Quick introduction to reverse engineering for beginners

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.text:005413A3 mov ecx, [esp+44h+password] .text:005413A7 push ecx ; password .text:005413A8 push esi ; aligned size .text:005413A9 push ebx ; buffer .text:005413AA call crypt ; do crypt Create output file. By the way, developer <strong>for</strong>got <strong>to</strong> check if it is was created correctly! File opening result is being checked though. .text:005413AF mov edx, [esp+50h+Filename] .text:005413B3 add esp, 40h .text:005413B6 push offset aWb ; "wb" .text:005413BB push edx ; Filename .text:005413BC call _fopen .text:005413C1 mov edi, eax Newly created file handle is in EDI register now. Write signature “QR9”. .text:005413C3 push edi ; File .text:005413C4 push 1 ; Count .text:005413C6 push 3 ; Size .text:005413C8 push offset aQr9 ; "QR9" .text:005413CD call _fwrite ; write file signature Write actual file size (not aligned): .text:005413D2 push edi ; File .text:005413D3 push 1 ; Count .text:005413D5 lea eax, [esp+30h+Str] .text:005413D9 push 4 ; Size .text:005413DB push eax ; Str .text:005413DC call _fwrite ; write original file size Write crypted buffer: .text:005413E1 push edi ; File .text:005413E2 push 1 ; Count .text:005413E4 push esi ; Size .text:005413E5 push ebx ; Str .text:005413E6 call _fwrite ; write crypted file Close file and free allocated buffer: .text:005413EB push edi ; File .text:005413EC call _fclose .text:005413F1 push ebx ; Memory .text:005413F2 call _free .text:005413F7 add esp, 40h .text:005413FA pop edi .text:005413FB pop esi .text:005413FC pop ebx .text:005413FD pop ebp .text:005413FE retn .text:005413FE crypt_file endp Here is reconstructed C-code: 163
void crypt_file(char *fin, char* fout, char *pw) { FILE *f; int flen, flen_aligned; BYTE *buf; }; f=fopen(fin, "rb"); if (f==NULL) { printf ("Cannot open input file!\n"); return; }; fseek (f, 0, SEEK_END); flen=ftell (f); fseek (f, 0, SEEK_SET); flen_aligned=(flen&0xFFFFFFC0)+0x40; buf=(BYTE*)malloc (flen_aligned); memset (buf, 0, flen_aligned); fread (buf, flen, 1, f); fclose (f); crypt (buf, flen_aligned, pw); f=fopen(fout, "wb"); fwrite ("QR9", 3, 1, f); fwrite (&flen, 4, 1, f); fwrite (buf, flen_aligned, 1, f); fclose (f); free (buf); Decrypting procedure is almost the same: .text:00541400 ; int __cdecl decrypt_file(char *Filename, int, void *Src) .text:00541400 decrypt_file proc near .text:00541400 .text:00541400 Filename = dword ptr 4 .text:00541400 arg_4 = dword ptr 8 .text:00541400 Src = dword ptr 0Ch .text:00541400 .text:00541400 mov eax, [esp+Filename] .text:00541404 push ebx .text:00541405 push ebp .text:00541406 push esi .text:00541407 push edi 164
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Quick introduction to reverse engin
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1.16 C++ classes . . . . . . . . .
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Preface Here (will be) some of my n
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1.1 Hello, world! Let’s start wit
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1.2 Stack Stack — is one of the m
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(_snprintf() function works just li
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1.3 printf() with several arguments
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1.4 scanf() Now let’s use scanf()
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GCC replaced first printf() call to
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}; return 0; printf ("What you ente
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1.5 Passing arguments via stack Now
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1.6 One more word about results ret
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push OFFSET $SG741 ; ’a
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1.8 switch()/case/default 1.8.1 Few
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1.8.2 A lot of cases If switch() st
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loc_804840C: ; DATA XREF: .rodata:0
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et 0 _main ENDP Nothing very specia
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add esp, 1Ch xor eax, eax ; return
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C/C++ standard defines char type as
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eax=eax+ecx return eax ... and this
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mov eax, ecx imul edx sar edx, 1 mo
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; current stack state: ST(0) = resu
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; in local stack here 8 bytes still
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fld QWORD PTR _a$[esp-4] ; current
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In other words, fnstsw ax / sahf in
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1.13 Arrays Array is just a set of
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mov [esp+70h+var_70], eax call _pri
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jge SHORT $LN1@main mov ecx, DWORD
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ebp 0x15 0x15 esi 0x0 0 edi 0x0 0 e
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1.14 Bit fields A lot of functions
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So, let’s download Linux Kernel 2
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There are some redundant code prese
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y 10 just by dropping last digit (3
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_key$ = 8 ; size = 4 _len$ = 12 ; s
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1.15 Structures It can be defined t
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call DWORD PTR __imp__GetSystemTime
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1.15.4 Fields packing in structure
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}; int b; struct outer_struct { cha
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}; printf ("stepping=%d\n", tmp->st
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call ___printf_chk mov eax, esi shr
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mov edx, DWORD PTR _t$[ebp] or edx,
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1.16 C++ classes I placed a C++ cla
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push ecx mov DWORD PTR _this$[ebp],
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call _ZN1c4dumpEv lea eax, [esp+20h
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1.17 Unions 1.17.1 Pseudo-random nu
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xor eax, eax pop esi mov esp, ebp p
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Let’s compile it in MSVC 2010 (I
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comp() function: public comp comp p
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Some compilers can do vectorization
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mov ecx, [esp+10h+var_10] mov edx,
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GCC In all other cases, non-SSE2 co
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add ebx, edx lea esi, [esi+0] loc_8
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movdqa xmm1, XMMWORD PTR [ecx+16] a
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1.20 x86-64 It’s a 64-bit extensi
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} x40 = a3 | x31; x41 = x24 & ~x37;
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or ebx, DWORD PTR _x6$[esp+36] mov
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and r11, r8 and rsi, r8 and r10, r1
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pop rdi pop rsi ret 0 s1 ENDP Nothi
Page 118 and 119: 2.1 LEA instruction LEA (Load Effec
Page 120 and 121: 2.3 npad It’s an assembler macro
Page 122 and 123: 2.4 Signed number representations T
Page 124 and 125: call function function: .. do somet
Page 126 and 127: 3.2 String Debugging messages are o
Page 128 and 129: .text:3011E91B DD 1E fstp qword ptr
Page 130 and 131: loc_80483F2: pop ebp retn _f endp 4
Page 132 and 133: loc_8048444: loc_8048448: loc_80484
Page 134 and 135: public f2 f2 proc near push ebp mov
Page 136 and 137: loc_804840A: loc_804842E: loc_80484
Page 138 and 139: mov esi, DWORD PTR _k$[esp+16] push
Page 140 and 141: or eax, edx retn f endp 4.1.8 Task
Page 142 and 143: 4.2 Middle level 4.2.1 Task 2.1 Wel
Page 144 and 145: mov [edx+eax], bl mov ebx, edi mov
Page 146 and 147: loc_80485AB: ; CODE XREF: f+1E0 ; f
Page 148 and 149: mov [ecx+eax], dl inc eax cmp ebx,
Page 150 and 151: Chapter 5 Tools ∙ IDA as disassem
Page 152 and 153: Chapter 7 More examples 147
Page 154 and 155: .text:00541050 arg_0 = dword ptr 4
Page 156 and 157: .text:005410F3 .text:005410F3 loc_5
Page 158 and 159: .text:005411A9 pop ebx .text:005411
Page 160 and 161: .text:00541249 .text:00541249 next_
Page 162 and 163: .text:005412FC mov esi, offset cube
Page 164 and 165: .text:005413DC call _fwrite ; write
Page 166 and 167: .text:005414A7 push ecx ; Filename
Page 170 and 171: .text:00541408 push offset aRb ; "r
Page 172 and 173: }; }; return; fseek (f, 0, SEEK_END
Page 174 and 175: .text:005411B5 mov ebp, eax Check f
Page 176 and 177: .text:0054124C inc ebp .text:005412
Page 178 and 179: This instruction clears bit, in oth
Page 180 and 181: .text:005410CD add esp, 40h .text:0
Page 182 and 183: }; for (i=0; i
Page 184 and 185: }; tmp[y][z]=get_bit (row, y, z); f
Page 186 and 187: }; fread (buf, flen, 1, f); fclose
Page 188 and 189: 7.2 SAP client network traffic comp
Page 190 and 191: .text:64413F68 .text:64413F68 loc_6
Page 192 and 193: .text:64405171 call dbg .text:64405
Page 194 and 195: .text:64404FF7 push offset aLogging
Page 196 and 197: .text:64405113 loc_64405113: .text:
Page 198 and 199: Now let’s dig deeper and find con
Page 200 and 201: .text:64411E0B .text:64411E0B loc_6
Page 202 and 203: Chapter 8 Other things 8.1 Compiler
Page 204 and 205: Chapter 9 Tasks solutions 9.1 Easy
Page 206 and 207: 9.1.5 Task 1.5 Hint #1: Keep in min
Page 208 and 209: } 9.1.8 Task 1.8 Solution: two 100*
Page 210 and 211: #define PUSH(low, high) ((void) ((t
Page 212 and 213: } } ignore one or both. Otherwise,
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