[Diy_efi] Re: Reversing EPROM decryption board

[Don Paauw]

Just to clarify, you will need two banks of NVRAM, one connected to the EPROM
data bus and the other connected to the processor data bus, both connected
to the address bus.  Thus, one will contain encrypted and the other
unencrypted
data.

-- Don



At 05:56 AM 6/13/06 +0100, you wrote:
>Hello Don,
>   
>  thanks for the answer.
>   
>  I tried to make the checksum of vectors correct by some trivial mehods -
assuming 8 and 16 bit ADD and 8 bit XOR is done on the area. That didn't
work, and I had always fear that they could check the vector table against
actual addresses of ISRs... (unused entrys point to the reset entry point).
But I will try again with the checksum, adding some bit shifting in
between....
>   
>  If address bus is not "walked" in correct sequence, PEELs do not decrypt
data anymore. And I do not know the sequence...
>   
>  As for the monitor board, could I do it like this:
>  1) use NVRAM
>  2) connect it in parallel to address and data bus
>  3) connect its /W to /R of EPROM
>  4) keep its /R high
>  5) connect its /CS to /CS of EPROM
>   
>  Regards,
>  NG