Nepctf2023 Review 出题思路及WP

原设计图


Flag格式

NepCTF{%s}
逻辑复原

main

UPX拆壳，main函数逻辑复原得到

• getInput
  
• checkInputFormat
  
• debuggerCheck
  
  
  
  • 如果存在，则加密一些数据
    
  
  
• cotIsDebuggerPresent++
  
• checkCRC
  
• checkBreakPoint1
  
• checkBreakPoint2
  
• thread3
  
• FinalCheck
  

checkCRC

• CRCcheck
  
• cotCRCcheck++
  
• create CRC thread
  
  
  
  • function: checkCRC
    
  
  

checkBreakPoint1

• BreakPoint则cotBreakPoint++
  
• 其它的则不管
  

checkBreakPoint2

• 检测cotBreakPoint个数，判断是否失败
  

thread3

• checkCRC
  
• keyTEA
  

1. keyTEA[0] = 0x19 * cotIsDebuggerPresent;
2. keyTEA[1] = 0x28 * cotBreakPoint1;
3. keyTEA[2] = 0x37 * cotCRCcheck;
4. keyTEA[3] = cotIsDebuggerPresent + cotCRCcheck + cotBreakPoint1

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• xxTEA加密，分析密钥结构可以知道是keyTEA
  
  
  
  • xxTEA可以看加密结构判断，TEA族加密可以通过魔数看出
    
  
  
• cotSame = flag经过xxTEA后得到的字符串中，相邻且相同的字符个数
  
• BitInvert
  
• keyAES
  

1. keyAES->m128i_i8[0] = 0x19 * cotIsDebuggerPresent;
2. keyAES->m128i_i8[1] = 5 * cotBreakPoint1 * 8;
3. keyAES->m128i_i8[2] = 0x37 * cotCRCcheck;
4. keyAES->m128i_i8[3] = cotSame + cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck;
5. keyAES->m128i_i8[4] = 0x19 * cotIsDebuggerPresent;
6. keyAES->m128i_i8[5] = 5 * cotBreakPoint1 * 8;
7. keyAES->m128i_i8[6] = 0x37 * cotCRCcheck;
8. keyAES->m128i_i8[7] = cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck - cotSame;
9. keyAES->m128i_i8[8] = 70 * cotIsDebuggerPresent;
10. keyAES->m128i_i8[9] = 85 * cotBreakPoint1;
11. keyAES->m128i_i8[10] = 100 * cotCRCcheck;
12. keyAES->m128i_i8[11] = cotSame + cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck;
13. keyAES->m128i_i8[12] = 70 * cotIsDebuggerPresent;
14. keyAES->m128i_i8[13] = 85 * cotBreakPoint1;
15. keyAES->m128i_i8[14] = 100 * cotCRCcheck;
16. keyAES->m128i_i8[15] = cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck - cotSame;

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• AES
  
• checkCRC
  

FinalCheck

• 检查加密后的数据和cmpVal是否相同
  
• 不同的话触发异常处理，要求输入不需要的password并且不可能正确
  

WP

只有cotSame是不确定的值，范围是0~47，后面爆破即可
cotXXX全部都是固定的，只有动态调试的时候会因为触发检测而错误，静态分析显然可以得到不同时刻的状态表

1. int main() {
2.     unsigned char flag[SIZE + 1] = "NepCTF{tEA_with_AES_by_mixing_antiDebug_hahaHah}";
3.     unsigned char comp[SIZE + 1] = { 0 };
4.     data.cot_Anti1 += 1;
5.     data.cot_Anti3 += 1;
6.     data.cot_Anti3 += 1;
7.     unsigned int keyTEA[4];
8.     keyTEA[0] = data.cot_Anti1 * 0x19;
9.     keyTEA[1] = data.cot_Anti2 * 0x28;
10.     keyTEA[2] = data.cot_Anti3 * 0x37;
11.     keyTEA[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3;
12.     xxtea_uint_encrypt((unsigned int*)flag, SIZE/4, (unsigned int*)keyTEA);
13.     data.cot_same = 0;
14.     data.cot_Anti2 += 1;
15.     for (int i = 0; i < SIZE; i++) {
16.         flag[i] = ~flag[i];
17.     }
18.     unsigned char* keyAES = new unsigned char[16];
19.     keyAES[0] = data.cot_Anti1 * 0x19;
20.     keyAES[1] = data.cot_Anti2 * 0x28;
21.     keyAES[2] = data.cot_Anti3 * 0x37;
22.     keyAES[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
23.     keyAES[4] = data.cot_Anti1 * 0x19;
24.     keyAES[5] = data.cot_Anti2 * 0x28;
25.     keyAES[6] = data.cot_Anti3 * 0x37;
26.     keyAES[7] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
27.     keyAES[8] = data.cot_Anti1 * 0x46;
28.     keyAES[9] = data.cot_Anti2 * 0x55;
29.     keyAES[10] = data.cot_Anti3 * 0x64;
30.     keyAES[11] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
31.     keyAES[12] = data.cot_Anti1 * 0x46;
32.     keyAES[13] = data.cot_Anti2 * 0x55;
33.     keyAES[14] = data.cot_Anti3 * 0x64;
34.     keyAES[15] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
35.     //for (int i = 0; i < 16; i++)
36.     //   printf("%hx", keyAES[i]);
37.     aesEncrypt((const unsigned char*)keyAES, 16, (const unsigned char*)flag, (unsigned char*)comp, SIZE);
38.     data.cot_Anti3 += 1;
39.     for (int i = 0; i < SIZE; i++)
40.         printf("%#hx, ", comp[i]);
41.     return 0;
42. }

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1. int main() {
2.         for (int i = 0; i < 48; i++) {
3.                 unsigned char flag[SIZE + 1] = { 0xf4, 0x9c, 0xdd, 0x41, 0x3, 0xdd, 0x5a, 0x13, 0x2e, 0x55, 0x97, 0x9e, 0xff, 0xd5, 0x8, 0xd9, 0xf6, 0xd1, 0x9, 0x8c, 0x68, 0x9e, 0x92, 0xff, 0x75, 0xf, 0x80, 0x95, 0x4b, 0x16, 0xb9, 0xc6, 0x7f, 0x54, 0x2e, 0x20, 0x35, 0xfc, 0x1b, 0x46, 0x14, 0xaa, 0xda, 0x5e, 0x4f, 0xbd, 0x59, 0x71 , 0 };
4.                 unsigned char comp[SIZE + 1] = { 0 };
5.                 data.cot_Anti1 = 1;
6.                 data.cot_Anti2 = 1;
7.                 data.cot_Anti3 = 2;
8.                 data.cot_same = i;
9.                 unsigned char* keyAES = new unsigned char[16];
10.                 keyAES[0] = data.cot_Anti1 * 0x19;
11.                 keyAES[1] = data.cot_Anti2 * 0x28;
12.                 keyAES[2] = data.cot_Anti3 * 0x37;
13.                 keyAES[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
14.                 keyAES[4] = data.cot_Anti1 * 0x19;
15.                 keyAES[5] = data.cot_Anti2 * 0x28;
16.                 keyAES[6] = data.cot_Anti3 * 0x37;
17.                 keyAES[7] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
18.                 keyAES[8] = data.cot_Anti1 * 0x46;
19.                 keyAES[9] = data.cot_Anti2 * 0x55;
20.                 keyAES[10] = data.cot_Anti3 * 0x64;
21.                 keyAES[11] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
22.                 keyAES[12] = data.cot_Anti1 * 0x46;
23.                 keyAES[13] = data.cot_Anti2 * 0x55;
24.                 keyAES[14] = data.cot_Anti3 * 0x64;
25.                 keyAES[15] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
26.                 aesDecrypt((const unsigned char*)keyAES, 16, (const unsigned char*)flag, (unsigned char*)comp, SIZE);
27.                 for (int i = 0; i < SIZE; i++) {
28.                         comp[i] = ~comp[i];
29.                 }
30.                 data.cot_Anti2 = 0;
31.                 unsigned int keyTEA[4];
32.                 keyTEA[0] = data.cot_Anti1 * 0x19;
33.                 keyTEA[1] = data.cot_Anti2 * 0x28;
34.                 keyTEA[2] = data.cot_Anti3 * 0x37;
35.                 keyTEA[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3;
36.                 xxtea_uint_decrypt((unsigned int*)comp, SIZE / 4, (unsigned int*)keyTEA);
37.                 for (int i = 0; i < SIZE; i++)
38.                         printf("%c", comp[i]);
39.                 printf("\n");
40.         }
41.         return 0;
42. }

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经测试，仅有cot_same==0时有合法flag

1. NepCTF{tEA_with_AES_by_mixing_antiDebug_hahaHah}

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