Zodiac Killer Cipher

The Zodiac cipher refers to a series of encryption systems used by a serial killer active in the San Francisco Bay Area (California, USA) in the late 1960s and early 1970s. The name Zodiac is the one the criminal gave himself by signing his letters with a symbol in the shape of a cross inscribed in a circle.

The Zodiac Killer is the nickname given to an unidentified criminal who committed a series of deadly attacks in the United States between 1968 and 1969.

His identity has never been confirmed despite numerous police investigations, from the FBI, and subsequent forensic analyses, and the case officially remains unsolved.

The Zodiac sent at least 17 letters to newspapers and local authorities. Some contained claims of responsibility or details about the crimes, and four of them included cryptograms composed of symbols: Z408, Z340, Z32, and Z13 (named after their length). Z408 was solved in 1969, Z340 in December 2020 by a team of cryptanalysts, while Z13 and Z32 remain unsolved.

Messages Z408 and Z340 are based on homophonic substitution: the same letter can be represented by several different symbols.

In Z340, this substitution is followed by a transposition of the characters into a 17-column grid, with reading performed along a predefined diagonal path.

The other cryptograms (Z32 and Z13) use visually similar symbols, but their alphabets and exact methods may differ.

The decipherment of Z408 consists of solving a homophonic substitution: each symbol corresponds to a plaintext letter, and several symbols can represent the same letter.

For Z340, apply the same homophonic substitution logic, then reorder the resulting letters using a diagonal transposition (knight's move) in a 17-column grid: the first letter of the first row, then the third letter of the second row, then the fifth letter of the third row, and so on.

The Z-408 cryptogram was sent on July 31, 1969, to three newspapers. It was solved within hours by Donald and Bettye Harden, a couple of teachers. The message was:

The translation that was made: 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 DANGEROUE ANAMAL 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 THAE 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 SLOI DOWN OR ATOP MY COLLECTIOG OF SLAVES FOR MY AFTERLIFE EBEORIETEMETHHPITI (solution found in August 1969, spelling mistakes left intact, the meaning of the last 18 characters never received a consensual interpretation).

Crypto Z-340 was sent on November 8, 1969, and solved 51 years later in December 2020 here

The ciphertext is arranged in a 17-column grid. After solving the homophonic substitution, the letters must be rearranged using a 20-line diagonal knight's move transposition to obtain the readable message.

The Z340 message has been translated I HOPE YOU ARE HAVING LOTS OF FAN IN TRYING TO CATCH ME THAT WASNT ME ON THE TV SHOW WHICH BRINGO UP A POINT ABOUT ME I AM NOT AFRAID OF THE GAS CHAMBER BECAASE IT WILL SEND ME TO PARADLCE ALL THE SOOHER BECAUSE E NOW HAVE ENOUGH SLAVES TO WORV FOR ME WHERE EVERY ONE ELSE HAS NOTHING WHEN THEY REACH PARADICE SO THEY ARE AFRAID OF DEATH I AM NOT AFRAID BECAUSE I VNOW THAT MY NEW LIFE IS LIFE WILL BE AN EASY ONE IN PARADICE DEATH (solution proposed in December 2020, spelling mistakes left intact)

It is again a homophonic substitution but this time, the letters of the message have been transposed so as to read diagonally on 9 lines: the reading order is:

The Z-13 cryptogram dates from April 20, 1970. It consists of only 13 symbols and is preceded by the phrase MY NAME IS, leading to the hypothesis that it might contain a name. It remains undeciphered to this day. The message is

The Z-32 cryptogram was sent on June 26, 1970, accompanied by a map of the San Francisco Bay Area bearing the Zodiac symbol, possibly indicating a location.

Investigators have theorized that Z-32 might encode digital geographic coordinates, but no definitive solution has been found. The message is:

Z408 and Z340 share the same family of methods (homophonic substitution), but their alphabets and lookup tables differ greatly.

	z408	z340		z408	z340		z408	z340		z408	z340
	W	D		L	E			Y		N	N
	E	R		S	F		A	T		T	I
	T			F	S		S	A		T	G
	H	O		E	E		N	A		I	I
	F			G	O		A	D		O	R
	I	S		B	O		E	W		O	R
	U	N		E	R			E		V	E
	O	L		C	X		D	B			P
	I	I		A	A		E	C		M	U
	R	Z		R	L			I		D	A
	O			L	T		L	T		I	S
	N	T		H	W		L	A		E	H
		S			N		K	U
		R			M			Y			E
	T	V		E	D		A	L		S	P
	I	N			T			T			I
	P	Q			N			J		S	U
				R	K		N	O		Y	B
		E

Despite the graphically similar symbols, only five symbol→letter correspondences coincide between the two ciphers: (=N), (=I), (=A), (=E), (=I).

The symbol present in Z13 does not appear in either of the two solved cryptograms.