suspect have all foundered on exactly this point: with so few symbols, you can torture almost any name out of Z13 if you're willing to bend the rules, and people have, repeatedly, for figures like Robert Graysmith's prime suspect and others. None survive scrutiny.

So the 340's fall changed the stakes in a strange way. It proved the Zodiac's encryption could be beaten by patience and the right math — and simultaneously proved that the two ciphers everyone most wants solved are the two least likely to ever be solved provably, precisely because they are small. The unresolved question isn't just "what do Z13 and Z32 say." It's whether they say anything at all — whether the man who wrote "My name is" left a real confession encrypted in those thirteen marks, or a final joke engineered to outlast him, his victims, and everyone still squinting at it today.