一個ReverseME破解 SynApsus's ReverseME #1 (13千字)

看雪資料發表於2015-11-15

SynApsus's ReverseME #1

感謝hello兄提供的NCG的CrackMe,正為CCG的CrackMe鬱悶,先找幾個簡單的下手.從其中挑了一個ReverseMe.這個要求修改程式,使其在輸入NAME的同時把正確的SERAIL在下面顯示出來.

關鍵程式碼如下:

:004011E8 6A32                    push 00000032
:004011EA 6825304000              push 00403025 <-NAME放在403025
:004011EF FF3505304000            push dword ptr [00403005] <-這是NAME文字框的控制程式碼

* Reference To: USER32.GetWindowTextA, Ord:015Bh
                                  |
:004011F5 E808010000              Call 00401302
:004011FA A315304000              mov dword ptr [00403015], eax
:004011FF 3B0519304000            cmp eax, dword ptr [00403019]
:00401205 0F84DD000000            je 004012E8 <-???莫名其妙,其實大有用處
:0040120B 803D2530400000          cmp byte ptr [00403025], 00
:00401212 0F84D0000000            je 004012E8 <-判斷是否為空
:00401218 6A05                    push 00000005

* Reference To: KERNEL32.Sleep, Ord:0273h
                                  |
:0040121A E807010000              Call 00401326
:0040121F 6A32                    push 00000032
:00401221 6857304000              push 00403057 <-Serial放在403057
:00401226 FF3509304000            push dword ptr [00403009] <-這是Serial文字框的控制程式碼

* Reference To: USER32.GetWindowTextA, Ord:015Bh
                                  |
:0040122C E8D1000000              Call 00401302
:00401231 A31D304000              mov dword ptr [0040301D], eax
:00401236 3B0521304000            cmp eax, dword ptr [00403021]
:0040123C 0F84A6000000            je 004012E8
:00401242 803D5730400000          cmp byte ptr [00403057], 00
:00401249 0F8499000000            je 004012E8
:0040124F 33C0                    xor eax, eax
:00401251 33DB                    xor ebx, ebx
:00401253 33C9                    xor ecx, ecx
:00401255 8D0525304000            lea eax, dword ptr [00403025]

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:0040126B(C)
|
:0040125B 8A8B25304000            mov cl, byte ptr [ebx+00403025] <-把NAME依次取出
:00401261 03C1                    add eax, ecx <-和403025h累加起來
:00401263 43                      inc ebx
:00401264 80BB2530400000          cmp byte ptr [ebx+00403025], 00
:0040126B 75EE                    jne 0040125B <-迴圈計算
:0040126D BB09030000              mov ebx, 00000309 
:00401272 F7E3                    mul ebx <-再乘以309h
:00401274 03C3                    add eax, ebx <-再加倍
:00401276 33DB                    xor ebx, ebx
:00401278 33D2                    xor edx, edx
:0040127A 33F6                    xor esi, esi

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:00401290(U)
|
:0040127C B906000000              mov ecx, 00000006 
:00401281 F7E1                    mul ecx <-乘6迴圈累加
:00401283 8A9E25304000            mov bl, byte ptr [esi+00403025]
:00401289 84DB                    test bl, bl
:0040128B 7405                    je 00401292
:0040128D 03C3                    add eax, ebx
:0040128F 46                      inc esi
:00401290 EBEA                    jmp 0040127C <-迴圈計算,得到EAX為最終結果(計算過程在這裡並不重要)

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:0040128B(C)
|
:00401292 50                      push eax <-結果入棧
:00401293 33C0                    xor eax, eax 
:00401295 8D3557304000            lea esi, dword ptr [00403057] <-取出Serial
:0040129B 8A1E                    mov bl, byte ptr [esi]
:0040129D 46                      inc esi

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:004012AF(C)
|
:0040129E 80EB30                  sub bl, 30 <-每一位減30h
:004012A1 BF0A000000              mov edi, 0000000A <-乘以A
:004012A6 F7E7                    mul edi
:004012A8 03C3                    add eax, ebx <-累加
:004012AA 8A1E                    mov bl, byte ptr [esi]
:004012AC 46                      inc esi
:004012AD 84DB                    test bl, bl
:004012AF 75ED                    jne 0040129E <-迴圈計算,可以看出實際上是把輸入的SERIAL當成十進位制數轉為了數值
:004012B1 5B                      pop ebx <-前面NAME的計算結果出棧
:004012B2 8D0D15304000            lea ecx, dword ptr [00403015]
:004012B8 8D151D304000            lea edx, dword ptr [0040301D]
:004012BE 890D19304000            mov dword ptr [00403019], ecx
:004012C4 891521304000            mov dword ptr [00403021], edx
:004012CA 3BC3                    cmp eax, ebx <-把NAME的結果和SERAIL的結果比較
:004012CC 751A                    jne 004012E8 <-關鍵跳轉
:004012CE C6050030400001          mov byte ptr [00403000], 01
:004012D5 6A00                    push 00000000

* Possible StringData Ref from Data Obj ->"Good serial !!!"
                                  |
:004012D7 68E0304000              push 004030E0

* Possible StringData Ref from Data Obj ->"Good Job Cracker ! But it's not "
                                        ->"enough, a serial to find... reverse "
                                        ->"this program now !"
                                  |
:004012DC 6889304000              push 00403089
:004012E1 6A00                    push 00000000

* Reference To: USER32.MessageBoxA, Ord:01BBh
                                  |
:004012E3 E826000000              Call 0040130E <-對話方塊出現,說這樣還不夠,要讓程式自己出現註冊碼

* Referenced by a (U)nconditional or (C)onditional Jump at Addresses:
|:004011DB(C), :00401205(C), :00401212(C), :004012CC(C), :00401357(U)
|
:004012E8 61                      popad
:004012E9 33C0                    xor eax, eax
:004012EB C9                      leave
:004012EC C21000                  ret 0010

先整理一下思路,程式把NAME經過一番計算後與十進位制的SERIAL比較,所以我們要把NAME的計算結果直接轉成字串顯示出來就OK了.

溫習一下程式設計,把數值轉成字串用什麼函式呢?當然是wsprintf啦.還要把字串顯示出來,就用SetWindowText吧.用LordPE給它加一個IID陣列(人家說了: u can use all the tools you need.) ,加上USER32.DLL裡的這兩個函式,看看ThunkRVA是多少,我的是604C和6050.(作者還專門給我們留了一個空塊放東西,有趣)

一開始改好後,一進入就會陷入死迴圈,我猜想程式是透過處理WM_這樣的系統訊息來執行這段程式碼,而我們寫進去的東西要把正確的註冊碼顯示在文字框,必然要再次觸發這條訊息,這樣形成死迴圈.苦思冥想N十分鐘後,想起程式裡有兩行很奇怪,就是在讀出NAME和SERAIL以後有一個CMP是判斷是否為空,還有一個卻不知所云.原來這就是關鍵的地方,只要在這裡設一個標誌就OK了.

現在祭出HIEW,壘程式碼吧.還得來回轉換什麼檔案偏移記憶體偏移,好痛苦.修改以後的程式碼如下:

:004011E8 6A32                    push 00000032
:004011EA 6825304000              push 00403025
:004011EF FF3505304000            push dword ptr [00403005]
:004011F5 E808010000              call 00401302 
:004011FA A315304000              mov dword ptr [00403015], eax 
:004011FF 3B0519304000            cmp eax, dword ptr [00403019] <-就是上面兩句,EAX是文字的字元個數,把它放在[403015]以後,再把它和[403019]處的數比較,如果相同就跳走不做處理.就是說我們可以在每次顯示出正確註冊碼後把[403015]處和[403019]處設為相同,這樣如果NAME的位數不變,程式就不會處理.而每次NAME改變時EAX就會變化,程式繼續處理新的NAME.(我考試時腦袋有這時的一半靈活就好了)
:00401205 0F84DD000000            je 004012E8
:0040120B 803D2530400000          cmp byte ptr [00403025], 00
:00401212 0F84D0000000            je 004012E8
:00401218 6A05                    push 00000005
:0040121A E807010000              call 00401326 <-上面沒有變化,讀取NAME
:0040121F 6A32                    push 00000032
:00401221 6857304000              push 00403057
:00401226 FF3509304000            push dword ptr [00403009]
:0040122C E8D1000000              call 00401302 <-讀取SERIAL
:00401231 A31D304000              mov dword ptr [0040301D], eax
:00401236 3B0521304000            cmp eax, dword ptr [00403021]
:0040123C 90                      nop <-把這裡對SERIAL的判斷去掉
:0040123D 90                      nop                   <-不然如果SERIAL為空就永遠會跳走
:0040123E 90                      nop
:0040123F 90                      nop
:00401240 90                      nop
:00401241 90                      nop
:00401242 803D5730400000          cmp byte ptr [00403057], 00
:00401249 90                      nop <-NOP大法非常實用:)
:0040124A 90                      nop
:0040124B 90                      nop
:0040124C 90                      nop
:0040124D 90                      nop
:0040124E 90                      nop
:0040124F 33C0                    xor eax, eax
:00401251 33DB                    xor ebx, ebx
:00401253 33C9                    xor ecx, ecx
:00401255 8D0525304000            lea eax, dword ptr [00403025]

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:0040126B(C)
|
:0040125B 8A8B25304000            mov cl, byte ptr [ebx+00403025]
:00401261 03C1                    add eax, ecx
:00401263 43                      inc ebx
:00401264 80BB2530400000          cmp byte ptr [ebx+00403025], 00
:0040126B 75EE                    jne 0040125B
:0040126D BB09030000              mov ebx, 00000309
:00401272 F7E3                    mul ebx
:00401274 03C3                    add eax, ebx
:00401276 33DB                    xor ebx, ebx
:00401278 33D2                    xor edx, edx
:0040127A 33F6                    xor esi, esi

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:00401290(U)
|
:0040127C B906000000              mov ecx, 00000006
:00401281 F7E1                    mul ecx
:00401283 8A9E25304000            mov bl, byte ptr [esi+00403025]
:00401289 84DB                    test bl, bl
:0040128B 7405                    je 00401292
:0040128D 03C3                    add eax, ebx
:0040128F 46                      inc esi
:00401290 EBEA                    jmp 0040127C <-上面是對NAME的計算,我們知道結果在EAX就行了

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:0040128B(C)
|
:00401292 E995000000              jmp 0040132C <-跳到自己寫的程式碼裡去
:00401297 90                      nop
:00401298 90                      nop <-這裡沒用了,NOP掉好看一些
:00401299 90                      nop
:0040129A 90                      nop
:0040129B 8A1E                    mov bl, byte ptr [esi]
:0040129D 46                      inc esi

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:004012AF(C)
|
:0040129E 80EB30                  sub bl, 30
:004012A1 BF0A000000              mov edi, 0000000A
:004012A6 F7E7                    mul edi
:004012A8 03C3                    add eax, ebx
:004012AA 8A1E                    mov bl, byte ptr [esi]
:004012AC 46                      inc esi
:004012AD 84DB                    test bl, bl
:004012AF 75ED                    jne 0040129E
:004012B1 5B                      pop ebx
:004012B2 8D0D15304000            lea ecx, dword ptr [00403015]
:004012B8 8D151D304000            lea edx, dword ptr [0040301D]
:004012BE 890D19304000            mov dword ptr [00403019], ecx
:004012C4 891521304000            mov dword ptr [00403021], edx
:004012CA 3BC3                    cmp eax, ebx
:004012CC 751A                    jne 004012E8
:004012CE C6050030400001          mov byte ptr [00403000], 01
:004012D5 6A00                    push 00000000

* Possible StringData Ref from Data Obj ->"Good serial !!!"
                                  |
:004012D7 68E0304000              push 004030E0

* Possible StringData Ref from Data Obj ->"Good Job Cracker ! But it's not "
                                        ->"enough, a serial to find... reverse "
                                        ->"this program now !"
                                  |
:004012DC 6889304000              push 00403089
:004012E1 6A00                    push 00000000
:004012E3 E826000000              call 0040130E

* Referenced by a (U)nconditional or (C)onditional Jump at Addresses:
|:004011DB(C), :00401205(C), :00401212(C), :004012CC(C), :00401357(U)
|
:004012E8 61                      popad
:004012E9 33C0                    xor eax, eax
:004012EB C9                      leave
:004012EC C21000                  ret 0010

......

(自己的程式碼)

* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:00401292(U) <-第一次用匯編寫這麼長,很爛,高手莫笑
|
:0040132C 60                      pushad <-保險起見 
:0040132D 50                      push eax <-這就是上面的計算結果

* Possible StringData Ref from Code Obj ->"%lu" <-wsprintf的引數(我查了半天書才查到)
                                  |
:0040132E 6860134000              push 00401360 <-直接把它扔在後面了,一大片空地
:00401333 57                      push edi <-用EDI儲存字串的地址(?)
:00401334 8B0D15304000            mov ecx, dword ptr [00403015] <-這裡是關鍵的兩句
:0040133A 890D19304000            mov dword ptr [00403019], ecx <-後加的這兩句插在這裡了
:00401340 FF1550604000            call dword ptr [00406050] <-這個CALL就是wsprintfA了
:00401346 83C40C                  add esp, 0000000C <-平衡堆疊
:00401349 57                      push edi <-轉換得到的字串
:0040134A FF3509304000            push dword ptr [00403009] <-這是那個輸入SERIAL文字框的控制程式碼
:00401350 FF154C604000            call dword ptr [0040604C] <-這個CALL是SetWindowTextA
:00401356 61                      popad 
:00401357 E98CFFFFFF              jmp 004012E8 <-終於完工嘍
:0040135C 00000000                BYTE  4 DUP(0)

菜鳥問題:我想把得到的字串放在401370處,不知為什麼卻不成功,後來只好蒙了一個EDI來放.
第一次搞這種東東(好像還稱不上"逆向工程"),請各位高手指教.

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