说实话，今天被自己蠢哭了
因为看多了一个字符，以为是输入字符变形后的base64编码，也怪自己没大致看过base64汇编形式，把base64跟完了用py实现完算法才意思到是base64，这是题外话
本人初学者，两天或一天一个cm练练，大家可以与我交流akkumans@qq.com，我博客
上面的题外话就是今天搞的一个cm，被自己蠢哭了，不过也算是base64编码流程无比清晰了，不算是无用功
这个cm是一个控制台的，丢到xp无法运行，本机只装了x64dbg(x32dbg)，用这个调试软件来试试吧
C:UsersAdministratorDesktop>reverse3.exe
Please enter the flag:97103012
wrong input
字符串搜索，找到判断的地方
01241269 | 85 C0 | test eax,eax | zf=1 => eax=0
0124126B | 75 07 | jne reverse3.1241274 |
0124126D | 68 78 21 24 01 | push reverse3.1242178 | 1242178:"this is the right flag"
01241272 | EB 05 | jmp reverse3.1241279 |
01241274 | 68 90 21 24 01 | push reverse3.1242190 | 1242190:"wrong input"
01241279 | FF 15 B0 20 24 01 | call dword ptr ds:[<&puts>] |
0124127F | 8B 4D FC | mov ecx,dword ptr ss:[ebp-4] |
01241282 | 83 C4 04 | add esp,4 |
01241285 | 33 CD | xor ecx,ebp |
01241287 | 33 C0 | xor eax,eax | eax:"OTacMDMzMTI="
01241289 | E8 92 00 00 00 | call reverse3.1241320 |
0124128E | 8B E5 | mov esp,ebp |
01241290 | 5D | pop ebp |
01241291 | C3 | ret |
可以看到要得到flag，jne就不能跳，也就是test eax,eax后的ZF=1，也就是eax=0
那这个eax=0从何而来？我们接着往上看
012411A0 | 55 | push ebp |
012411A1 | 8B EC | mov ebp,esp |
012411A3 | 83 EC 44 | sub esp,44 |
012411A6 | A1 04 30 24 01 | mov eax,dword ptr ds:[1243004] | eax:"OTacMDMzMTI="
012411AB | 33 C5 | xor eax,ebp |
012411AD | 89 45 FC | mov dword ptr ss:[ebp-4],eax |
012411B0 | 0F 57 C0 | xorps xmm0,xmm0 |
012411B3 | C7 45 F8 00 00 00 00 | mov dword ptr ss:[ebp-8],0 |
012411BA | 68 58 21 24 01 | push reverse3.1242158 | 1242158:"Please enter the flag:"
012411BF | 0F 11 45 E8 | movups xmmword ptr ss:[ebp-18],xmm0 |
012411C3 | 0F 11 45 C0 | movups xmmword ptr ss:[ebp-40],xmm0 |
012411C7 | 0F 11 45 D0 | movups xmmword ptr ss:[ebp-30],xmm0 |
012411CB | 66 0F D6 45 E0 | movq qword ptr ss:[ebp-20],xmm0 |
012411D0 | E8 1B 01 00 00 | call reverse3.12412F0 |
012411D5 | 8D 45 E8 | lea eax,dword ptr ss:[ebp-18] |
012411D8 | 50 | push eax | eax:"OTacMDMzMTI="
012411D9 | 68 70 21 24 01 | push reverse3.1242170 | 1242170:"%20s"
012411DE | E8 CD 00 00 00 | call reverse3.12412B0 |
012411E3 | 8D 4D E8 | lea ecx,dword ptr ss:[ebp-18] | 你的输入 -> ecx
012411E6 | 83 C4 0C | add esp,C |
012411E9 | 8D 51 01 | lea edx,dword ptr ds:[ecx+1] | 你的输入减第一个字节 -> edx
012411EC | 0F 1F 40 00 | nop dword ptr ds:[eax] | eax:"OTacMDMzMTI="
012411F0 | 8A 01 | mov al,byte ptr ds:[ecx] |
012411F2 | 41 | inc ecx |
012411F3 | 84 C0 | test al,al |
012411F5 | 75 F9 | jne reverse3.12411F0 |
012411F7 | 2B CA | sub ecx,edx | 你的输入的长度 -> ecx
012411F9 | 8D 55 E8 | lea edx,dword ptr ss:[ebp-18] | 输入 -> edx
012411FC | 56 | push esi | esi:"TacMDMzMTI="
012411FD | 51 | push ecx |
012411FE | 51 | push ecx |
012411FF | 8D 4D C0 | lea ecx,dword ptr ss:[ebp-40] |
01241202 | E8 F9 FD FF FF | call reverse3.1241000 | base64(你的输入) -> [ebp - 0x40]
01241207 | 8D 4D C0 | lea ecx,dword ptr ss:[ebp-40] |
0124120A | 83 C4 08 | add esp,8 |
0124120D | 33 D2 | xor edx,edx |
0124120F | 8D 71 01 | lea esi,dword ptr ds:[ecx+1] | esi:"TacMDMzMTI="
01241212 | 8A 01 | mov al,byte ptr ds:[ecx] |
01241214 | 41 | inc ecx |
01241215 | 84 C0 | test al,al |
01241217 | 75 F9 | jne reverse3.1241212 |
01241219 | 2B CE | sub ecx,esi | 长度（base64你的输入） -> ecx
0124121B | 74 37 | je reverse3.1241254 |
0124121D | 0F 1F 00 | nop dword ptr ds:[eax] | eax:"OTacMDMzMTI="
01241220 | 8A 4C 15 C0 | mov cl,byte ptr ss:[ebp+edx-40] |
01241224 | 33 C0 | xor eax,eax | eax:"OTacMDMzMTI="
01241226 | 3A 88 08 21 24 01 | cmp cl,byte ptr ds:[eax+1242108] |
0124122C | 74 08 | je reverse3.1241236 |
0124122E | 40 | inc eax | eax:"OTacMDMzMTI="
0124122F | 83 F8 1A | cmp eax,1A | eax:"OTacMDMzMTI="
01241232 | 72 F2 | jb reverse3.1241226 |
01241234 | EB 0A | jmp reverse3.1241240 |
01241236 | 8A 80 24 21 24 01 | mov al,byte ptr ds:[eax+1242124] |
0124123C | 88 44 15 C0 | mov byte ptr ss:[ebp+edx-40],al |
01241240 | 8D 4D C0 | lea ecx,dword ptr ss:[ebp-40] |
01241243 | 42 | inc edx |
01241244 | 8D 71 01 | lea esi,dword ptr ds:[ecx+1] | esi:"TacMDMzMTI="
01241247 | 8A 01 | mov al,byte ptr ds:[ecx] |
01241249 | 41 | inc ecx |
0124124A | 84 C0 | test al,al |
0124124C | 75 F9 | jne reverse3.1241247 |
0124124E | 2B CE | sub ecx,esi | esi:"TacMDMzMTI="
01241250 | 3B D1 | cmp edx,ecx |
01241252 | 72 CC | jb reverse3.1241220 |
01241254 | 6A 14 | push 14 |
01241256 | 8D 45 C0 | lea eax,dword ptr ss:[ebp-40] |
01241259 | 68 40 21 24 01 | push reverse3.1242140 | 1242140:"o2Ffx3V0OjJtYW5spQ=="
0124125E | 50 | push eax | eax:"OTacMDMzMTI="
0124125F | FF 15 C4 20 24 01 | call dword ptr ds:[<&strncmp>] | 经过处理的base64与内置base64值比较，相等=>eax=0
01241265 | 83 C4 0C | add esp,C |
01241268 | 5E | pop esi | esi:"TacMDMzMTI="
01241269 | 85 C0 | test eax,eax | zf=1 => eax=0
0124126B | 75 07 | jne reverse3.1241274 |
0124126D | 68 78 21 24 01 | push reverse3.1242178 | 1242178:"this is the right flag"
01241272 | EB 05 | jmp reverse3.1241279 |
01241274 | 68 90 21 24 01 | push reverse3.1242190 | 1242190:"wrong input"
01241279 | FF 15 B0 20 24 01 | call dword ptr ds:[<&puts>] |
0124127F | 8B 4D FC | mov ecx,dword ptr ss:[ebp-4] |
01241282 | 83 C4 04 | add esp,4 |
01241285 | 33 CD | xor ecx,ebp |
01241287 | 33 C0 | xor eax,eax | eax:"OTacMDMzMTI="
01241289 | E8 92 00 00 00 | call reverse3.1241320 |
0124128E | 8B E5 | mov esp,ebp |
01241290 | 5D | pop ebp |
01241291 | C3 | ret |
看来是这几行做了手脚，压入了两个参数
01241259 | 68 40 21 24 01 | push reverse3.1242140 | 1242140:"o2Ffx3V0OjJtYW5spQ=="
0124125E | 50 | push eax | eax:"OTacMDMzMTI="
0124125F | FF 15 C4 20 24 01 | call dword ptr ds:[<&strncmp>] | 经过处理的base64与内置base64值比较，相等=>eax=0
我们跟这个call进去看看
6C2F8C30 | 53 | push ebx |
6C2F8C31 | 56 | push esi | esi:"TacMDMzMTI="
6C2F8C32 | 8B 4C 24 0C | mov ecx,dword ptr ss:[esp+C] | 我们输入的base64变形后的值
6C2F8C36 | 8B 54 24 10 | mov edx,dword ptr ss:[esp+10] | 内置base64值
6C2F8C3A | 8B 5C 24 14 | mov ebx,dword ptr ss:[esp+14] |
6C2F8C3E | F7 C3 FF FF FF FF | test ebx,FFFFFFFF |
6C2F8C44 | 74 50 | je ucrtbase.6C2F8C96 |
6C2F8C46 | 2B CA | sub ecx,edx |
6C2F8C48 | F7 C2 03 00 00 00 | test edx,3 |
6C2F8C4E | 74 17 | je ucrtbase.6C2F8C67 |
6C2F8C50 | 0F B6 04 0A | movzx eax,byte ptr ds:[edx+ecx] | edx+ecx*1:"OTacMDMzMTI="
6C2F8C54 | 3A 02 | cmp al,byte ptr ds:[edx] |
6C2F8C56 | 75 48 | jne ucrtbase.6C2F8CA0 |
6C2F8C58 | 85 C0 | test eax,eax | eax:"OTacMDMzMTI="
6C2F8C5A | 74 3A | je ucrtbase.6C2F8C96 |
6C2F8C5C | 42 | inc edx |
6C2F8C5D | 83 EB 01 | sub ebx,1 |
6C2F8C60 | 76 34 | jbe ucrtbase.6C2F8C96 |
6C2F8C62 | F6 C2 03 | test dl,3 |
6C2F8C65 | 75 E9 | jne ucrtbase.6C2F8C50 |
6C2F8C67 | 8D 04 0A | lea eax,dword ptr ds:[edx+ecx] | eax:"OTacMDMzMTI="
6C2F8C6A | 25 FF 0F 00 00 | and eax,FFF | eax:"OTacMDMzMTI="
6C2F8C6F | 3D FC 0F 00 00 | cmp eax,FFC | eax:"OTacMDMzMTI="
6C2F8C74 | 77 DA | ja ucrtbase.6C2F8C50 |
6C2F8C76 | 8B 04 0A | mov eax,dword ptr ds:[edx+ecx] | eax:"OTacMDMzMTI="
6C2F8C79 | 3B 02 | cmp eax,dword ptr ds:[edx] | eax:"OTacMDMzMTI="
6C2F8C7B | 75 D3 | jne ucrtbase.6C2F8C50 |
6C2F8C7D | 83 EB 04 | sub ebx,4 |
6C2F8C80 | 76 14 | jbe ucrtbase.6C2F8C96 |
6C2F8C82 | 8D B0 FF FE FE FE | lea esi,dword ptr ds:[eax-1010101] | esi:"TacMDMzMTI="
6C2F8C88 | 83 C2 04 | add edx,4 |
6C2F8C8B | F7 D0 | not eax | eax:"OTacMDMzMTI="
6C2F8C8D | 23 C6 | and eax,esi | eax:"OTacMDMzMTI=", esi:"TacMDMzMTI="
6C2F8C8F | A9 80 80 80 80 | test eax,80808080 | eax:"OTacMDMzMTI="
6C2F8C94 | 74 D1 | je ucrtbase.6C2F8C67 |
6C2F8C96 | 33 C0 | xor eax,eax | eax:"OTacMDMzMTI="
6C2F8C98 | 5E | pop esi | esi:"TacMDMzMTI="
6C2F8C99 | 5B | pop ebx |
6C2F8C9A | C3 | ret |
6C2F8C9B | EB 03 | jmp ucrtbase.6C2F8CA0 |
6C2F8C9D | CC | int3 |
6C2F8C9E | CC | int3 |
6C2F8C9F | CC | int3 |
6C2F8CA0 | 1B C0 | sbb eax,eax | eax:"OTacMDMzMTI="
6C2F8CA2 | 83 C8 01 | or eax,1 | eax:"OTacMDMzMTI="
6C2F8CA5 | 5E | pop esi | esi:"TacMDMzMTI="
6C2F8CA6 | 5B | pop ebx |
6C2F8CA7 | C3 | ret |
这段代码的跳转比较复杂，我们主要看这段
6C2F8C50 | 0F B6 04 0A | movzx eax,byte ptr ds:[edx+ecx] | edx+ecx*1:"OTacMDMzMTI="
6C2F8C54 | 3A 02 | cmp al,byte ptr ds:[edx] | edx:"o2Ffx3V0OjJtYW5spQ=="
6C2F8C56 | 75 48 | jne ucrtbase.6C2F8CA0 |
6C2F8C58 | 85 C0 | test eax,eax |
6C2F8C5A | 74 3A | je ucrtbase.6C2F8C96 |
6C2F8C5C | 42 | inc edx | edx:"o2Ffx3V0OjJtYW5spQ=="
6C2F8C5D | 83 EB 01 | sub ebx,1 |
6C2F8C60 | 76 34 | jbe ucrtbase.6C2F8C96 |
6C2F8C62 | F6 C2 03 | test dl,3 |
6C2F8C65 | 75 E9 | jne ucrtbase.6C2F8C50 |
6C2F8C67 | 8D 04 0A | lea eax,dword ptr ds:[edx+ecx] | edx+ecx*1:"OTacMDMzMTI="
6C2F8C6A | 25 FF 0F 00 00 | and eax,FFF |
6C2F8C6F | 3D FC 0F 00 00 | cmp eax,FFC |
6C2F8C74 | 77 DA | ja ucrtbase.6C2F8C50 |
通读可以发现就是把我们输入的base64变形后的值(OTacMDMzMTI=)按字节取出来一一和内置的o2Ffx3V0OjJtYW5spQ==做比较，只有当全部相等才跳到这把eax置零
6C2F8C96 | 33 C0 | xor eax,eax
然后退出函数
那么这个OTacMDMzMTI=是个什么呢？看着是个base64，但是我们解出来是96?3312，完全不是我们输入的97103012了，这个只怎么来的呢？我们继续看这段
012411FF | 8D 4D C0 | lea ecx,dword ptr ss:[ebp-40] |
01241202 | E8 F9 FD FF FF | call reverse3.1241000 |base64(你的输入) -> [ebp - 0x40]
01241207 | 8D 4D C0 | lea ecx,dword ptr ss:[ebp-40] |
0124120A | 83 C4 08 | add esp,8 |
0124120D | 33 D2 | xor edx,edx | edx:"97103012"
0124120F | 8D 71 01 | lea esi,dword ptr ds:[ecx+1] |
01241212 | 8A 01 | mov al,byte ptr ds:[ecx] |
01241214 | 41 | inc ecx |
01241215 | 84 C0 | test al,al |
01241217 | 75 F9 | jne reverse3.1241212 |
01241219 | 2B CE | sub ecx,esi | 长度（base64你的输入） -> ecx
0124121B | 74 37 | je reverse3.1241254 |
0124121D | 0F 1F 00 | nop dword ptr ds:[eax] |
01241220 | 8A 4C 15 C0 | mov cl,byte ptr ss:[ebp+edx-40] |
01241224 | 33 C0 | xor eax,eax |
01241226 | 3A 88 08 21 24 01 | cmp cl,byte ptr ds:[eax+1242108] | eax+1242108:"abcdefghijklmnopqrstuvwxyz"
0124122C | 74 08 | je reverse3.1241236 |
0124122E | 40 | inc eax |
0124122F | 83 F8 1A | cmp eax,1A |
01241232 | 72 F2 | jb reverse3.1241226 |
01241234 | EB 0A | jmp reverse3.1241240 |
01241236 | 8A 80 24 21 24 01 | mov al,byte ptr ds:[eax+1242124] | eax+1242124:"wxabopdefghijklqrstuvyzcmn"
0124123C | 88 44 15 C0 | mov byte ptr ss:[ebp+edx-40],al |
01241240 | 8D 4D C0 | lea ecx,dword ptr ss:[ebp-40] |
01241243 | 42 | inc edx | edx:"97103012"
01241244 | 8D 71 01 | lea esi,dword ptr ds:[ecx+1] |
01241247 | 8A 01 | mov al,byte ptr ds:[ecx] |
01241249 | 41 | inc ecx |
0124124A | 84 C0 | test al,al |
0124124C | 75 F9 | jne reverse3.1241247 |
0124124E | 2B CE | sub ecx,esi |
01241250 | 3B D1 | cmp edx,ecx | edx:"97103012"
01241252 | 72 CC | jb reverse3.1241220 |
01241254 | 6A 14 | push 14 |
下面这行代码有兴趣的可以跟进去看看，其实就是base64编码，苦逼的我傻乎乎地跟完了
01241202 | E8 F9 FD FF FF | call reverse3.1241000 |base64(你的输入) -> [ebp - 0x40]
那97103012的base64是OTcxMDMwMTI=呀，这个OTacMDMzMTI=是怎么来的呢？我们看着一段
01241220 | 8A 4C 15 C0 | mov cl,byte ptr ss:[ebp+edx-40] |
01241224 | 33 C0 | xor eax,eax |
01241226 | 3A 88 08 21 24 01 | cmp cl,byte ptr ds:[eax+1242108] | eax+1242108:"abcdefghijklmnopqrstuvwxyz"
0124122C | 74 08 | je reverse3.1241236 |
0124122E | 40 | inc eax |
0124122F | 83 F8 1A | cmp eax,1A |
01241232 | 72 F2 | jb reverse3.1241226 |
01241234 | EB 0A | jmp reverse3.1241240 |
01241236 | 8A 80 24 21 24 01 | mov al,byte ptr ds:[eax+1242124] | eax+1242124:"wxabopdefghijklqrstuvyzcmn"
0124123C | 88 44 15 C0 | mov byte ptr ss:[ebp+edx-40],al |
01241240 | 8D 4D C0 | lea ecx,dword ptr ss:[ebp-40] |
01241243 | 42 | inc edx | edx:"97103012"
01241244 | 8D 71 01 | lea esi,dword ptr ds:[ecx+1] |
01241247 | 8A 01 | mov al,byte ptr ds:[ecx] |
01241249 | 41 | inc ecx |
0124124A | 84 C0 | test al,al |
0124124C | 75 F9 | jne reverse3.1241247 |
0124124E | 2B CE | sub ecx,esi |
01241250 | 3B D1 | cmp edx,ecx | edx:"97103012"
01241252 | 72 CC | jb reverse3.1241220 |
这一段的工作大家跟跟就知道，就是通过一次次循环将OTcxMDMwMTI=中的值通过下面这个对应关系一一置换
abcdefghijklmnopqrstuvwxyz
wxabopdefghijklqrstuvyzcmn
所以OTcxMDMwMTI=变成了OTacMDMzMTI=
好的，我们看到了这里，相信已经知道密码是什么了，也就是我们变形后的base64值要等于o2Ffx3V0OjJtYW5spQ==
那就倒着置换呗，得出来正确的base64是e2Fib3V0OmJsYW5rfQ==，解码为{about:blank}
