> Are there some opcodes which can used for decryption?
yes, it's well documented on his site ( fwiw I came up with the pea/odd vector method charles used, he credited me by my first name for some reason ).
unfortunately there seems to be a point where the encryption changes. it could be a one time thing ( i.e. boot code is encrypted one way & the rest is encrypted another ) or different parts of the game may be encrypted differently.
after the ( from memory ):
move.w #$2700, SR
cmp.l #$52ffff, d0
The encryption changes. I sent charles some more things to try but haven't heard back.