Change value based on register value

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swank

Joined: 07 Nov 2018
Posts: 6

swank 02 Feb 2019, 17:15

Hello,

I have this code:

macro rndNum {
rdtsc
.if byte[eax]<=$CC
num = $FF
.else
num = $AA
.endif
}

rndNum

My idea is to define num as $FF or $AA based on AL value. But I got always the same value. Can somebody give me a light?

Thanks!

02 Feb 2019, 17:15

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20624
Location: In your JS exploiting you and your system

revolution 02 Feb 2019, 17:37

RDTSC returns values in the registers directly. So you also need to read them directly.

Code:

macro rndNum {
        local   .higher, .done
        rdtsc
        cmp     al,0xcc
        ja      .higher
        mov     [num],0xff
        jmp     .done
    .higher:
        mov     [num],0xaa
    .done:
}

02 Feb 2019, 17:37

swank

Joined: 07 Nov 2018
Posts: 6

swank 02 Feb 2019, 20:19

Thanks for the fast reply revolution!

The idea is to create a constant in the binary that changes every time it is compiled (preprocess).

But the code always returns 0xaa, with this code:

Code:

macro rndNum {
    local   .higher, .done
    rdtsc
    cmp     al,0xcc
    ja      .higher
    ;mov     [num],0xff
    num = 0xff
    jmp     .done
  .higher:
    ;mov     [num],0xaa
    num = 0xaa
  .done:
}
rndNum

Do I have to use match if I wan't preprocess conditional?

02 Feb 2019, 20:19

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20624
Location: In your JS exploiting you and your system

revolution 03 Feb 2019, 01:42

The assembler doesn't execute the RDTSC instruction. It is only compiled.

One way to get a changing value during assembly is the %t value. It is the current local time in seconds since 1970-01-01.

Code:

macro rndNum {
        if %t and 0xff <= 0xcc
                num = 0xff
        else
                num = 0xaa
        end if
}

You don't need the macro wrapper BTW. It can be assembled directly also.

03 Feb 2019, 01:42

swank

Joined: 07 Nov 2018
Posts: 6

swank 03 Feb 2019, 03:17

Thanks mate! Works like a charm!!

With your help, I can generate a random 1byte number preprocessed with just one line!!

Code:

key = %t and 0xff

Thanks for the patience!

03 Feb 2019, 03:17

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