PIC单片机芯片的解密原理介绍 (上)

PIC单片机芯片的解密原理介绍 (上)
In this file you will find out more about the theory on which my CrackPic software is based.
Theory on which CrackPic is based
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1. Some of the PIC models allow scrambled data reading when they are protected, according to the formula:
s = a NXOR b 1.) where: a = higher 7 bits from 14-bit word
b = lower 7 bits from 14-bit word
NXOR Table:
x y |z
------
0 0 |1
1 0 |0
0 1 |0
1 1 |1
2. Program is stored into EPROM. When this EPROM is erased, he has all of its bits set to 1, which means that value of all words stored into it is 3fffh. During the EPROM programming it is ONLY possible to change 1 into 0, AND NOT the other way around.
Combining 1. and 2. You can conclude that to unscramble the word it is enough to erase lower 7 bits (b=0), which makes:
for equation 1. if b=0
s1 = a NXOR 0 = NOT a
s1 = NOT a 2.)
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equation 1. can be written as
s = (NOT a) XOR b 3.)
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Combining equations 2 and 3
s = s1 XOR b
b = (s XOR s1) AND 127 4.) (AND 127 extracts 7th bit)
======================
a = (NOT s1) AND 127
====================
The original word is:
w = a*128 + b
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* w = ((NOT s1) AND 127)*128 + ((s XOR s1 ) AND 127) *
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"s" is obtained by simple reading the PIC.
Obtaining "s1" is a problem.
Some PIC`s (16c71,61,.) allow writing into first 64 words, so it is enough
to write 3f80h (11 1111 1000 000 bin) into every of these words,
thus making b=0.
The rest of the memory is protected from the writing, so it is necessary
to find a way to do it somehow.
I accidentally discovered the way to write the following word into the
rest of the EPROM: (xx xxxx x x 00 0000 bin), thus making b=(x00 0000)bin (More about you can find in CrackPic.a32 writen in ASM).
I cannot reset the highest bit in b, and from the previous equations it is obvious that it influences 14th and 7th bit in the original word. This
means that for a given "s" we can calculate two pairs of different combinations for one instruction code (since bit 7 in b can be either 0 or 1): possible value
bit for the bit
7 "s" 14 7 "w"
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0 0 1 code 1
1 0 code 2
1 1 1 code 1
0 0 code 2
Because of that you will get 2 codes of which one is the correct. You will have to manually determine the correct instruction, but they differs a
lot, and it should be easy to filter out wrong ones. I have made software (CrackPic.exe) that generate DEX.LST file which contains pairs of instructions, and enables you to easy pick the correct ones.
Some helpfull notes
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If 7th bit of "s" equals 0, and you're cracking the PIC that has up to 3fh files (16c71,61,84, etc.), than it is probably code 1 0, because
instructions with 0 1 code works with files 40h - 7fh, which are not supported in these PIC`s. Of course, after reading this text anyone can
use this kind of instruction which has no other purpose but to fool you. Anyway, this program is not something special, but if you need inverse engineering of the PIC`s, and you`re experienced PIC programmer who
can not pay 1000$ - 5000$ to the professionals, I believe that this FREE software will be of some use to you ;)。