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finding out members of a C struct from disassembled ASM functions

Posted on 2003-10-26
Medium Priority
Last Modified: 2010-08-05
Hi, i need to find out the members of a C struct from ASM functions, but i don't know ASM, the functions does an MD5Update. (This is not for a crack!).

DEBUG :: void __cdecl appMD5Init(struct FMD5Context *)
:1012D390 8B442404                mov eax, dword[esp+04]
:1012D394 33C9                    xor ecx, ecx
:1012D396 894814                  mov dword[eax+14], ecx
:1012D399 894810                  mov dword[eax+10], ecx
:1012D39C C70001234567            mov dword[eax], 67452301
:1012D3A2 C7400489ABCDEF          mov dword[eax+04], EFCDAB89
:1012D3A9 C74008FEDCBA98          mov dword[eax+08], 98BADCFE
:1012D3B0 C7400C76543210          mov dword[eax+0C], 10325476
:1012D3B7 53                      push ebx
:1012D3B8 83C018                  add eax, 018
:1012D3BB 57                      push edi
:1012D3BC 8944240C                mov dword[esp+0C], eax
:1012D3C0 B940000000              mov ecx, 00000040
:1012D3C5 8B7C240C                mov edi, dword[esp+0C]
:1012D3C9 33C0                    xor eax, eax
:1012D3CB 8BD9                    mov ebx, ecx
:1012D3CD C1E902                  shr ecx, 02
:1012D3D0 83E303                  and ebx, 003
:1012D3D3 F3AB                    rep stosd
:1012D3D5 8BCB                    mov ecx, ebx
:1012D3D7 F3AA                    rep stosb
:1012D3D9 5F                      pop edi
:1012D3DA 5B                      pop ebx
:1012D3DB C3                      ret

DEBUG :: void __cdecl appMD5Update(struct FMD5Context *,unsigned char *,int)
:1012E5E0 53                      push ebx
:1012E5E1 55                      push ebp
:1012E5E2 56                      push esi
:1012E5E3 8B742410                mov esi, dword[esp+10]
:1012E5E7 8B4E10                  mov ecx, dword[esi+10]
:1012E5EA 8BC1                    mov eax, ecx
:1012E5EC 57                      push edi
:1012E5ED 8B7C241C                mov edi, dword[esp+1C]
:1012E5F1 C1E803                  shr eax, 03
:1012E5F4 8D0CF9                  lea ecx, dword[ecx+8*edi]
:1012E5F7 8D14FD00000000          lea edx, dword[8*edi+00000000]
:1012E5FE 83E03F                  and eax, 03F
:1012E601 3BCA                    cmp ecx, edx
:1012E603 894E10                  mov dword[esi+10], ecx
:1012E606 7303                    jae 1012E60B
:1012E608 FF4614                  inc dword[esi+14]
:1012E60B 8B5E14                  mov ebx, dword[esi+14]
:1012E60E 8BCF                    mov ecx, edi
:1012E610 C1E91D                  shr ecx, 1D
:1012E613 03D9                    add ebx, ecx
:1012E615 895E14                  mov dword[esi+14], ebx
:1012E618 BB40000000              mov ebx, 00000040
:1012E61D 2BD8                    sub ebx, eax
:1012E61F 3BFB                    cmp edi, ebx
:1012E621 7C4D                    jl 1012E670
:1012E623 8B542418                mov edx, dword[esp+18]
:1012E627 53                      push ebx
:1012E628 52                      push edx
:1012E629 8D443018                lea eax, dword[eax+esi+18]
:1012E62D 50                      push eax
:1012E62E E8CD29FDFF              call 10101000
:1012E633 8D4E18                  lea ecx, dword[esi+18]
:1012E636 51                      push ecx
:1012E637 56                      push esi
:1012E638 E893F4FFFF              call 1012DAD0 {void __cdecl appMD5Transform(unsigned long *,unsigned char *)}  {void __cdecl appMD5Transform(unsigned long *,unsigned char *)}
:1012E63D 8BEB                    mov ebp, ebx
:1012E63F 83C33F                  add ebx, 03F
:1012E642 83C414                  add esp, 014
:1012E645 3BDF                    cmp ebx, edi
:1012E647 7D23                    jge 1012E66C
:1012E649 8DA42400000000          lea esp, dword[esp+00000000]
:1012E650 8B542418                mov edx, dword[esp+18]
:1012E654 8D441AC1                lea eax, dword[edx+ebx-3F]
:1012E658 50                      push eax
:1012E659 56                      push esi
:1012E65A E871F4FFFF              call 1012DAD0 {void __cdecl appMD5Transform(unsigned long *,unsigned char *)}  {void __cdecl appMD5Transform(unsigned long *,unsigned char *)}
:1012E65F 83C340                  add ebx, 040
:1012E662 83C408                  add esp, 008
:1012E665 83C540                  add ebp, 040
:1012E668 3BDF                    cmp ebx, edi
:1012E66A 7CE4                    jl 1012E650
:1012E66C 33C0                    xor eax, eax
:1012E66E EB02                    jmp 1012E672
:1012E670 33ED                    xor ebp, ebp
:1012E672 8B4C2418                mov ecx, dword[esp+18]
:1012E676 2BFD                    sub edi, ebp
:1012E678 57                      push edi
:1012E679 03E9                    add ebp, ecx
:1012E67B 8D543018                lea edx, dword[eax+esi+18]
:1012E67F 55                      push ebp
:1012E680 52                      push edx
:1012E681 E87A29FDFF              call 10101000
:1012E686 83C40C                  add esp, 00C
:1012E689 5F                      pop edi
:1012E68A 5E                      pop esi
:1012E68B 5D                      pop ebp
:1012E68C 5B                      pop ebx
:1012E68D C3                      ret

DEBUG :: void __cdecl appMD5Final(unsigned char *,struct FMD5Context *)
:1012E690 83EC08                  sub esp, 008
:1012E693 56                      push esi
:1012E694 8B742414                mov esi, dword[esp+14]
:1012E698 57                      push edi
:1012E699 33C9                    xor ecx, ecx
:1012E69B 8D4612                  lea eax, dword[esi+12]
:1012E69E 8BFF                    mov edi, edi
:1012E6A0 8A50FE                  mov dl, byte[eax-02]
:1012E6A3 88540C08                mov byte[esp+ecx+08], dl
:1012E6A7 8A50FF                  mov dl, byte[eax-01]
:1012E6AA 88540C09                mov byte[esp+ecx+09], dl
:1012E6AE 8A10                    mov dl, byte[eax]
:1012E6B0 88540C0A                mov byte[esp+ecx+0A], dl
:1012E6B4 8A5001                  mov dl, byte[eax+01]
:1012E6B7 88540C0B                mov byte[esp+ecx+0B], dl
:1012E6BB 83C104                  add ecx, 004
:1012E6BE 83C004                  add eax, 004
:1012E6C1 83F908                  cmp ecx, 008
:1012E6C4 7CDA                    jl 1012E6A0
:1012E6C6 8B4E10                  mov ecx, dword[esi+10]
:1012E6C9 C1E903                  shr ecx, 03
:1012E6CC 83E13F                  and ecx, 03F
:1012E6CF 83F938                  cmp ecx, 038
:1012E6D2 B838000000              mov eax, 00000038
:1012E6D7 7C05                    jl 1012E6DE
:1012E6D9 B878000000              mov eax, 00000078
:1012E6DE 2BC1                    sub eax, ecx
:1012E6E0 50                      push eax
:1012E6E1 68201E1910              push 10191E20
:1012E6E6 56                      push esi
:1012E6E7 E8F4FEFFFF              call 1012E5E0 {void __cdecl appMD5Update(struct FMD5Context *,unsigned char *,int)}  {void __cdecl appMD5Update(struct FMD5Context *,unsigned char *,int)}
:1012E6EC 6A08                    push 008
:1012E6EE 8D442418                lea eax, dword[esp+18]
:1012E6F2 50                      push eax
:1012E6F3 56                      push esi
:1012E6F4 E8E7FEFFFF              call 1012E5E0 {void __cdecl appMD5Update(struct FMD5Context *,unsigned char *,int)}  {void __cdecl appMD5Update(struct FMD5Context *,unsigned char *,int)}
:1012E6F9 8B4C242C                mov ecx, dword[esp+2C]
:1012E6FD 83C418                  add esp, 018
:1012E700 41                      inc ecx
:1012E701 8D4602                  lea eax, dword[esi+02]
:1012E704 BF04000000              mov edi, 00000004
:1012E709 8DA42400000000          lea esp, dword[esp+00000000]
:1012E710 8A50FE                  mov dl, byte[eax-02]
:1012E713 8851FF                  mov byte[ecx-01], dl
:1012E716 8A50FF                  mov dl, byte[eax-01]
:1012E719 8811                    mov byte[ecx], dl
:1012E71B 8A10                    mov dl, byte[eax]
:1012E71D 885101                  mov byte[ecx+01], dl
:1012E720 8A5001                  mov dl, byte[eax+01]
:1012E723 885102                  mov byte[ecx+02], dl
:1012E726 83C004                  add eax, 004
:1012E729 83C104                  add ecx, 004
:1012E72C 4F                      dec edi
:1012E72D 75E1                    jne 1012E710
:1012E72F 6A58                    push 058
:1012E731 6A00                    push 000
:1012E733 56                      push esi
:1012E734 E83753FEFF              call 10113A70 {void __cdecl appMemset(void *,int,int)}  {void __cdecl appMemset(void *,int,int)}
:1012E739 83C40C                  add esp, 00C
:1012E73C 5F                      pop edi
:1012E73D 5E                      pop esi
:1012E73E 83C408                  add esp, 008
:1012E741 C3                      ret
i need to know the c variable types and what they would contain.

Thank You.
Question by:MudBlud
  • 3
  • 3

Expert Comment

ID: 9623837
You definitely can't find out the members of the original C structs from an assembly dump. However, the code you posted is the well-known MD5 algorithm. The corresponding C code is published in here:


Author Comment

ID: 9626316
i just need to know if the FMD5Context is the same as the original, so if that code is the same i guess it must be ;/

Accepted Solution

mtmike earned 1200 total points
ID: 9626428
The MD5Init in the given dump is nearly the same as the RFC C code, except for the fact that it also clears the buffer. Fields are identical.

Context structure offsets:
0x00 state[0]
0x04 state[1]
0x08 state[2]
0x12 state[3]
0x10 count[0]
0x14 count[1]
0x18 buffer

Annotated MD5Init:

mov eax, dword[esp+04] ; eax = context
xor ecx, ecx
mov dword[eax+14], ecx ; context->count[1] = 0
mov dword[eax+10], ecx ; context->count(0) = 0
mov dword[eax], 67452301 ; context->state[0] = 0x67452301
mov dword[eax+04], EFCDAB89 ; context->state[1] = 0xEFCDAB89
mov dword[eax+08], 98BADCFE ; context->state[2] = 0x98BADCFE
mov dword[eax+0C], 10325476 ; context->state[3] = 0x10325476

; memset(context->buffer, 0, 64)
push ebx
add eax, 018
push edi
mov dword[esp+0C], eax
mov ecx, 00000040 ; 0x40 == 64
mov edi, dword[esp+0C]
xor eax, eax
mov ebx, ecx
shr ecx, 02
and ebx, 003
rep stosd
mov ecx, ebx
rep stosb
pop edi
pop ebx

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Author Comment

ID: 9629919
hmm, i already accepted it but i have one more question :(

how am i meant  to get the final hash when the buffer is cleared?

Expert Comment

ID: 9630084
By calling MD5Final(digest, context). The context is cleared, but the message digest is stored in a user-supplied buffer.

Sample function to compute the 16 byte message digest:

void MessageDigest(unsigned char *digest, char *message, int length)
  MD5_CTX context;

  MD5Update(&context, message, length);
  MD5Final(digest, &context);

Author Comment

ID: 9630941
oh.. k, thanks

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