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
keyTEA[0] = 0x19 * cotIsDebuggerPresent;
keyTEA[1] = 0x28 * cotBreakPoint1;
keyTEA[2] = 0x37 * cotCRCcheck;
keyTEA[3] = cotIsDebuggerPresent + cotCRCcheck + cotBreakPoint1
- 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
keyAES->m128i_i8[0] = 0x19 * cotIsDebuggerPresent;
keyAES->m128i_i8[1] = 5 * cotBreakPoint1 * 8;
keyAES->m128i_i8[2] = 0x37 * cotCRCcheck;
keyAES->m128i_i8[3] = cotSame + cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck;
keyAES->m128i_i8[4] = 0x19 * cotIsDebuggerPresent;
keyAES->m128i_i8[5] = 5 * cotBreakPoint1 * 8;
keyAES->m128i_i8[6] = 0x37 * cotCRCcheck;
keyAES->m128i_i8[7] = cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck - cotSame;
keyAES->m128i_i8[8] = 70 * cotIsDebuggerPresent;
keyAES->m128i_i8[9] = 85 * cotBreakPoint1;
keyAES->m128i_i8[10] = 100 * cotCRCcheck;
keyAES->m128i_i8[11] = cotSame + cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck;
keyAES->m128i_i8[12] = 70 * cotIsDebuggerPresent;
keyAES->m128i_i8[13] = 85 * cotBreakPoint1;
keyAES->m128i_i8[14] = 100 * cotCRCcheck;
keyAES->m128i_i8[15] = cotIsDebuggerPresent + cotBreakPoint1 + cotCRCcheck - cotSame;
- 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
int main() {
unsigned char flag[SIZE + 1] = "NepCTF{tEA_with_AES_by_mixing_antiDebug_hahaHah}";
unsigned char comp[SIZE + 1] = { 0 };
data.cot_Anti1 += 1;
data.cot_Anti3 += 1;
data.cot_Anti3 += 1;
unsigned int keyTEA[4];
keyTEA[0] = data.cot_Anti1 * 0x19;
keyTEA[1] = data.cot_Anti2 * 0x28;
keyTEA[2] = data.cot_Anti3 * 0x37;
keyTEA[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3;
xxtea_uint_encrypt((unsigned int*)flag, SIZE/4, (unsigned int*)keyTEA);
data.cot_same = 0;
data.cot_Anti2 += 1;
for (int i = 0; i < SIZE; i++) {
flag[i] = ~flag[i];
}
unsigned char* keyAES = new unsigned char[16];
keyAES[0] = data.cot_Anti1 * 0x19;
keyAES[1] = data.cot_Anti2 * 0x28;
keyAES[2] = data.cot_Anti3 * 0x37;
keyAES[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
keyAES[4] = data.cot_Anti1 * 0x19;
keyAES[5] = data.cot_Anti2 * 0x28;
keyAES[6] = data.cot_Anti3 * 0x37;
keyAES[7] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
keyAES[8] = data.cot_Anti1 * 0x46;
keyAES[9] = data.cot_Anti2 * 0x55;
keyAES[10] = data.cot_Anti3 * 0x64;
keyAES[11] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
keyAES[12] = data.cot_Anti1 * 0x46;
keyAES[13] = data.cot_Anti2 * 0x55;
keyAES[14] = data.cot_Anti3 * 0x64;
keyAES[15] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
//for (int i = 0; i < 16; i++)
// printf("%hx", keyAES[i]);
aesEncrypt((const unsigned char*)keyAES, 16, (const unsigned char*)flag, (unsigned char*)comp, SIZE);
data.cot_Anti3 += 1;
for (int i = 0; i < SIZE; i++)
printf("%#hx, ", comp[i]);
return 0;
}
int main() {
for (int i = 0; i < 48; i++) {
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 };
unsigned char comp[SIZE + 1] = { 0 };
data.cot_Anti1 = 1;
data.cot_Anti2 = 1;
data.cot_Anti3 = 2;
data.cot_same = i;
unsigned char* keyAES = new unsigned char[16];
keyAES[0] = data.cot_Anti1 * 0x19;
keyAES[1] = data.cot_Anti2 * 0x28;
keyAES[2] = data.cot_Anti3 * 0x37;
keyAES[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
keyAES[4] = data.cot_Anti1 * 0x19;
keyAES[5] = data.cot_Anti2 * 0x28;
keyAES[6] = data.cot_Anti3 * 0x37;
keyAES[7] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
keyAES[8] = data.cot_Anti1 * 0x46;
keyAES[9] = data.cot_Anti2 * 0x55;
keyAES[10] = data.cot_Anti3 * 0x64;
keyAES[11] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 + data.cot_same;
keyAES[12] = data.cot_Anti1 * 0x46;
keyAES[13] = data.cot_Anti2 * 0x55;
keyAES[14] = data.cot_Anti3 * 0x64;
keyAES[15] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3 - data.cot_same;
aesDecrypt((const unsigned char*)keyAES, 16, (const unsigned char*)flag, (unsigned char*)comp, SIZE);
for (int i = 0; i < SIZE; i++) {
comp[i] = ~comp[i];
}
data.cot_Anti2 = 0;
unsigned int keyTEA[4];
keyTEA[0] = data.cot_Anti1 * 0x19;
keyTEA[1] = data.cot_Anti2 * 0x28;
keyTEA[2] = data.cot_Anti3 * 0x37;
keyTEA[3] = data.cot_Anti1 + data.cot_Anti2 + data.cot_Anti3;
xxtea_uint_decrypt((unsigned int*)comp, SIZE / 4, (unsigned int*)keyTEA);
for (int i = 0; i < SIZE; i++)
printf("%c", comp[i]);
printf("\n");
}
return 0;
}
经测试，仅有cot_same==0时有合法flag
NepCTF{tEA_with_AES_by_mixing_antiDebug_hahaHah}