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 

int main() {
	for (int i = 0; i 

经测试，仅有cot_same==0时有合法flag

NepCTF{tEA_with_AES_by_mixing_antiDebug_hahaHah}

机房租用，北京机房托管，大带宽租用，IDC机房服务器主机租用托管-价格及服务咨询 www.e1idc.net