RE FORUM

[REVERSE ENGINEERING] => Off topic => Topic started by: DARKER on October 20, 2015, 07:20:21 AM

Title: Reversing Shift-XOR operation
Post by: DARKER on October 20, 2015, 07:20:21 AM

Minule som nieco podobne potreboval, mozno sa to niekomu hodi. Ako vypocitat povodnu hodnotu na ktoru bol pouzity SHR a XOR
https://krypt05.blogspot.in/2015/10/reversing-shift-xor-operation.html

Title: Re: Reversing Shift-XOR operation
Post by: pr0p4g4nd4 on October 20, 2015, 08:09:42 AM

 ;D az sa mi nechce verit, ze niektomu by nebolo jasne, ze ako je v tom linku shr s hodnotou 10, to je len delenie hodnotou 1024 a xor je len viacmenej scitavanie.

Title: Re: Reversing Shift-XOR operation
Post by: DARKER on October 20, 2015, 08:33:28 AM

No nie celkom, nezabudaj ze pri SHR sa ti stracaju byty

Priklad SHR(10110011010011111100010010011111, 10)
1011001101001111110001 > 0010011111 - 0010011111 je stratene
0000000000 > 1011001101001111110001 - 0000000000 sa prida na zaciatok

teda napriklad 2 rozne cisla budu mat ten isty vysledok
1011001101001111110001-0010011111 = 0000000000-1011001101001111110001
1011001101001111110001-1111111111 = 0000000000-1011001101001111110001

cize 2^10 cisel bude mat ten isty vysledok (1024 hodnot)
jedina sanca je je tych 10 bytov vybrtutovat... XOR je jasny tam vies zistit povodnu hodnotu ...

Title: Re: Reversing Shift-XOR operation
Post by: pr0p4g4nd4 on October 20, 2015, 09:00:53 AM

samozrejme, ze som reagoval na ten konkretny priklad v clanku, tam je to presne tak ako pisem. v pripade, ze sa shr-uje nejake nie konstantne cislo, tak sa cast informacie strati (resp moze stratit).

Title: Re: Reversing Shift-XOR operation
Post by: Kockatá hlava on October 20, 2015, 11:25:22 AM

DARKERe, zalozil sis blog? Vsichni ted teda cekame dalsi zajimavy clanky! ;)

EDIT: hmm, nevypada to, ze to ses ty, tak nic...

Title: Re: Reversing Shift-XOR operation
Post by: DARKER on October 20, 2015, 11:40:04 AM

Nie, nie :-) ziadny blog, iba zuajimavy clanok z netu :-) Ale obcas si vravim ze by nebolo zle sa obcas podelit so skusenostami :-) Mozno vam sem hodim nabuduce skusenosti so Z3 SMT Solver
https://github.com/Z3Prover/z3 - veeelmi dobra vec na riesenie niektorych problemov v RE.

Title: Re: Reversing Shift-XOR operation
Post by: Kockatá hlava on October 20, 2015, 09:17:22 PM

Uz jsem rikal RubberDuckovi a dalsim, ze to v CZ/SK s publikovanim okolo RE a asm dost skomira, takze co se tyce me, rad si prectu vsechno, co napises. Treba ty solvery jsou hodne zajimava vec.