The Zodiac may have disguised the key somewhere in his many cards and letters, possibly indicating something with his photocopied keys in the Eureka Card for example, but the search goes on into its 44th year.
On the right is an example he may have created, but this just shows how HER> at the start of the 340 code, deciphered to THIS ( is the Zodiac speaking ).
My key is Below, Below, -9, S,
The first letter H in the cipher is treated by my first key BELOW, which in my design is T.
The second letter E is treated by my second key BELOW, which is H.
Then the third letter R is taken back 9 positions on my grid to the letter I.
The fourth symbol > is the enclosed S to the right.
But of course there could be any number of keys and each or all grids could be rotated 90 degrees or more after each
The possibilities are infinite, but realistically the Zodiac must have known that an impossible cipher is pointless, so if it were genuine, a key would be attainable and it could be resting harmlessly somewhere within the letters or cards, it is just knowing where to look.
PIGPEN CIPHER TOOL
MASONIC SYMBOLS VIDEO at 18 minutes into video
FBI CODE ANALYSIS