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AhMunRa
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 Posted: Thu Nov 04, 2010 10:53 am Post subject: ASM Instruction Question |
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I know the NOT instruction takes 1 operand. What I am not comprehending about it is that it takes a value say: 0x52
| Code: |
EAX = 00000052
NOT EAX;
EAX then becomes FFFFFFAD
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My question is exactly what has happened here?
Some links to a good explanation are welcome as well.
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<Wiccaan> Bah that was supposed to say 'not saying its dead' lol. Fixing >.> |
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tombana
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| Posted: Thu Nov 04, 2010 11:14 am Post subject: |
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| The NOT instruction will invert all bits.
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AhMunRa
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| Posted: Thu Nov 04, 2010 11:29 am Post subject: |
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Right, my trouble is that I am trying to mathematically reproduce this using base 10, and it's not working.
Got it. In C#
| Code: |
for( int x = 1; x < len; x++ )
{
value = arrStr[x] + 1;
value = System.Math.Abs(value) * (-1);
}
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If arrstr[x] = 52
value = arrStr[52] + 1;
value = -53
Converted to hex is FFFFFFAD
which is the same as NOT 52
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AtheistCrusader
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| Posted: Thu Nov 04, 2010 1:07 pm Post subject: |
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Go over each one
15-x=inverted
15-2 = 13, in hex is D
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Jani
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| Posted: Fri Nov 05, 2010 2:25 am Post subject: |
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| AhMunRa wrote: | | Right, my trouble is that I am trying to mathematically reproduce this using base 10, and it's not working. |
Mathematically? Bitwise much? | Code: | int i = 0x52;
i = ~i;
// i is now 0xFFFFFFAD |
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AhMunRa
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| Posted: Fri Nov 05, 2010 8:05 am Post subject: |
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Yeah already sorted that.
That works better than what I was using Jani, now though instead of actually being 0xFFAD the value is -53.
I really hate that you can't work with actual hex values. This would be so much easier.
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Jani
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| Posted: Sat Nov 06, 2010 7:57 am Post subject: |
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| AhMunRa wrote: | | That works better than what I was using Jani, now though instead of actually being 0xFFAD the value is -53. |
-53 is the same as 0xFFAD, if the type is signed short.
| AhMunRa wrote: | | I really hate that you can't work with actual hex values. This would be so much easier. |
What do you mean? int i = 170 and i is 0xAA. Everything is stored as binary, hex and dec are just representation forms for us humans to make it easier to read.
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AhMunRa
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| Posted: Mon Nov 08, 2010 12:00 pm Post subject: |
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The type is not singed short it is int, was running into conversion errors due to every 3rd line having to convert between decimal and hex.
I've already manually solved the problem, but I have not completed the task and written code to perform it for me.
All my values should be no larger than dword, yet some are overflowing and causing errors during byte conversion.
I'm starting to think it's the language I've chosen to write this in is too constrained.
Right Jani, but it sucks that the computer will treat cin.get('H'); as 72 and not 0x48 unless you specify 0x
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hcavolsdsadgadsg
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| Posted: Mon Nov 08, 2010 12:20 pm Post subject: |
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they are the same thing.
72 is 0x48
0x48 is 72
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AhMunRa
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| Posted: Mon Nov 08, 2010 3:13 pm Post subject: |
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Thank you all for the help, in this case I couldn't see the forest for the tree. My value kept coming out wrong due to me not setting it properly before moving on in my code. I was setting the hex value to a var prevInt in which I was using it later as a decimal instead of hex, was causing my screw up.
Final result
| Code: |
for (int x = 0; x < len; x++)
{
if (x < 1)
{
byteStr = arrStr[x].ToString();
bByte = Convert.ToByte(byteStr);
bInt = ~bByte;
int test = Convert.ToInt32(byteStr);
test = test / 2;
value2 = bInt & test;
outPut = String.Format("{0:x2}", value2);
prevInt = Convert.ToInt32(outPut, 16);
}
else
{
value = arrStr[x] - prevInt;
value2 = value / 2;
prevInt = Convert.ToInt32(value2.ToString("X"), 16);
if (x == 2 || x == 4 || x == 6 || x == 8 || x == 10 || x == 12 || x == 14 || x == 16)
{
outPut += String.Format("-{0:x2}", value2);
}
else
{
outPut += String.Format("{0:x2}", value2);
}
}
}
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If someone can offer a better solution to add the hyphen in there I'm willing to listen. Best I could come up with is that hack.
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hcavolsdsadgadsg
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| Posted: Mon Nov 08, 2010 3:47 pm Post subject: |
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| what exactly are you trying to do?
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AhMunRa
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| Posted: Mon Nov 08, 2010 4:31 pm Post subject: |
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It's a keygen for a crackme. The code works properly, takes a username then derives a key from it.
The bit shifting in the if loop is unnecessary, I was just wondering if there is a prettier way to do the formatting to add the hyphens every 2 bytes. Other than if (x == 2 || x == 4 || x == 6 || x == 8 || x == 10 || x == 12 || x == 14 || x == 16)
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atom0s
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| Posted: Mon Nov 08, 2010 4:43 pm Post subject: |
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You can check if the number is even rather then a huge if.
| Code: | public bool IsEven(int nInput)
{
return ((nInput & 1) == 0);
} |
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AhMunRa
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| Posted: Tue Nov 09, 2010 10:05 am Post subject: |
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Wiccan you are the best. I never woulda thought of that.
Final function working keygen.
C# Solution
| Code: |
private string GenKey(string user, int len)
{
arrStr = new int[len];
string outPut = "";
int y = 0;
int prevInt = 0;
long lval;
foreach (char c in user)
{
arrStr[y] = Convert.ToInt32(c);
y++;
}
for (int x = 0; x < len; x++)
{
if (x < 1)
{
outPut = String.Format("{0:x2}", ~Convert.ToByte(arrStr[x].ToString()) & (Convert.ToInt32(arrStr[x].ToString()) / 2));
prevInt = Convert.ToInt32(outPut, 16);
}
else
{
lval = arrStr[x];
for (int n = 0; n < prevInt; n++)
{
lval = lval * 2;
}
if (lval > 4294967295)
{
lval = lval / 4294967295;
if (lval < 57)
{
lval = 0;
}
}
long value2 = ((arrStr[x] - prevInt) / 2) & ~Convert.ToInt64(lval);
prevInt = Convert.ToInt32(String.Format("{0:x2}", value2), 16);
if ((x & 1) == 0)
{
outPut += String.Format("-{0:x2}", value2);
}
else
{
outPut += String.Format("{0:x2}", value2);
}
}
}
return outPut.ToUpper();
}
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Code criticism is welcome.
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<Wiccaan> Bah that was supposed to say 'not saying its dead' lol. Fixing >.> |
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