Methods to form and check bitmasks

This most likely has been asked and answered before, but my searches was futile.

Question is about bits, bytes masks and checking.

Say one have two "triggers" 0xC4 and 0xC5:

196: 1100 0100  0xc4
197: 1100 0101  0xc5

The simple way of checking if var is either would be:

if (var == 0xc5 || var == 0xc4) {

}

But sometimes one see this (or the like):

if ( ((var ^ magic) & mask) == 0)  {

}

My question is how to find magic and mask. What methods, procedures, tricks etc. is to be utilized to form these values and to assert if any exists?

EDIT:

To clarify. Yes, in this exact example the former would be better then the latter, but my question is more as in general of generating and checking these kinds of masks. Bit twiddling in general. I omitted a lot and tried to make the question simple. But ...

As an example I had a look at the source of OllyDbg decompiler source where one find:

if (((code ^ pd->code) & pd->mask) == 0) 
    FOUND

Where code is 0 - 3 bytes of command cast from instruction.

unsigned long code = 0;
if (size > 0) *(((char *)&code) + 0) = cmd[0];
if (size > 1) *(((char *)&code) + 1) = cmd[1];
if (size > 2) *(((char *)&code) + 2) = cmd[2];

As in masking against only bytes part of cmd

And pd is part of:

struct t_cmddata {
    uint32_t mask;          Mask for first 4 bytes of the command
    uint32_t code;          Compare masked bytes with this
        ...
}

holding a long array as:

const t_cmddata cmddata[] = {
/*      mask      code  */
  { 0x0000FF, 0x000090, 1,00,  NNN,NNN,NNN, C_CMD+0,        "NOP" },
  { 0x0000FE, 0x00008A, 1,WW,  REG,MRG,NNN, C_CMD+0,        "MOV" },
  { 0x0000F8, 0x000050, 1,00,  RCM,NNN,NNN, C_PSH+0,        "PUSH" },
  { 0x0000FE, 0x000088, 1,WW,  MRG,REG,NNN, C_CMD+0,        "MOV" },
  { 0x0000FF, 0x0000E8, 1,00,  JOW,NNN,NNN, C_CAL+0,        "CALL" },
  { 0x0000FD, 0x000068, 1,SS,  IMM,NNN,NNN, C_PSH+0,        "PUSH" },
  { 0x0000FF, 0x00008D, 1,00,  REG,MMA,NNN, C_CMD+0,        "LEA" },
  { 0x0000FF, 0x000074, 1,CC,  JOB,NNN,NNN, C_JMC+0,        "JE,JZ" },
  { 0x0000F8, 0x000058, 1,00,  RCM,NNN,NNN, C_POP+0,        "POP" },
  { 0x0038FC, 0x000080, 1,WS,  MRG,IMM,NNN, C_CMD+1,        "ADD" },
  { 0x0000FF, 0x000075, 1,CC,  JOB,NNN,NNN, C_JMC+0,        "JNZ,JNE" },
  { 0x0000FF, 0x0000EB, 1,00,  JOB,NNN,NNN, C_JMP+0,        "JMP" },
  { 0x0000FF, 0x0000E9, 1,00,  JOW,NNN,NNN, C_JMP+0,        "JMP" },
  { 0x0000FE, 0x000084, 1,WW,  MRG,REG,NNN, C_CMD+0,        "TEST" },
  { 0x0038FE, 0x0000C6, 1,WW,  MRG,IMM,NNN, C_CMD+1,        "MOV" },
  { 0x0000FE, 0x000032, 1,WW,  REG,MRG,NNN, C_CMD+0,        "XOR" },
  ...

That would be a typical live example of usage. So again: methods for this. Have been looking at Karnaugh map etc. – but thought there was other and so on method for the same district of operation.

Given your two values,

196: 1100 0100  0xc4
197: 1100 0101  0xc5

you'd want to mask-off the bits that differ, in this case bit 0. So the mask value would be the inverse of 0x01, 0xFE.

ie. 0xC4 & 0xFE == 0xC4, and 0xC5 & 0xFE == 0xC4.

That means both values become 0xC4. Then you can check for 0xC4 by xor-ing with the exact bit pattern that should remain.

     1100 0100  0xC4

ie. 0xC4 ^ 0xC4 == 0.

     1100 0100    1100 0101
   & 1111 1110    1111 1110 
     ---- ----    ---- ----
     1100 0100    1100 0100
   ^ 1100 0100
     ---- ----
     0000 0000

Mask first, or risk utter confusion.

Looking through the actual source file, I kinda think he is trying to be obfuscated. Many of the functions want factoring.

I assume your question is: given a set of "triggers", can we find a mask and magic that the triggers can be checked by the following code

if ( ((var ^ magic) & mask) == 0)  {
}

or it is the same as

if ((var & mask) == (magic & mask))  {
}

An example of "triggers" is like

196: 1100 0100  0xc4
197: 1100 0101  0xc5
204: 1100 1100  0xcc
205: 1100 1101  0xcd

If it is feasible, the bits of "triggers" should be classified into 2 types: "specific bits" and "arbitrary bits". Like the first 4 bits and the 6th and 7th bits, specific bits are the same in each trigger. If your change an arbitrary bit of an trigger, it's still an trigger.

So there are exactly 2^N triggers where N denotes the number of arbitrary bits.

This is my first answer on stackoverflow. I'm not sure if I understand your question correctly. Or are you asking other bit twiddling methods?

来源：https://stackoverflow.com/questions/17487121/methods-to-form-and-check-bitmasks

标签

bit-manipulation

bitmask