MFC逆向之CrackMe Level3 过反调试 + 写注册机

今天我来分享一下,过反调试的方法以及使用IDA还原代码 + 写注册机的过程

由于内容太多,我准备分为两个帖子写,这个帖子主要是写IDA还原代码,下一个帖子是写反调试的分析以及过反调试和异常

这个CrackMe Level3是一个朋友发我的,我也不知道他在哪里弄的,我感觉挺好玩的,对反调试异常以及代码还原的学习有一些帮助

调试器:X64和OD

反编译工具:IDA

PE工具:Detect It Easy

反调试插件:OD的StrongOD和虫子的ScyllaHide

ARK工具:Pchunter

MFC工具:MfcSpy

初步分析

第一步先看看PE文件工具说:软件是使用2008的MFC写的

上面得知是MFC写的软件,我们可以利用MFC的RTTI(动态类型识别)获取信息,如果不是MFC写的,我们可以运行一下软件看看,有没有报错信息

当然这个软件,输入注册码点击注册没有任何提示

打开调试器附加看看字符串,结果软件会闪退,无法附加,估计是有反调试,遇到这种情况我有三种方式可以解决

第一种:静态分析解决反调试

第二种:使用ARK工具把软件暂停,然后在用调试器附加

第三种:直接使用调试器打开程序,让软件断到入口或者系统断点

我们先使用第二种吧,因为X32调试器对中文字符串的支持不是很好,我没有装插件,所以先用OD看看字符串吧

验证函数中有一个异常 OD的反汇编已经出现了BUG,可以使用删除分析,我这里就不删除了直接打开IDA F5吧

虽然不能F5 但是我们可以很清晰的看到,两个GetWindowText 估计一个是获取用户名一个是获取密码

至于为什么不能F5 是因为有一个jmp $ebx + 0xXXX$ 这样的代码 IDA在解析代码的时候它不知道跳到那里去导致无法解析

我们直接把这个Jmp $ebp + var_8A8$ 给nop掉就好了然后删除这个函数的分析接着在把这个函数分析为代码即可

我们可以向上看,在OD反汇编的时候也是到这个位置反汇编失败了

下面是IDA F5后的代码很明显这IDA还原出来的根本不能用,这是需要我们进行修理变量的,另外我们给它重新命名一下

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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 | int __thiscall sub_401810(CWnd *``this``) { ``CWnd *DlgItem; ``// eax ``CWnd *v2; ``// eax ``int result; ``// eax ``char *v4; ``// [esp+10h] [ebp-8C8h] ``CHAR *v5; ``// [esp+14h] [ebp-8C4h] ``unsigned ``int v6; ``// [esp+18h] [ebp-8C0h] ``unsigned ``__int8 v8; ``// [esp+21h] [ebp-8B7h] ``unsigned ``__int8 v9; ``// [esp+22h] [ebp-8B6h] ``unsigned ``__int8 v10; ``// [esp+23h] [ebp-8B5h] ``int k; ``// [esp+24h] [ebp-8B4h] ``int j; ``// [esp+28h] [ebp-8B0h] ``int i; ``// [esp+2Ch] [ebp-8ACh] ``int v14; ``// [esp+34h] [ebp-8A4h] ``int v15; ``// [esp+34h] [ebp-8A4h] ``char v16; ``// [esp+38h] [ebp-8A0h] BYREF ``int v17[6]; ``// [esp+39h] [ebp-89Fh] BYREF ``char v18; ``// [esp+51h] [ebp-887h] ``char v19; ``// [esp+53h] [ebp-885h] ``char v20; ``// [esp+54h] [ebp-884h] ``int v21; ``// [esp+55h] [ebp-883h] ``int v22[5]; ``// [esp+59h] [ebp-87Fh] BYREF ``char v23; ``// [esp+6Dh] [ebp-86Bh] ``int v24; ``// [esp+70h] [ebp-868h] ``char v25; ``// [esp+74h] [ebp-864h] ``int v26; ``// [esp+75h] [ebp-863h] ``int v27; ``// [esp+79h] [ebp-85Fh] ``int v28; ``// [esp+7Dh] [ebp-85Bh] ``int v29; ``// [esp+81h] [ebp-857h] ``int v30; ``// [esp+85h] [ebp-853h] ``int v31; ``// [esp+89h] [ebp-84Fh] ``char v32; ``// [esp+8Dh] [ebp-84Bh] ``char v33; ``// [esp+93h] [ebp-845h] ``char v34; ``// [esp+94h] [ebp-844h] ``int v35; ``// [esp+95h] [ebp-843h] ``int v36[5]; ``// [esp+99h] [ebp-83Fh] BYREF ``char v37; ``// [esp+ADh] [ebp-82Bh] ``CHAR String[1028]; ``// [esp+B0h] [ebp-828h] BYREF ``int v39; ``// [esp+4B4h] [ebp-424h] ``CHAR v40[1028]; ``// [esp+4B8h] [ebp-420h] BYREF ``CPPEH_RECORD ms_exc; ``// [esp+8C0h] [ebp-18h] ``MEMORY[0] = 6530; ``ms_exc.registration.TryLevel = -2; ``v33 = sub_402D60(); ``v14 = sub_402C60(v33); ``sub_402750(v14); ``v26 = 0; ``v27 = 0; ``v28 = 0; ``v29 = 0; ``v30 = 0; ``v31 = 0; ``v32 = 0; ``v35 = 0; ``memset``(v36, 0, ``sizeof``(v36)); ``v37 = 0; ``v21 = 0; ``memset``(v22, 0, ``sizeof``(v22)); ``v23 = 0; ``v25 = 48; ``v34 = 66; ``v20 = 98; ``for ( i = 1; i < 26; ++i ) ``{ ``*(&v25 + i) = *((_BYTE *)&v24 + i + 3) + 1; ``*(&v34 + i) = *(&v33 + i) + 1; ``*(&v20 + i) = *(&v19 + i) + 1; ``} ``CWnd::UpdateData(``this``, 1); ``memset``(String, 0, 1024); ``memset``(v40, 0, 1024); ``ms_exc.registration.Next = (``struct _EH3_EXCEPTION_REGISTRATION *)1024; ``ms_exc.exc_ptr = (EXCEPTION_POINTERS *)String; ``DlgItem = CWnd::GetDlgItem(``this``, 1000); ``CWnd::GetWindowTextA(DlgItem, (``LPSTR``)ms_exc.exc_ptr, (``int``)ms_exc.registration.Next); ``ms_exc.registration.Next = (``struct _EH3_EXCEPTION_REGISTRATION *)1024; ``ms_exc.exc_ptr = (EXCEPTION_POINTERS *)v40; ``v2 = CWnd::GetDlgItem(``this``, 1001); ``result = CWnd::GetWindowTextA(v2, (``LPSTR``)ms_exc.exc_ptr, (``int``)ms_exc.registration.Next); ``if ( String[7] ) ``{ ``result = String[8]; ``if ( !String[8] && v40[23] && !v40[24] ) ``{ ``v39 = 16; ``v24 = 32; ``for ( j = 0; j < 8; ++j ) ``{ ``String[j] ^= j; ``String[j] ^= *(_BYTE *)(j + v39); ``String[j] ^= *(_BYTE *)(j + v24); ``} ``v16 = 0; ``memset``(v17, 0, ``sizeof``(v17)); ``v18 = 0; ``for ( k = 0; k < 8; ++k ) ``{ ``v10 = (unsigned ``__int8``)(String[k] & 0xE0) / 32; ``v8 = (String[k] & 0x1C) / 4; ``v9 = String[k] & 3; ``if ( k % 3 == 2 ) ``{ ``*(&v16 + 3 * k) = *(&v25 + v9); ``*((_BYTE *)v17 + 3 * k) = *((_BYTE *)v36 + v10 + 3); ``*((_BYTE *)v17 + 3 * k + 1) = *((_BYTE *)&v22[2] + v8 + 3); ``} ``if ( k % 3 == 1 ) ``{ ``*(&v16 + 3 * k) = *((_BYTE *)&v36[2] + v10 + 3); ``*((_BYTE *)v17 + 3 * k) = *((_BYTE *)v22 + v8 + 3); ``*((_BYTE *)v17 + 3 * k + 1) = *(&v25 + v9); ``} ``if ( !(k % 3) ) ``{ ``*(&v16 + 3 * k) = *((_BYTE *)&v36[2] + v8 + 3); ``*((_BYTE *)v17 + 3 * k) = *((_BYTE *)v22 + v9 + 3); ``*((_BYTE *)v17 + 3 * k + 1) = *(&v25 + v10); ``} ``} ``sub_401790(); ``v33 = sub_402D90(); ``v15 = sub_402C60(v33); ``sub_4028A0(v15); ``v6 = 24; ``v5 = v40; ``v4 = &v16; ``while ( v6 >= 4 ) ``{ ``result = (``int``)v5; ``if ( *(_DWORD *)v4 != *(_DWORD *)v5 ) ``return result; ``v6 -= 4; ``v5 += 4; ``v4 += 4; ``} ``return CWnd::MessageBoxA(``this``, &Text, 0, 0); ``} ``} ``return result; } |

1.根据循环可以知道对应数组大小和数组首地址

2.根据MemSet也可以知道对应数组的大小和数组首地址

3.根据GetWindowsText也可以知道数组的大小

修正变量要结合代码怎么去操作这个变量的根据它的逻辑来猜测大小

这是变量重命名后的汇编代码

经过修理后的IDA F5代码可以看到基本上很清晰了效果跟源代码应该很接近

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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 | int __thiscall sub_401810(CWnd *``this``) { ``CWnd *DlgItem; ``// eax ``CWnd *v2; ``// eax ``int result; ``// eax ``char *pFinalKey_1; ``// [esp+10h] [ebp-8C8h] ``char *pGetWindowTextString1_1; ``// [esp+14h] [ebp-8C4h] ``unsigned ``int Var24_1; ``// [esp+18h] [ebp-8C0h] ``unsigned ``__int8 Var8_1; ``// [esp+21h] [ebp-8B7h] ``unsigned ``__int8 Var9_1; ``// [esp+22h] [ebp-8B6h] ``unsigned ``__int8 Var10_1; ``// [esp+23h] [ebp-8B5h] ``int K; ``// [esp+24h] [ebp-8B4h] ``int J; ``// [esp+28h] [ebp-8B0h] ``int I; ``// [esp+2Ch] [ebp-8ACh] ``int Unknown2_1; ``// [esp+34h] [ebp-8A4h] ``int unknown1; ``// [esp+34h] [ebp-8A4h] ``char FinalKey_1[26]; ``// [esp+38h] [ebp-8A0h] BYREF ``char SecretKey3[28]; ``// [esp+54h] [ebp-884h] BYREF ``int v19; ``// [esp+70h] [ebp-868h] ``char SecretKey1[31]; ``// [esp+74h] [ebp-864h] BYREF ``char Unknown3_1; ``// [esp+93h] [ebp-845h] ``char SecretKey2[28]; ``// [esp+94h] [ebp-844h] BYREF ``char GetWindowTextString2_1[1028]; ``// [esp+B0h] [ebp-828h] BYREF ``int v24; ``// [esp+4B4h] [ebp-424h] ``char GetWindowTextString1[1028]; ``// [esp+4B8h] [ebp-420h] BYREF ``CPPEH_RECORD ms_exc; ``// [esp+8C0h] [ebp-18h] ``MEMORY[0] = 6530; ``ms_exc.registration.TryLevel = -2; ``Unknown3_1 = sub_402D60(); ``Unknown2_1 = sub_402C60(&dword_42FA04, Unknown3_1); ``sub_402750(Unknown2_1); ``memset``(&SecretKey1[1], 0, 25); ``memset``(&SecretKey2[1], 0, 25); ``memset``(&SecretKey3[1], 0, 25); ``SecretKey1[0] = 48; ``SecretKey2[0] = 66; ``SecretKey3[0] = 98; ``for ( I = 1; I < 26; ++I ) ``{ ``SecretKey1[I] = SecretKey1[I - 1] + 1; ``SecretKey2[I] = SecretKey2[I - 1] + 1; ``SecretKey3[I] = SecretKey3[I - 1] + 1; ``} ``CWnd::UpdateData(``this``, 1); ``memset``(GetWindowTextString2_1, 0, 1024); ``memset``(GetWindowTextString1, 0, 1024); ``ms_exc.registration.Next = (``struct _EH3_EXCEPTION_REGISTRATION *)1024; ``ms_exc.exc_ptr = (EXCEPTION_POINTERS *)GetWindowTextString2_1; ``DlgItem = CWnd::GetDlgItem(``this``, 1000); ``CWnd::GetWindowTextA(DlgItem, (``LPSTR``)ms_exc.exc_ptr, (``int``)ms_exc.registration.Next); ``ms_exc.registration.Next = (``struct _EH3_EXCEPTION_REGISTRATION *)1024; ``ms_exc.exc_ptr = (EXCEPTION_POINTERS *)GetWindowTextString1; ``v2 = CWnd::GetDlgItem(``this``, 1001); ``result = CWnd::GetWindowTextA(v2, (``LPSTR``)ms_exc.exc_ptr, (``int``)ms_exc.registration.Next); ``if ( GetWindowTextString2_1[7] ) ``{ ``result = GetWindowTextString2_1[8]; ``if ( !GetWindowTextString2_1[8] && GetWindowTextString1[23] && !GetWindowTextString1[24] ) ``{ ``v24 = 16; ``v19 = 32; ``for ( J = 0; J < 8; ++J ) ``{ ``GetWindowTextString2_1[J] ^= J; ``GetWindowTextString2_1[J] ^= *(_BYTE *)(J + v24); ``GetWindowTextString2_1[J] ^= *(_BYTE *)(J + v19); ``} ``memset``(FinalKey_1, 0, ``sizeof``(FinalKey_1)); ``for ( K = 0; K < 8; ++K ) ``{ ``Var10_1 = (unsigned ``__int8``)(GetWindowTextString2_1[K] & 0xE0) / 32; ``Var8_1 = (GetWindowTextString2_1[K] & 0x1C) / 4; ``Var9_1 = GetWindowTextString2_1[K] & 3; ``if ( K % 3 == 2 ) ``{ ``FinalKey_1[3 * K] = SecretKey1[Var9_1]; ``FinalKey_1[3 * K + 1] = SecretKey2[Var10_1 + 8]; ``FinalKey_1[3 * K + 2] = SecretKey3[Var8_1 + 16]; ``} ``if ( K % 3 == 1 ) ``{ ``FinalKey_1[3 * K] = SecretKey2[Var10_1 + 16]; ``FinalKey_1[3 * K + 1] = SecretKey3[Var8_1 + 8]; ``FinalKey_1[3 * K + 2] = SecretKey1[Var9_1]; ``} ``if ( !(K % 3) ) ``{ ``FinalKey_1[3 * K] = SecretKey2[Var8_1 + 16]; ``FinalKey_1[3 * K + 1] = SecretKey3[Var9_1 + 8]; ``FinalKey_1[3 * K + 2] = SecretKey1[Var10_1]; ``} ``} ``sub_401790(); ``Unknown3_1 = sub_402D90(); ``unknown1 = sub_402C60(&dword_42FA04, Unknown3_1); ``sub_4028A0(unknown1); ``Var24_1 = 24; ``pGetWindowTextString1_1 = GetWindowTextString1; ``pFinalKey_1 = FinalKey_1; ``while ( Var24_1 >= 4 ) ``{ ``result = (``int``)pGetWindowTextString1_1; ``if ( *(_DWORD *)pFinalKey_1 != *(_DWORD *)pGetWindowTextString1_1 ) ``return result; ``Var24_1 -= 4; ``pGetWindowTextString1_1 += 4; ``pFinalKey_1 += 4; ``} ``return CWnd::MessageBoxA(``this``, &Text, 0, 0); ``} ``} ``return result; } |

我们可以看到上面的代码中还是有两个变量的值是错误的一个是V24 一个是V19 为什么是错误的呢?

我们可以结合下面的循环代码,下面代码中用到V24 是*(BYTE*)(J+V24); 用到V19 是*(BYTE*)(J+V19);

这很明显是一个数组大小是8个字节

看下面代码中 V19和V24是来源于AbnormalVariable这个变量的值 + 16和+32的位置的

这也就是下面代码中V19和V24数组的值的来源

最终的C代码算法还原完成

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