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[原创]看雪CTF2018 - 密界寻踪 WriteUp
发表于: 2018-6-23 12:49 3199

[原创]看雪CTF2018 - 密界寻踪 WriteUp

2018-6-23 12:49
3199

        第一次做密码类的CrackMe,踩了不少坑。以下该题的解题思路:

        上网搜了一下miracl库,把相关的signature文件下载下来,加载到 IDA pro 中成功识别100多个函数

二、主要功能代码分析

.text:004031FA 68 18 61 48 00                          push    offset aS_0     ; "%s"
.text:004031FF E8 BC B1 02 00                          call    _scanf          ; 输入key
.text:00403204 83 C4 0C                                add     esp, 0Ch
.text:00403207 8D 55 C8                                lea     edx, [ebp-38h]
.text:0040320A 52                                      push    edx             ; char *
.text:0040320B E8 B0 A4 02 00                          call    _strlen
.text:00403210 83 C4 04                                add     esp, 4
.text:00403213 83 F8 17                                cmp     eax, 17h        ; 长度不能超过0x17
.text:00403216 76 13                                   jbe     short loc_40322B ; 取第4 - 23字节的字符串
.text:00403218 8D 45 E0                                lea     eax, [ebp-20h]  ; error
.text:0040321B 50                                      push    eax             ; char *
.text:0040321C E8 1F A5 02 00                          call    _printf
.text:00403221 83 C4 04                                add     esp, 4
.text:00403224 6A 00                                   push    0               ; int
.text:00403226 E8 05 B0 02 00                          call    _exit
.text:0040322B                         ; ---------------------------------------------------------------------------
.text:0040322B
.text:0040322B                         loc_40322B:                             ; CODE XREF: sub_403160+B6j
.text:0040322B 8D 4D CB                                lea     ecx, [ebp-35h]  ; 取第4 - 23字节的字符串
.text:0040322E 51                                      push    ecx             ; char *
.text:0040322F E8 8C A4 02 00                          call    _strlen
.text:00403234 83 C4 04                                add     esp, 4
.text:00403237 50                                      push    eax
.text:00403238 68 60 56 49 00                          push    offset unk_495660
.text:0040323D 8D 55 CB                                lea     edx, [ebp-35h]
.text:00403240 52                                      push    edx
.text:00403241 E8 2C DF FF FF                          call    j_ascii2hex     ; 转换成十六进制数组存储在unk_495660
.text:00403246 83 C4 0C                                add     esp, 0Ch
.text:00403249 E8 0F E0 FF FF                          call    j_powmod_check  ; 将十六进制数组进行模幂运算,与存储在程序中的key进行对比
.text:0040324E 89 45 FC                                mov     [ebp-4], eax
.text:00403251 6A 03                                   push    3               ; size_t
.text:00403253 8D 45 C8                                lea     eax, [ebp-38h]
.text:00403256 50                                      push    eax             ; void *
.text:00403257 8D 4D C4                                lea     ecx, [ebp-3Ch]
.text:0040325A 51                                      push    ecx             ; void *
.text:0040325B E8 60 A5 02 00                          call    _memcpy
.text:00403260 83 C4 0C                                add     esp, 0Ch
.text:00403263 8D 55 C4                                lea     edx, [ebp-3Ch]
.text:00403266 52                                      push    edx
.text:00403267 E8 20 DE FF FF                          call    j_check_first_3_char_digit ; 检查输入key的前三个字节是否为数字
.text:0040326C 83 C4 04                                add     esp, 4
.text:0040326F 25 FF 00 00 00                          and     eax, 0FFh
.text:00403274 85 C0                                   test    eax, eax
.text:00403276 74 11                                   jz      short loc_403289 ; error
.text:00403278 8D 45 C4                                lea     eax, [ebp-3Ch]
.text:0040327B 50                                      push    eax
.text:0040327C E8 0E E0 FF FF                          call    j_aes_check     ; 将前三个字符串作一定变换后,计算aes加密,最后对比结果
       主要功能是将输入的Key取第4-23字节,然后转换成十六进制数组,进行RSA加密,与隐藏在程序中的值进行比较,通过后,再检查前三个字节是否为数字,并将其进行一定变换,最后进行aes加密,并与程序中的隐藏的值对比,如果一致,则成功。

三、RSA加密及逆元d求解

       现在来看一下j_powmod_check函数,反求Key的4-23字节。模幂运算主要是基于这个公式,c = (m ^ e) mod n; 
.text:00402A42 8D 85 34 FF FF FF                       lea     eax, [ebp-0CCh]
.text:00402A48 50                                      push    eax
.text:00402A49 E8 2A E6 FF FF                          call    j_xor_string_transform ; hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304
.text:00402A4E 83 C4 04                                add     esp, 4
.text:00402A51 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402A57 51                                      push    ecx
.text:00402A58 E8 1B E6 FF FF                          call    j_xor_string_transform ; n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585
.text:00402A5D 83 C4 04                                add     esp, 4
.text:00402A60 8B 55 FC                                mov     edx, [ebp-4]
.text:00402A63 C7 82 34 02 00 00 10 00+                mov     dword ptr [edx+234h], 10h
.text:00402A6D 6A 00                                   push    0
.text:00402A6F E8 DC 68 00 00                          call    _mirvar
.text:00402A74 83 C4 04                                add     esp, 4
.text:00402A77 89 85 D8 FC FF FF                       mov     [ebp-328h], eax
.text:00402A7D 6A 00                                   push    0
.text:00402A7F E8 CC 68 00 00                          call    _mirvar
.text:00402A84 83 C4 04                                add     esp, 4
.text:00402A87 89 85 D4 FC FF FF                       mov     [ebp-32Ch], eax
.text:00402A8D 6A 00                                   push    0
.text:00402A8F E8 BC 68 00 00                          call    _mirvar
.text:00402A94 83 C4 04                                add     esp, 4
.text:00402A97 89 85 CC FC FF FF                       mov     [ebp-334h], eax
.text:00402A9D 6A 00                                   push    0
.text:00402A9F E8 AC 68 00 00                          call    _mirvar
.text:00402AA4 83 C4 04                                add     esp, 4
.text:00402AA7 89 85 D0 FC FF FF                       mov     [ebp-330h], eax
.text:00402AAD 68 60 56 49 00                          push    offset unk_495660 ; m = hex of user input
.text:00402AB2 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402AB8 50                                      push    eax
.text:00402AB9 E8 22 A7 00 00                          call    _cinstr
.text:00402ABE 83 C4 08                                add     esp, 8
.text:00402AC1 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402AC7 51                                      push    ecx
.text:00402AC8 8B 95 D8 FC FF FF                       mov     edx, [ebp-328h]
.text:00402ACE 52                                      push    edx
.text:00402ACF E8 0C A7 00 00                          call    _cinstr
.text:00402AD4 83 C4 08                                add     esp, 8
.text:00402AD7 68 A0 60 48 00                          push    offset a3e9     ; "3e9"
.text:00402ADC 8B 85 D4 FC FF FF                       mov     eax, [ebp-32Ch] ; e = 0x3e9
.text:00402AE2 50                                      push    eax
.text:00402AE3 E8 F8 A6 00 00                          call    _cinstr
.text:00402AE8 83 C4 08                                add     esp, 8
.text:00402AEB 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402AF1 51                                      push    ecx
.text:00402AF2 8B 95 CC FC FF FF                       mov     edx, [ebp-334h]
.text:00402AF8 52                                      push    edx
.text:00402AF9 E8 C2 77 00 00                          call    _mr_compare     ; mr_compare(m, n)
.text:00402AFE 83 C4 08                                add     esp, 8
.text:00402B01 83 F8 FF                                cmp     eax, -1
.text:00402B04 0F 85 CA 00 00 00                       jnz     loc_402BD4
.text:00402B0A 8B 85 D0 FC FF FF                       mov     eax, [ebp-330h]
.text:00402B10 50                                      push    eax
.text:00402B11 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402B17 51                                      push    ecx
.text:00402B18 8B 95 D4 FC FF FF                       mov     edx, [ebp-32Ch]
.text:00402B1E 52                                      push    edx
.text:00402B1F 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402B25 50                                      push    eax
.text:00402B26 E8 E5 95 00 00                          call    _powmod         ; (m ^ e) mod n
       根据代码分析可知,0x00402A49处为隐藏Key,即 hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304,0x00402A58处为模数n,即 n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585, 0x002AD7处为指数 e = 0x3e9,需要求出 m 并于hidden_key进行比较。由于已知m,可以直接算出c。
       相关原理为,加密过程:c = (m^e) mod n;解密过程:m = (c ^d) mod n;需要求出逆元d,首先需要将n进行分解质因数,n = 208096057845685678782766058500526476379 * 273086345401562743300402731618892888991,即 p = 208096057845685678782766058500526476379, q = 273086345401562743300402731618892888991,在求(p-1)*(q-1),最后根据"扩展欧几里得算法"求解 d = 21005425588345339621950762401208703877439204233991217052799016669569632540401,最后根据公式反求 m = 0x69616d6168616e64736f6d656775796861686131,即字符串“iamahandsomeguyhaha1”

四、前三个字节求解

    前三个字节的求解主要在j_aes_check函数中,相关代码分析如下:
.text:00402E5A 8D 95 74 FC FF FF                       lea     edx, [ebp-38Ch]
.text:00402E60 52                                      push    edx
.text:00402E61 E8 12 E2 FF FF                          call    j_xor_string_transform ; "831GD47;?K2M=8:&U#$V#T\"-+\\*)*X'e" ==> "912CA2036A9A0656D17B6B552F157F8E"
.text:00402E66 83 C4 04                                add     esp, 4
.text:00402E69 8D 85 D0 FA FF FF                       lea     eax, [ebp-530h]
.text:00402E6F 50                                      push    eax
.text:00402E70 E8 03 E2 FF FF                          call    j_xor_string_transform ; "123567389:;<=>?" ==> "0001314000000000"
.text:00402E75 83 C4 04                                add     esp, 4
.text:00402E78 8B 4D 08                                mov     ecx, [ebp+8]
.text:00402E7B 8A 11                                   mov     dl, [ecx]
.text:00402E7D 88 95 D0 FA FF FF                       mov     [ebp-530h], dl  ; 第一个字节
.text:00402E83 8B 45 08                                mov     eax, [ebp+8]
.text:00402E86 8A 48 01                                mov     cl, [eax+1]
.text:00402E89 88 8D D1 FA FF FF                       mov     [ebp-52Fh], cl  ; 第二个字节
.text:00402E8F 8B 55 08                                mov     edx, [ebp+8]
.text:00402E92 0F BE 42 02                             movsx   eax, byte ptr [edx+2]
.text:00402E96 03 05 28 57 49 00                       add     eax, dword_495728 ; eax += 1
.text:00402E9C 88 85 D2 FA FF FF                       mov     [ebp-52Eh], al  ; 第三个字节+1
.text:00402EA2 68 FC 01 00 00                          push    1FCh            ; size_t
.text:00402EA7 6A 00                                   push    0               ; int
.text:00402EA9 8D 8D 04 FE FF FF                       lea     ecx, [ebp-1FCh]
.text:00402EAF 51                                      push    ecx             ; void *
.text:00402EB0 E8 4B AD 02 00                          call    _memset
.text:00402EB5 83 C4 0C                                add     esp, 0Ch
.text:00402EB8 6A 00                                   push    0
.text:00402EBA 8D 95 D0 FA FF FF                       lea     edx, [ebp-530h]
.text:00402EC0 52                                      push    edx
.text:00402EC1 6A 10                                   push    10h
.text:00402EC3 6A 00                                   push    0
.text:00402EC5 8D 85 04 FE FF FF                       lea     eax, [ebp-1FCh] ; 猜测一下,前三个输入字节"520", aes的密钥为:"52113140000000000"
.text:00402ECB 50                                      push    eax
.text:00402ECC E8 8F A8 00 00                          call    _aes_init       ; aes_init("52113140000000000" 0, 0x10, 0);
.text:00402ED1 83 C4 14                                add     esp, 14h
.text:00402ED4 8D 8D AC FB FF FF                       lea     ecx, [ebp-454h]
.text:00402EDA 51                                      push    ecx
.text:00402EDB 8D 95 04 FE FF FF                       lea     edx, [ebp-1FCh]
.text:00402EE1 52                                      push    edx
.text:00402EE2 E8 59 AD 00 00                          call    _aes_encrypt    ; ase_encrypt("pediy", "52113140000000000");
        主要将输入的第一、二个字节填充到“0001314000000000”到前两个字节,第三个字节加1,填充到对应第三字节位置,将填充的字符串作为key对“pediy”进行aes加密,并与隐藏值0x912CA2036A9A0656D17B6B552F157F8E进行比较,通过则成功。顺便提一句,当大家看到“1314”的时候,估计已经猜到前三个字节为“520”了吧,顺手试一下,果然成功。
最后,完整的key为:520iamahandsomeguyhaha1

四、总结几点经验教训

        踩坑1. 首次逆密码类的CrackMe,没有经验,居然没想到去先找找常用密码库的信息,后来逆着逆着感觉不对劲,赶紧又去翻翻程序中的字符串,才发现了miracl库。由于对密码学不熟悉,完全不知道有这个库。所以以后对于这类逆向问题,首相应该去识别密码算法库,找Signature文件和相应文档。
        踩坑2. 同样由于不会密码学的缘故,对RSA算法主要流程的不是很了解,导致模幂求逆上都卡了好一阵子,翻了好多资料后,才大致知道咋回事。以后一定要恶补密码学
        上网搜了一下miracl库,把相关的signature文件下载下来,加载到 IDA pro 中成功识别100多个函数
.text:004031FA 68 18 61 48 00                          push    offset aS_0     ; "%s"
.text:004031FF E8 BC B1 02 00                          call    _scanf          ; 输入key
.text:00403204 83 C4 0C                                add     esp, 0Ch
.text:00403207 8D 55 C8                                lea     edx, [ebp-38h]
.text:0040320A 52                                      push    edx             ; char *
.text:0040320B E8 B0 A4 02 00                          call    _strlen
.text:00403210 83 C4 04                                add     esp, 4
.text:00403213 83 F8 17                                cmp     eax, 17h        ; 长度不能超过0x17
.text:00403216 76 13                                   jbe     short loc_40322B ; 取第4 - 23字节的字符串
.text:00403218 8D 45 E0                                lea     eax, [ebp-20h]  ; error
.text:0040321B 50                                      push    eax             ; char *
.text:0040321C E8 1F A5 02 00                          call    _printf
.text:00403221 83 C4 04                                add     esp, 4
.text:00403224 6A 00                                   push    0               ; int
.text:00403226 E8 05 B0 02 00                          call    _exit
.text:0040322B                         ; ---------------------------------------------------------------------------
.text:0040322B
.text:0040322B                         loc_40322B:                             ; CODE XREF: sub_403160+B6j
.text:0040322B 8D 4D CB                                lea     ecx, [ebp-35h]  ; 取第4 - 23字节的字符串
.text:0040322E 51                                      push    ecx             ; char *
.text:0040322F E8 8C A4 02 00                          call    _strlen
.text:00403234 83 C4 04                                add     esp, 4
.text:00403237 50                                      push    eax
.text:00403238 68 60 56 49 00                          push    offset unk_495660
.text:0040323D 8D 55 CB                                lea     edx, [ebp-35h]
.text:00403240 52                                      push    edx
.text:00403241 E8 2C DF FF FF                          call    j_ascii2hex     ; 转换成十六进制数组存储在unk_495660
.text:00403246 83 C4 0C                                add     esp, 0Ch
.text:00403249 E8 0F E0 FF FF                          call    j_powmod_check  ; 将十六进制数组进行模幂运算,与存储在程序中的key进行对比
.text:0040324E 89 45 FC                                mov     [ebp-4], eax
.text:00403251 6A 03                                   push    3               ; size_t
.text:00403253 8D 45 C8                                lea     eax, [ebp-38h]
.text:00403256 50                                      push    eax             ; void *
.text:00403257 8D 4D C4                                lea     ecx, [ebp-3Ch]
.text:0040325A 51                                      push    ecx             ; void *
.text:0040325B E8 60 A5 02 00                          call    _memcpy
.text:00403260 83 C4 0C                                add     esp, 0Ch
.text:00403263 8D 55 C4                                lea     edx, [ebp-3Ch]
.text:00403266 52                                      push    edx
.text:00403267 E8 20 DE FF FF                          call    j_check_first_3_char_digit ; 检查输入key的前三个字节是否为数字
.text:0040326C 83 C4 04                                add     esp, 4
.text:0040326F 25 FF 00 00 00                          and     eax, 0FFh
.text:00403274 85 C0                                   test    eax, eax
.text:00403276 74 11                                   jz      short loc_403289 ; error
.text:00403278 8D 45 C4                                lea     eax, [ebp-3Ch]
.text:0040327B 50                                      push    eax
.text:0040327C E8 0E E0 FF FF                          call    j_aes_check     ; 将前三个字符串作一定变换后,计算aes加密,最后对比结果
       主要功能是将输入的Key取第4-23字节,然后转换成十六进制数组,进行RSA加密,与隐藏在程序中的值进行比较,通过后,再检查前三个字节是否为数字,并将其进行一定变换,最后进行aes加密,并与程序中的隐藏的值对比,如果一致,则成功。

三、RSA加密及逆元d求解

       现在来看一下j_powmod_check函数,反求Key的4-23字节。模幂运算主要是基于这个公式,c = (m ^ e) mod n; 
.text:00402A42 8D 85 34 FF FF FF                       lea     eax, [ebp-0CCh]
.text:00402A48 50                                      push    eax
.text:00402A49 E8 2A E6 FF FF                          call    j_xor_string_transform ; hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304
.text:00402A4E 83 C4 04                                add     esp, 4
.text:00402A51 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402A57 51                                      push    ecx
.text:00402A58 E8 1B E6 FF FF                          call    j_xor_string_transform ; n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585
.text:00402A5D 83 C4 04                                add     esp, 4
.text:00402A60 8B 55 FC                                mov     edx, [ebp-4]
.text:00402A63 C7 82 34 02 00 00 10 00+                mov     dword ptr [edx+234h], 10h
.text:00402A6D 6A 00                                   push    0
.text:00402A6F E8 DC 68 00 00                          call    _mirvar
.text:00402A74 83 C4 04                                add     esp, 4
.text:00402A77 89 85 D8 FC FF FF                       mov     [ebp-328h], eax
.text:00402A7D 6A 00                                   push    0
.text:00402A7F E8 CC 68 00 00                          call    _mirvar
.text:00402A84 83 C4 04                                add     esp, 4
.text:00402A87 89 85 D4 FC FF FF                       mov     [ebp-32Ch], eax
.text:00402A8D 6A 00                                   push    0
.text:00402A8F E8 BC 68 00 00                          call    _mirvar
.text:00402A94 83 C4 04                                add     esp, 4
.text:00402A97 89 85 CC FC FF FF                       mov     [ebp-334h], eax
.text:00402A9D 6A 00                                   push    0
.text:00402A9F E8 AC 68 00 00                          call    _mirvar
.text:00402AA4 83 C4 04                                add     esp, 4
.text:00402AA7 89 85 D0 FC FF FF                       mov     [ebp-330h], eax
.text:00402AAD 68 60 56 49 00                          push    offset unk_495660 ; m = hex of user input
.text:00402AB2 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402AB8 50                                      push    eax
.text:00402AB9 E8 22 A7 00 00                          call    _cinstr
.text:00402ABE 83 C4 08                                add     esp, 8
.text:00402AC1 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402AC7 51                                      push    ecx
.text:00402AC8 8B 95 D8 FC FF FF                       mov     edx, [ebp-328h]
.text:00402ACE 52                                      push    edx
.text:00402ACF E8 0C A7 00 00                          call    _cinstr
.text:00402AD4 83 C4 08                                add     esp, 8
.text:00402AD7 68 A0 60 48 00                          push    offset a3e9     ; "3e9"
.text:00402ADC 8B 85 D4 FC FF FF                       mov     eax, [ebp-32Ch] ; e = 0x3e9
.text:00402AE2 50                                      push    eax
.text:00402AE3 E8 F8 A6 00 00                          call    _cinstr
.text:00402AE8 83 C4 08                                add     esp, 8
.text:00402AEB 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402AF1 51                                      push    ecx
.text:00402AF2 8B 95 CC FC FF FF                       mov     edx, [ebp-334h]
.text:00402AF8 52                                      push    edx
.text:00402AF9 E8 C2 77 00 00                          call    _mr_compare     ; mr_compare(m, n)
.text:00402AFE 83 C4 08                                add     esp, 8
.text:00402B01 83 F8 FF                                cmp     eax, -1
.text:00402B04 0F 85 CA 00 00 00                       jnz     loc_402BD4
.text:00402B0A 8B 85 D0 FC FF FF                       mov     eax, [ebp-330h]
.text:00402B10 50                                      push    eax
.text:00402B11 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402B17 51                                      push    ecx
.text:00402B18 8B 95 D4 FC FF FF                       mov     edx, [ebp-32Ch]
.text:00402B1E 52                                      push    edx
.text:00402B1F 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402B25 50                                      push    eax
.text:00402B26 E8 E5 95 00 00                          call    _powmod         ; (m ^ e) mod n
       根据代码分析可知,0x00402A49处为隐藏Key,即 hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304,0x00402A58处为模数n,即 n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585, 0x002AD7处为指数 e = 0x3e9,需要求出 m 并于hidden_key进行比较。由于已知m,可以直接算出c。
       相关原理为,加密过程:c = (m^e) mod n;解密过程:m = (c ^d) mod n;需要求出逆元d,首先需要将n进行分解质因数,n = 208096057845685678782766058500526476379 * 273086345401562743300402731618892888991,即 p = 208096057845685678782766058500526476379, q = 273086345401562743300402731618892888991,在求(p-1)*(q-1),最后根据"扩展欧几里得算法"求解 d = 21005425588345339621950762401208703877439204233991217052799016669569632540401,最后根据公式反求 m = 0x69616d6168616e64736f6d656775796861686131,即字符串“iamahandsomeguyhaha1”

四、前三个字节求解

    前三个字节的求解主要在j_aes_check函数中,相关代码分析如下:
.text:00402E5A 8D 95 74 FC FF FF                       lea     edx, [ebp-38Ch]
.text:00402E60 52                                      push    edx
.text:00402E61 E8 12 E2 FF FF                          call    j_xor_string_transform ; "831GD47;?K2M=8:&U#$V#T\"-+\\*)*X'e" ==> "912CA2036A9A0656D17B6B552F157F8E"
.text:00402E66 83 C4 04                                add     esp, 4
.text:00402E69 8D 85 D0 FA FF FF                       lea     eax, [ebp-530h]
.text:00402E6F 50                                      push    eax
.text:00402E70 E8 03 E2 FF FF                          call    j_xor_string_transform ; "123567389:;<=>?" ==> "0001314000000000"
.text:00402E75 83 C4 04                                add     esp, 4
.text:00402E78 8B 4D 08                                mov     ecx, [ebp+8]
.text:00402E7B 8A 11                                   mov     dl, [ecx]
.text:00402E7D 88 95 D0 FA FF FF                       mov     [ebp-530h], dl  ; 第一个字节
.text:00402E83 8B 45 08                                mov     eax, [ebp+8]
.text:00402E86 8A 48 01                                mov     cl, [eax+1]
.text:00402E89 88 8D D1 FA FF FF                       mov     [ebp-52Fh], cl  ; 第二个字节
.text:00402E8F 8B 55 08                                mov     edx, [ebp+8]
.text:00402E92 0F BE 42 02                             movsx   eax, byte ptr [edx+2]
.text:00402E96 03 05 28 57 49 00                       add     eax, dword_495728 ; eax += 1
.text:00402E9C 88 85 D2 FA FF FF                       mov     [ebp-52Eh], al  ; 第三个字节+1
.text:00402EA2 68 FC 01 00 00                          push    1FCh            ; size_t
.text:00402EA7 6A 00                                   push    0               ; int
.text:00402EA9 8D 8D 04 FE FF FF                       lea     ecx, [ebp-1FCh]
.text:00402EAF 51                                      push    ecx             ; void *
.text:00402EB0 E8 4B AD 02 00                          call    _memset
.text:00402EB5 83 C4 0C                                add     esp, 0Ch
.text:00402EB8 6A 00                                   push    0
.text:00402EBA 8D 95 D0 FA FF FF                       lea     edx, [ebp-530h]
.text:00402EC0 52                                      push    edx
.text:00402EC1 6A 10                                   push    10h
.text:00402EC3 6A 00                                   push    0
.text:00402EC5 8D 85 04 FE FF FF                       lea     eax, [ebp-1FCh] ; 猜测一下,前三个输入字节"520", aes的密钥为:"52113140000000000"
.text:00402ECB 50                                      push    eax
.text:00402ECC E8 8F A8 00 00                          call    _aes_init       ; aes_init("52113140000000000" 0, 0x10, 0);
.text:00402ED1 83 C4 14                                add     esp, 14h
.text:00402ED4 8D 8D AC FB FF FF                       lea     ecx, [ebp-454h]
.text:00402EDA 51                                      push    ecx
.text:00402EDB 8D 95 04 FE FF FF                       lea     edx, [ebp-1FCh]
.text:00402EE1 52                                      push    edx
.text:00402EE2 E8 59 AD 00 00                          call    _aes_encrypt    ; ase_encrypt("pediy", "52113140000000000");
        主要将输入的第一、二个字节填充到“0001314000000000”到前两个字节,第三个字节加1,填充到对应第三字节位置,将填充的字符串作为key对“pediy”进行aes加密,并与隐藏值0x912CA2036A9A0656D17B6B552F157F8E进行比较,通过则成功。顺便提一句,当大家看到“1314”的时候,估计已经猜到前三个字节为“520”了吧,顺手试一下,果然成功。
最后,完整的key为:520iamahandsomeguyhaha1
.text:004031FA 68 18 61 48 00                          push    offset aS_0     ; "%s"
.text:004031FF E8 BC B1 02 00                          call    _scanf          ; 输入key
.text:00403204 83 C4 0C                                add     esp, 0Ch
.text:00403207 8D 55 C8                                lea     edx, [ebp-38h]
.text:0040320A 52                                      push    edx             ; char *
.text:0040320B E8 B0 A4 02 00                          call    _strlen
.text:00403210 83 C4 04                                add     esp, 4
.text:00403213 83 F8 17                                cmp     eax, 17h        ; 长度不能超过0x17
.text:00403216 76 13                                   jbe     short loc_40322B ; 取第4 - 23字节的字符串
.text:00403218 8D 45 E0                                lea     eax, [ebp-20h]  ; error
.text:0040321B 50                                      push    eax             ; char *
.text:0040321C E8 1F A5 02 00                          call    _printf
.text:00403221 83 C4 04                                add     esp, 4
.text:00403224 6A 00                                   push    0               ; int
.text:00403226 E8 05 B0 02 00                          call    _exit
.text:0040322B                         ; ---------------------------------------------------------------------------
.text:0040322B
.text:0040322B                         loc_40322B:                             ; CODE XREF: sub_403160+B6j
.text:0040322B 8D 4D CB                                lea     ecx, [ebp-35h]  ; 取第4 - 23字节的字符串
.text:0040322E 51                                      push    ecx             ; char *
.text:0040322F E8 8C A4 02 00                          call    _strlen
.text:00403234 83 C4 04                                add     esp, 4
.text:00403237 50                                      push    eax
.text:00403238 68 60 56 49 00                          push    offset unk_495660
.text:0040323D 8D 55 CB                                lea     edx, [ebp-35h]
.text:00403240 52                                      push    edx
.text:00403241 E8 2C DF FF FF                          call    j_ascii2hex     ; 转换成十六进制数组存储在unk_495660
.text:00403246 83 C4 0C                                add     esp, 0Ch
.text:00403249 E8 0F E0 FF FF                          call    j_powmod_check  ; 将十六进制数组进行模幂运算,与存储在程序中的key进行对比
.text:0040324E 89 45 FC                                mov     [ebp-4], eax
.text:00403251 6A 03                                   push    3               ; size_t
.text:00403253 8D 45 C8                                lea     eax, [ebp-38h]
.text:00403256 50                                      push    eax             ; void *
.text:00403257 8D 4D C4                                lea     ecx, [ebp-3Ch]
.text:0040325A 51                                      push    ecx             ; void *
.text:0040325B E8 60 A5 02 00                          call    _memcpy
.text:00403260 83 C4 0C                                add     esp, 0Ch
.text:00403263 8D 55 C4                                lea     edx, [ebp-3Ch]
.text:00403266 52                                      push    edx
.text:00403267 E8 20 DE FF FF                          call    j_check_first_3_char_digit ; 检查输入key的前三个字节是否为数字
.text:0040326C 83 C4 04                                add     esp, 4
.text:0040326F 25 FF 00 00 00                          and     eax, 0FFh
.text:00403274 85 C0                                   test    eax, eax
.text:00403276 74 11                                   jz      short loc_403289 ; error
.text:00403278 8D 45 C4                                lea     eax, [ebp-3Ch]
.text:0040327B 50                                      push    eax
.text:0040327C E8 0E E0 FF FF                          call    j_aes_check     ; 将前三个字符串作一定变换后,计算aes加密,最后对比结果
       主要功能是将输入的Key取第4-23字节,然后转换成十六进制数组,进行RSA加密,与隐藏在程序中的值进行比较,通过后,再检查前三个字节是否为数字,并将其进行一定变换,最后进行aes加密,并与程序中的隐藏的值对比,如果一致,则成功。
       现在来看一下j_powmod_check函数,反求Key的4-23字节。模幂运算主要是基于这个公式,c = (m ^ e) mod n; 
.text:00402A42 8D 85 34 FF FF FF                       lea     eax, [ebp-0CCh]
.text:00402A48 50                                      push    eax
.text:00402A49 E8 2A E6 FF FF                          call    j_xor_string_transform ; hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304
.text:00402A4E 83 C4 04                                add     esp, 4
.text:00402A51 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402A57 51                                      push    ecx
.text:00402A58 E8 1B E6 FF FF                          call    j_xor_string_transform ; n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585
.text:00402A5D 83 C4 04                                add     esp, 4
.text:00402A60 8B 55 FC                                mov     edx, [ebp-4]
.text:00402A63 C7 82 34 02 00 00 10 00+                mov     dword ptr [edx+234h], 10h
.text:00402A6D 6A 00                                   push    0
.text:00402A6F E8 DC 68 00 00                          call    _mirvar
.text:00402A74 83 C4 04                                add     esp, 4
.text:00402A77 89 85 D8 FC FF FF                       mov     [ebp-328h], eax
.text:00402A7D 6A 00                                   push    0
.text:00402A7F E8 CC 68 00 00                          call    _mirvar
.text:00402A84 83 C4 04                                add     esp, 4
.text:00402A87 89 85 D4 FC FF FF                       mov     [ebp-32Ch], eax
.text:00402A8D 6A 00                                   push    0
.text:00402A8F E8 BC 68 00 00                          call    _mirvar
.text:00402A94 83 C4 04                                add     esp, 4
.text:00402A97 89 85 CC FC FF FF                       mov     [ebp-334h], eax
.text:00402A9D 6A 00                                   push    0
.text:00402A9F E8 AC 68 00 00                          call    _mirvar
.text:00402AA4 83 C4 04                                add     esp, 4
.text:00402AA7 89 85 D0 FC FF FF                       mov     [ebp-330h], eax
.text:00402AAD 68 60 56 49 00                          push    offset unk_495660 ; m = hex of user input
.text:00402AB2 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402AB8 50                                      push    eax
.text:00402AB9 E8 22 A7 00 00                          call    _cinstr
.text:00402ABE 83 C4 08                                add     esp, 8
.text:00402AC1 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402AC7 51                                      push    ecx
.text:00402AC8 8B 95 D8 FC FF FF                       mov     edx, [ebp-328h]
.text:00402ACE 52                                      push    edx
.text:00402ACF E8 0C A7 00 00                          call    _cinstr
.text:00402AD4 83 C4 08                                add     esp, 8
.text:00402AD7 68 A0 60 48 00                          push    offset a3e9     ; "3e9"
.text:00402ADC 8B 85 D4 FC FF FF                       mov     eax, [ebp-32Ch] ; e = 0x3e9
.text:00402AE2 50                                      push    eax
.text:00402AE3 E8 F8 A6 00 00                          call    _cinstr
.text:00402AE8 83 C4 08                                add     esp, 8
.text:00402AEB 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402AF1 51                                      push    ecx
.text:00402AF2 8B 95 CC FC FF FF                       mov     edx, [ebp-334h]
.text:00402AF8 52                                      push    edx
.text:00402AF9 E8 C2 77 00 00                          call    _mr_compare     ; mr_compare(m, n)
.text:00402AFE 83 C4 08                                add     esp, 8
.text:00402B01 83 F8 FF                                cmp     eax, -1
.text:00402B04 0F 85 CA 00 00 00                       jnz     loc_402BD4
.text:00402B0A 8B 85 D0 FC FF FF                       mov     eax, [ebp-330h]
.text:00402B10 50                                      push    eax
.text:00402B11 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402B17 51                                      push    ecx
.text:00402B18 8B 95 D4 FC FF FF                       mov     edx, [ebp-32Ch]
.text:00402B1E 52                                      push    edx
.text:00402B1F 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402B25 50                                      push    eax
.text:00402B26 E8 E5 95 00 00                          call    _powmod         ; (m ^ e) mod n
       根据代码分析可知,0x00402A49处为隐藏Key,即 hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304,0x00402A58处为模数n,即 n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585, 0x002AD7处为指数 e = 0x3e9,需要求出 m 并于hidden_key进行比较。由于已知m,可以直接算出c。
       相关原理为,加密过程:c = (m^e) mod n;解密过程:m = (c ^d) mod n;需要求出逆元d,首先需要将n进行分解质因数,n = 208096057845685678782766058500526476379 * 273086345401562743300402731618892888991,即 p = 208096057845685678782766058500526476379, q = 273086345401562743300402731618892888991,在求(p-1)*(q-1),最后根据"扩展欧几里得算法"求解 d = 21005425588345339621950762401208703877439204233991217052799016669569632540401,最后根据公式反求 m = 0x69616d6168616e64736f6d656775796861686131,即字符串“iamahandsomeguyhaha1”
.text:00402A42 8D 85 34 FF FF FF                       lea     eax, [ebp-0CCh]
.text:00402A48 50                                      push    eax
.text:00402A49 E8 2A E6 FF FF                          call    j_xor_string_transform ; hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304
.text:00402A4E 83 C4 04                                add     esp, 4
.text:00402A51 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402A57 51                                      push    ecx
.text:00402A58 E8 1B E6 FF FF                          call    j_xor_string_transform ; n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585
.text:00402A5D 83 C4 04                                add     esp, 4
.text:00402A60 8B 55 FC                                mov     edx, [ebp-4]
.text:00402A63 C7 82 34 02 00 00 10 00+                mov     dword ptr [edx+234h], 10h
.text:00402A6D 6A 00                                   push    0
.text:00402A6F E8 DC 68 00 00                          call    _mirvar
.text:00402A74 83 C4 04                                add     esp, 4
.text:00402A77 89 85 D8 FC FF FF                       mov     [ebp-328h], eax
.text:00402A7D 6A 00                                   push    0
.text:00402A7F E8 CC 68 00 00                          call    _mirvar
.text:00402A84 83 C4 04                                add     esp, 4
.text:00402A87 89 85 D4 FC FF FF                       mov     [ebp-32Ch], eax
.text:00402A8D 6A 00                                   push    0
.text:00402A8F E8 BC 68 00 00                          call    _mirvar
.text:00402A94 83 C4 04                                add     esp, 4
.text:00402A97 89 85 CC FC FF FF                       mov     [ebp-334h], eax
.text:00402A9D 6A 00                                   push    0
.text:00402A9F E8 AC 68 00 00                          call    _mirvar
.text:00402AA4 83 C4 04                                add     esp, 4
.text:00402AA7 89 85 D0 FC FF FF                       mov     [ebp-330h], eax
.text:00402AAD 68 60 56 49 00                          push    offset unk_495660 ; m = hex of user input
.text:00402AB2 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402AB8 50                                      push    eax
.text:00402AB9 E8 22 A7 00 00                          call    _cinstr
.text:00402ABE 83 C4 08                                add     esp, 8
.text:00402AC1 8D 8D DC FC FF FF                       lea     ecx, [ebp-324h]
.text:00402AC7 51                                      push    ecx
.text:00402AC8 8B 95 D8 FC FF FF                       mov     edx, [ebp-328h]
.text:00402ACE 52                                      push    edx
.text:00402ACF E8 0C A7 00 00                          call    _cinstr
.text:00402AD4 83 C4 08                                add     esp, 8
.text:00402AD7 68 A0 60 48 00                          push    offset a3e9     ; "3e9"
.text:00402ADC 8B 85 D4 FC FF FF                       mov     eax, [ebp-32Ch] ; e = 0x3e9
.text:00402AE2 50                                      push    eax
.text:00402AE3 E8 F8 A6 00 00                          call    _cinstr
.text:00402AE8 83 C4 08                                add     esp, 8
.text:00402AEB 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402AF1 51                                      push    ecx
.text:00402AF2 8B 95 CC FC FF FF                       mov     edx, [ebp-334h]
.text:00402AF8 52                                      push    edx
.text:00402AF9 E8 C2 77 00 00                          call    _mr_compare     ; mr_compare(m, n)
.text:00402AFE 83 C4 08                                add     esp, 8
.text:00402B01 83 F8 FF                                cmp     eax, -1
.text:00402B04 0F 85 CA 00 00 00                       jnz     loc_402BD4
.text:00402B0A 8B 85 D0 FC FF FF                       mov     eax, [ebp-330h]
.text:00402B10 50                                      push    eax
.text:00402B11 8B 8D D8 FC FF FF                       mov     ecx, [ebp-328h]
.text:00402B17 51                                      push    ecx
.text:00402B18 8B 95 D4 FC FF FF                       mov     edx, [ebp-32Ch]
.text:00402B1E 52                                      push    edx
.text:00402B1F 8B 85 CC FC FF FF                       mov     eax, [ebp-334h]
.text:00402B25 50                                      push    eax
.text:00402B26 E8 E5 95 00 00                          call    _powmod         ; (m ^ e) mod n
       根据代码分析可知,0x00402A49处为隐藏Key,即 hidden_key = 0x208CBB7CD6ECC64516D07D978F5F0681F534EAD235D5C49ADD72D2DB840D5304,0x00402A58处为模数n,即 n = 0x7da39de66016477b1afc3dc8e309dc429b5de855f0d616d225b570b68b88a585, 0x002AD7处为指数 e = 0x3e9,需要求出 m 并于hidden_key进行比较。由于已知m,可以直接算出c。
       相关原理为,加密过程:c = (m^e) mod n;解密过程:m = (c ^d) mod n;需要求出逆元d,首先需要将n进行分解质因数,n = 208096057845685678782766058500526476379 * 273086345401562743300402731618892888991,即 p = 208096057845685678782766058500526476379, q = 273086345401562743300402731618892888991,在求(p-1)*(q-1),最后根据"扩展欧几里得算法"求解 d = 21005425588345339621950762401208703877439204233991217052799016669569632540401,最后根据公式反求 m = 0x69616d6168616e64736f6d656775796861686131,即字符串“iamahandsomeguyhaha1”

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最后于 2018-6-23 13:15 被Iamakiller编辑 ,原因:
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