Mathematical impossibility?

[squareD]

There is a given IdentCode calculated by ProductId of system.

Author of program compares HexCodes of IdentCode with RegistrationCode by doing this:

1st byte of RegCode -> NEG -> ROR 1 = value
...
8th byte of RegCode -> NEG -> ROR 8 = value
9th byte of RegCode -> NEG -> ROR 1 = value
...
16th byte of RegCode -> NEG -> ROR 8 = value

This means:

IdentCode................ = C5CA0E7A3B0433BD
HexCode of IdentCode = 43 35 43 41 30 45 37 41 33 42 30 34 33 33 42 44
Found RegCode......... = 7A 2C E6 EC FA ?? ?? BF 9A F7 7F BD 9A 34 DF BC

Not found:

?? -> NEG -> ROR 6 = 45
?? -> NEG -> ROR 7 = 37
?? -> NEG -> ROR 3 = 30 FOUND!!

In my opinion it's impossible to get out these values by using only one byte.
But may be that I'm wrong.

Program is cracked in meanwhile, but anyway I'm interested in figure out, how this guy would register this program, if I had bought it.

Thus I present here for discussion.

EDIT: Just found corresponding byte for 30...

Regards,
squareD