环境：
Windows xp sp3

工具：
ollydbg
exeinfope

0x00 查壳

无壳的程序

0x01 分析

运行后随便输入点东西，

OD载入：

00401127   > /6A 00         push 0x0                                 ; /lParam = 0
00401129   . |6A 00         push 0x0                                 ; |wParam = 0
0040112B   . |6A 0E         push 0xE                                 ; |Message = WM_GETTEXTLENGTH
0040112D   . |6A 03         push 0x3                                 ; |ControlID = 3
0040112F   . |FF75 08       push dword ptr ss:[ebp+0x8]              ; |hWnd
00401132   . |E8 41020000   call <jmp.&USER32.SendDlgItemMessageA>   ; \SendDlgItemMessageA
00401137   . |A3 AF214000   mov dword ptr ds:[0x4021AF],eax
0040113C   . |83F8 00       cmp eax,0x0                              ;  不能为空
0040113F   . |0F84 D5000000 je DueList_.0040121A
00401145   . |83F8 08       cmp eax,0x8                              ;  大于8个就跳
00401148   . |0F8F CC000000 jg DueList_.0040121A
0040114E   . |8BF0          mov esi,eax
00401150   . |6A 00         push 0x0                                 ; /lParam = 0
00401152   . |6A 00         push 0x0                                 ; |wParam = 0
00401154   . |6A 0E         push 0xE                                 ; |Message = WM_GETTEXTLENGTH
00401156   . |6A 04         push 0x4                                 ; |ControlID = 4
00401158   . |FF75 08       push dword ptr ss:[ebp+0x8]              ; |hWnd
0040115B   . |E8 18020000   call <jmp.&USER32.SendDlgItemMessageA>   ; \SendDlgItemMessageA
00401160   . |83F8 00       cmp eax,0x0
00401163   . |0F84 B1000000 je DueList_.0040121A
00401169   . |3BF0          cmp esi,eax
0040116B   . |0F85 A9000000 jnz DueList_.0040121A                    ;  serial与name的长度不同就跳
00401171   . |68 60214000   push DueList_.00402160                   ; /lParam = 402160
00401176   . |6A 08         push 0x8                                 ; |wParam = 8
00401178   . |6A 0D         push 0xD                                 ; |Message = WM_GETTEXT
0040117A   . |6A 03         push 0x3                                 ; |ControlID = 3
0040117C   . |FF75 08       push dword ptr ss:[ebp+0x8]              ; |hWnd
0040117F   . |E8 F4010000   call <jmp.&USER32.SendDlgItemMessageA>   ; \SendDlgItemMessageA
00401184   . |68 79214000   push DueList_.00402179                   ; /lParam = 402179
00401189   . |6A 10         push 0x10                                ; |wParam = 10
0040118B   . |6A 0D         push 0xD                                 ; |Message = WM_GETTEXT
0040118D   . |6A 04         push 0x4                                 ; |ControlID = 4
0040118F   . |FF75 08       push dword ptr ss:[ebp+0x8]              ; |hWnd
00401192   . |E8 E1010000   call <jmp.&USER32.SendDlgItemMessageA>   ; \SendDlgItemMessageA
00401197   . |B9 FFFFFFFF   mov ecx,-0x1
0040119C   > |41            inc ecx
0040119D   . |0FBE81 602140>movsx eax,byte ptr ds:[ecx+0x402160]
004011A4   . |83F8 00       cmp eax,0x0                              ;  Switch (cases 0..7A)
004011A7   . |74 32         je XDueList_.004011DB
004011A9   . |BE FFFFFFFF   mov esi,-0x1
004011AE   . |83F8 41       cmp eax,0x41
004011B1   . |7C 67         jl XDueList_.0040121A                    ;  小于0x41的直接错误
004011B3   . |83F8 7A       cmp eax,0x7A
004011B6   . |77 62         ja XDueList_.0040121A                    ;  大于0x7a的就直接错误
004011B8   . |83F8 5A       cmp eax,0x5A
004011BB   . |7C 03         jl XDueList_.004011C0
004011BD   . |83E8 20       sub eax,0x20                             ;  Cases 5A ('Z'),5B ('['),5C ('\'),5D (']'),5E ('^'),5F ('_'),60 ('`'),61 ('a'),62 ('b'),63 ('c'),64 ('d'),65 ('e'),66 ('f'),67 ('g'),68 ('h'),69 ('i'),6A ('j'),6B ('k'),6C ('l'),6D ('m')... of switch 004011A4
004011C0   > |46            inc esi                                  ;  Cases 41 ('A'),42 ('B'),43 ('C'),44 ('D'),45 ('E'),46 ('F'),47 ('G'),48 ('H'),49 ('I'),4A ('J'),4B ('K'),4C ('L'),4D ('M'),4E ('N'),4F ('O'),50 ('P'),51 ('Q'),52 ('R'),53 ('S'),54 ('T')... of switch 004011A4
004011C1   . |0FBE96 172040>movsx edx,byte ptr ds:[esi+0x402017]     ;  找到相同的值
004011C8   . |3BC2          cmp eax,edx
004011CA   .^|75 F4         jnz XDueList_.004011C0
004011CC     |0FBE86 3C2040>movsx eax,byte ptr ds:[esi+0x40203C]     ;  在另一个字符串中找到对应位置的值
004011D3   . |8981 94214000 mov dword ptr ds:[ecx+0x402194],eax      ;  这里就是正确的serial
004011D9   .^|EB C1         jmp XDueList_.0040119C
004011DB   > |FF35 AF214000 push dword ptr ds:[0x4021AF]             ;  Case 0 of switch 004011A4
004011E1   . |68 94214000   push DueList_.00402194
004011E6   . |68 79214000   push DueList_.00402179
004011EB   . |E8 54000000   call DueList_.00401244                   ;  比较
004011F0   . |83F8 01       cmp eax,0x1
004011F3   .^|0F84 DEFEFFFF je DueList_.004010D7

搜一下错误信息的字符串，往上翻一翻就有了。下个断点观察一下，不难看出这里就是算法了。

0x02 算法分析

这个算法也是比较简单的，将输入的内容转为大写字母，与：

A1LSK2DJF4HGP3QWO5EIR6UTYZ8MXN7CBV9

比较，找到相同字符的位置，然后再从：

SU7CSJKF09NCSDO9SDF09SDRLVK7809S4NF

找到对应位置的字符，就是所求的serial

存在一个问题就是如果输入的内容在a(0x61)和Z(0x5A)之间的话，程序会错误，因为在上面的字符串中找不到对应的字符，所以会没有正确的serial