Zodiac Killer Cipher

The Zodiac cipher is the generic name given to a series of ciphers used by a serial killer in the United States in the 1960s and 1970s. Zodiac is the name given to this serial killer who would be responsible for at least 7 victims in California, USA (according to the authorities/FBI because he claims 37 victims).

The murderer has never been apprehended and his story has been the subject of numerous reports, documentaries and movies.

The Zodiac sent at least 17 letters to local newspapers. These letters sometimes contained evidence of crimes and sometimes cryptograms. It is these 4 cryptograms (called Z408, Z340, Z32 and Z13 depending on their length) made up of symbols that contributed to the mysterious reputation of the killer. The first cryptogram Z408 was resolved quickly. The second cryptogram Z340 was solved in 2020 and the mystery remains unsolved for Z13 and Z32.

The Z408 and Z340 messages are homophonic substitution ciphers: the same letter can be encrypted by several different symbols.

The Z340 cryptogram also uses a character transposition, resembling a route cipher (reading diagonally).

All other cryptograms use common symbols but do not share the same alphabet and the encryption method may be different.

The decryption based on Z408 is that of a homophonic substitution, each character / symbol is associated with a plain letter, and several symbols can be associated with the same letter.

The decryption based on Z340 is also a homophonic substitution, but the resulting text must then undergo a transposition (mixing of letters). The rule is a diagonal reading (knight moves) in a grid of 17 columns. First letter of the first line, then third letter of the second line, then the 5th letter of the third line, etc.

Crypto Z-408 was sent on July 31, 1969 in 3 pages and was decoded in only 20 hours by a teacher Donald Harden and his wife Bettye. The message was:

The translation starts with words I like killing people because it is so much fun.…

The Crypto Z-340 was sent on November 8, 1969, it was resolved 51 years later in December 2020 here (link) The message is written in a grid of 17 columns:

The message begins with I hope you are having lots of fun to catch me…

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:

Crypto Z-13 is dated April 20, 1970, it is the shortest, it is preceded by MY NAME IS which suggests that the killer could reveal its identity. The message is:

The crypto Z-32 was sent on June 26, 1970, it was accompanied by a map of the San Francisco Bay.

The investigators assume that this cryptogram may contain numerical geographic coordinates.

By having deciphered 2 of the messages, a hope of understanding the last 2 arose. Unfortunately, although the symbols are similar, the transliterations common to the two cryptograms are rare. Here are the symbols and their correspondences:

	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

There are only 5 symbols with a common key: (=N), (=I), (=A), (=E), (=I).

The symbol in Z13 do not appear in any of these 2 decrypted messages.