Binary simplification

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AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 24 Mar 2008, 05:15

How would you simplify this:

Code:

    shr     ebp, 5
    add     ebp, 7
    shl     ebp, 2
    sub     ebp, 8

To make it use operators like 'and' and 'or'. I have read about this, but I can't understand how..

24 Mar 2008, 05:15

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20449
Location: In your JS exploiting you and your system

revolution 24 Mar 2008, 05:36

Code:

    shr     ebp, 5 ;ebp/32
    add     ebp, 7 ;ebp/32+7
    shl     ebp, 2 ;(ebp/8)&0xfffffffc+28
    sub     ebp, 8 ;ebp/8+20

So:

Code:

    shr ebp,3
    and ebp,0xfffffffc
    add ebp,20

24 Mar 2008, 05:36

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 24 Mar 2008, 05:46

Wow, thanks! I'll definitely start learning some of those tricks.

24 Mar 2008, 05:46

r22

Joined: 27 Dec 2004
Posts: 805

r22 24 Mar 2008, 14:12

Architecture wise you can simplify that instruction sequence even further with...

Code:

SHR ebp,5
LEA ebp,[ebp*4+20]

24 Mar 2008, 14:12

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 24 Mar 2008, 14:33

No, I think I'll stick with:

Code:

 shr   ebp, 3
 add  ebp, 20

24 Mar 2008, 14:33

r22

Joined: 27 Dec 2004
Posts: 805

r22 24 Mar 2008, 23:35

Correct(127):
127/32{SHR x,5} = 3
+ 7 {ADD x,7} = 10
* 4 {SHL x,2} = 40
- 8 {SUB x,8} = 32

Fail(127):
127/8 {SHR x,3} = 15
+ 20 {ADD x,20} = 35

AlexP
Its NOT equivalent to your initial sequence of instructions.

24 Mar 2008, 23:35

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 24 Mar 2008, 23:43

Should have specified, it fits the purpose of my code.

24 Mar 2008, 23:43

ic2

Joined: 19 Jan 2008
Posts: 75

ic2 25 Mar 2008, 03:54

I always thought the usage of LEA shows a lot of garbage code in the registers but provide a added special something specially when dealing with encryption. Just wondering out loud. I'll talk logic by September...

25 Mar 2008, 03:54

daniel.lewis

Joined: 28 Jan 2008
Posts: 92

daniel.lewis 25 Mar 2008, 05:27

I hope your idea is what you think it is.

I stopped worrying about encryption after I decided my Beale Ciphers were strong enough.

_________________
dd 0x90909090 ; problem solved.

25 Mar 2008, 05:27

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 25 Mar 2008, 18:36

Depends, what source are you using for key?

25 Mar 2008, 18:36

daniel.lewis

Joined: 28 Jan 2008
Posts: 92

daniel.lewis 25 Mar 2008, 23:36

If I told you I'd have to kill you.

Let me say though, that any data can be used to cipher something. Any data from anywhere on any disk or network. I recommend a widely varied set of data from different places if you really care.

_________________
dd 0x90909090 ; problem solved.

25 Mar 2008, 23:36

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