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CTF-RE 从0到N:Chacha20逆向实战 2024 强网杯青少年专项赛 EnterGame WP (END)

news 2025/7/27 19:39:58

只想解题的看最后就好了,前面是算法分析
Chacha20
c语言是如何利用逻辑运算符拆分变量和合并的

通过百度网盘分享的文件：EnterGame_9acdc7c33f85832082adc6a4e...
链接：https://pan.baidu.com/s/182SRj2Xemo63PCoaLNUsRQ?pwd=1111 
提取码：1111 
--来自百度网盘超级会员V2的分享

step1 主函数

int __fastcall main(int argc, const char **argv, const char **envp)unsigned int v3; // eax_WORD v5[20]; // [rsp+8h] [rbp-138h] BYREF_QWORD s2[2]; // [rsp+30h] [rbp-110h] BYREF_QWORD v7[4]; // [rsp+40h] [rbp-100h] BYREFchar s[112]; // [rsp+60h] [rbp-E0h] BYREF_BYTE s1[104]; // [rsp+D0h] [rbp-70h] BYREFunsigned __int64 v10; // [rsp+138h] [rbp-8h]v10 = __readfsqword(0x28u);puts("Welcome to the Youth Cybersecurity Challenge!");s2[0] = 0x7A2B7587D3AA135ELL;s2[1] = 0xD21D7E49A304161BLL;qmemcpy(v7, "k]X@^DcYHQ", 10);*(_QWORD *)((char *)&v7[1] + 2) = 0x82AD5855585E540DLL;*(_QWORD *)((char *)&v7[2] + 2) = 0xC1CE5D58ABE7DCAFLL;puts("Please enter the password to start the game:");fgets(s, 100, _bss_start);s[strcspn(s, "\n")] = 0;HIBYTE(v5[18]) = 0;v5[19] = 0;strcpy((char *)v5, "01234567Youth Strengthens the Nation");v3 = strlen(s);chacha20_encrypt(&v5[4], v5, s, s1, v3);if ( !memcmp(s1, s2, 0x2AuLL) ){puts("Password correct, you may start the game.");puts("With my strength, I secure the cyber frontier!");}else{puts("Incorrect password, cannot start the game.");}return 0;
}

直接定位到比较函数,在此处动态调试抽取s2的值

  if ( !memcmp(s1, s2, 0x2AuLL) ){puts("Password correct, you may start the game.");puts("With my strength, I secure the cyber frontier!");}else{puts("Incorrect password, cannot start the game.");}

建立数组使用插件dump数组备用
请添加图片描述

unsigned char data[42] = {0x5E, 0x13, 0xAA, 0xD3, 0x87, 0x75, 0x2B, 0x7A, 0x1B, 0x16, 0x04, 0xA3, 0x49, 0x7E, 0x1D, 0xD2, 0x6B, 0x5D, 0x58, 0x40, 0x5E, 0x44, 0x63, 0x59, 0x48, 0x51, 0x0D, 0x54, 0x5E, 0x58, 0x55, 0x58, 0xAD, 0x82, 0xAF, 0xDC, 0xE7, 0xAB, 0x58, 0x5D, 0xCE, 0xC1
};

step2 分析chacha20_encrypt参数

unsigned __int64 __fastcall chacha20_encrypt(int *a1, int *a2, __int64 a3, __int64 a4, int a5)
{int i; // [rsp+38h] [rbp-98h]int j; // [rsp+3Ch] [rbp-94h]int v11[12]; // [rsp+40h] [rbp-90h] BYREFint v12; // [rsp+70h] [rbp-60h]int v13; // [rsp+74h] [rbp-5Ch]int v14; // [rsp+78h] [rbp-58h]int v15; // [rsp+7Ch] [rbp-54h]char v16[72]; // [rsp+80h] [rbp-50h] BYREFunsigned __int64 v17; // [rsp+C8h] [rbp-8h]v17 = __readfsqword(0x28u);qmemcpy(v11, "expand 32-byte k", 16);v11[4] = *a1;v11[5] = a1[1];v11[6] = a1[2];v11[7] = a1[3];v11[8] = a1[4];v11[9] = a1[5];v11[10] = a1[6];v11[11] = a1[7];v12 = 0;v13 = 0;v14 = *a2;v15 = a2[1];for ( i = 0; i < a5; i += 64 ){chacha20_block(v16, v11);++v12;for ( j = 0; j <= 63 && a5 > i + j; ++j )*(_BYTE *)(i + j + a4) = v16[j] ^ *(_BYTE *)(i + j + a3);}return v17 - __readfsqword(0x28u);
}

无视换位直接动态调试在此句断下

  for ( i = 0; i < a5; i += 64 ){chacha20_block(v16, v11);++v12;for ( j = 0; j <= 63 && a5 > i + j; ++j )*(_BYTE *)(i + j + a4) = v16[j] ^ *(_BYTE *)(i + j + a3);}

化简,因为i + j + a4 等价于 a4[i + j]

  for ( i = 0; i < a5; i += 64 ){chacha20_block(v16, v11);++v12;for ( j = 0; j <= 63 && a5 > i + j; ++j )a4[i + j] = v16[j] ^ a3[i+j];}

对比标准函数

    while (in_len > 0) {// 生成一个加密块chacha20_block((uint32_t *)block, state);state[12]++;  // 每次加密后递增计数器size_t block_size = (in_len < 64) ? in_len : 64;  // 计算当前块的大小for (i = 0; i < block_size; i++) {out[i] = in[i] ^ block[i];  // 将输入数据与加密块异或得到密文}

step3 _block()分析

unsigned __int64 __fastcall chacha20_block(__int64 a1, const void *a2)
{int i; // [rsp+18h] [rbp-58h]int j; // [rsp+1Ch] [rbp-54h]unsigned int dest; // [rsp+20h] [rbp-50h] BYREFunsigned int v6; // [rsp+24h] [rbp-4Ch]unsigned int v7; // [rsp+28h] [rbp-48h]unsigned int v8; // [rsp+2Ch] [rbp-44h]unsigned int v9; // [rsp+30h] [rbp-40h]unsigned int v10; // [rsp+34h] [rbp-3Ch]unsigned int v11; // [rsp+38h] [rbp-38h]unsigned int v12; // [rsp+3Ch] [rbp-34h]unsigned int v13; // [rsp+40h] [rbp-30h]unsigned int v14; // [rsp+44h] [rbp-2Ch]unsigned int v15; // [rsp+48h] [rbp-28h]unsigned int v16; // [rsp+4Ch] [rbp-24h]unsigned int v17; // [rsp+50h] [rbp-20h]unsigned int v18; // [rsp+54h] [rbp-1Ch]unsigned int v19; // [rsp+58h] [rbp-18h]unsigned int v20; // [rsp+5Ch] [rbp-14h]unsigned __int64 v21; // [rsp+68h] [rbp-8h]v21 = __readfsqword(0x28u);memcpy(&dest, a2, 0x40uLL);for ( i = 0; i <= 9; ++i ){dest += v9;v17 = (dest << 16) ^ v17 | HIWORD(dest) ^ v17;v13 += v17;v9 = (v13 << 12) ^ v9 | (v13 >> 20) ^ v9;dest += v9;v17 = (dest << 8) ^ v17 | HIBYTE(dest) ^ v17;v13 += v17;v9 = (v13 << 7) ^ v9 | (v13 >> 25) ^ v9;v6 += v10;v18 = (v6 << 16) ^ v18 | HIWORD(v6) ^ v18;v14 += v18;v10 = (v14 << 12) ^ v10 | (v14 >> 20) ^ v10;v6 += v10;v18 = (v6 << 8) ^ v18 | HIBYTE(v6) ^ v18;v14 += v18;v10 = (v14 << 7) ^ v10 | (v14 >> 25) ^ v10;v7 += v11;v19 = (v7 << 16) ^ v19 | HIWORD(v7) ^ v19;v15 += v19;v11 = (v15 << 12) ^ v11 | (v15 >> 20) ^ v11;v7 += v11;v19 = (v7 << 8) ^ v19 | HIBYTE(v7) ^ v19;v15 += v19;v11 = (v15 << 7) ^ v11 | (v15 >> 25) ^ v11;v8 += v12;v20 = (v8 << 16) ^ v20 | HIWORD(v8) ^ v20;v16 += v20;v12 = (v16 << 12) ^ v12 | (v16 >> 20) ^ v12;v8 += v12;v20 = (v8 << 8) ^ v20 | HIBYTE(v8) ^ v20;v16 += v20;v12 = (v16 << 7) ^ v12 | (v16 >> 25) ^ v12;dest += v10;v20 = (dest << 16) ^ v20 | HIWORD(dest) ^ v20;v15 += v20;v10 = (v15 << 12) ^ v10 | (v15 >> 20) ^ v10;dest += v10;v20 = (dest << 8) ^ v20 | HIBYTE(dest) ^ v20;v15 += v20;v10 = (v15 << 7) ^ v10 | (v15 >> 25) ^ v10;v6 += v11;v17 = (v6 << 16) ^ v17 | HIWORD(v6) ^ v17;v16 += v17;v11 = (v16 << 12) ^ v11 | (v16 >> 20) ^ v11;v6 += v11;v17 = (v6 << 8) ^ v17 | HIBYTE(v6) ^ v17;v16 += v17;v11 = (v16 << 7) ^ v11 | (v16 >> 25) ^ v11;v7 += v12;v18 = (v7 << 16) ^ v18 | HIWORD(v7) ^ v18;v13 += v18;v12 = (v13 << 12) ^ v12 | (v13 >> 20) ^ v12;v7 += v12;v18 = (v7 << 8) ^ v18 | HIBYTE(v7) ^ v18;v13 += v18;v12 = (v13 << 7) ^ v12 | (v13 >> 25) ^ v12;v8 += v9;v19 = (v8 << 16) ^ v19 | HIWORD(v8) ^ v19;v14 += v19;v9 = (v14 << 12) ^ v9 | (v14 >> 20) ^ v9;v8 += v9;v19 = (v8 << 8) ^ v19 | HIBYTE(v8) ^ v19;v14 += v19;v9 = (v14 << 7) ^ v9 | (v14 >> 25) ^ v9;}for ( j = 0; j <= 15; ++j )*(4LL * j + a1) = *(&dest + j) + *(a2 + j);return v21 - __readfsqword(0x28u);
}

将其与标准Chacha20代码对比

void chacha20_block(uint32_t output[16], const uint32_t input[16]) {int i;uint32_t x[16];  // 创建一个32位的数组用于存储状态memcpy(x, input, sizeof(x));  // 将输入状态复制到数组x// 进行20轮加密操作，每轮执行四分之一轮操作for (i = 0; i < 10; i++) {// 奇数轮QR(x[0], x[4], x[8], x[12]);QR(x[1], x[5], x[9], x[13]);QR(x[2], x[6], x[10], x[14]);QR(x[3], x[7], x[11], x[15]);// 偶数轮QR(x[0], x[5], x[10], x[15]);QR(x[1], x[6], x[11], x[12]);QR(x[2], x[7], x[8], x[13]);QR(x[3], x[4], x[9], x[14]);}// 将加密结果与原始输入状态相加，输出最终结果for (i = 0; i < 16; ++i) {output[i] = x[i] + input[i];}
}

继续对比

#define QR(a, b, c, d) ( \a += b, d ^= a, d = ROTL(d, 16), \c += d, b ^= c, b = ROTL(b, 12), \a += b, d ^= a, d = ROTL(d, 8), \c += d, b ^= c, b = ROTL(b, 7))

此加密函数外层循环包含重复的此块8次,与此对应分析

        QR(x[0], x[4], x[8], x[12]);QR(x[1], x[5], x[9], x[13]);QR(x[2], x[6], x[10], x[14]);QR(x[3], x[7], x[11], x[15]);// 偶数轮QR(x[0], x[5], x[10], x[15]);QR(x[1], x[6], x[11], x[12]);QR(x[2], x[7], x[8], x[13]);QR(x[3], x[4], x[9], x[14]);

    dest += v9;v17 = (dest << 16) ^ v17 | HIWORD(dest) ^ v17;v13 += v17;v9 = (v13 << 12) ^ v9 | (v13 >> 20) ^ v9;dest += v9;v17 = (dest << 8) ^ v17 | HIBYTE(dest) ^ v17;v13 += v17;v9 = (v13 << 7) ^ v9 | (v13 >> 25) ^ v9;v6 += v10;

退化ida的HIWORD

dest += v9;
v17 = (dest << 16) ^ v17 | (dest >> 16) ^ v17;
v13 += v17;
v9 = (v13 << 12) ^ v9 | (v13 >> 20) ^ v9;
dest += v9;
v17 = (dest << 8) ^ v17 | (dest >> 24) ^ v17;
v13 += v17;
v9 = (v13 << 7) ^ v9 | (v13 >> 25) ^ v9;
v6 += v10;

分析

变量映射：
- 我们来对比两个代码片段中的变量：
  - a 对应 dest
  - b 对应 v9
  - c 对应 v13
  - d 对应 v17
操作分析：
- 第一轮：
  - 第一个代码片段：
    1. a += b
    2. d ^= a
    3. d = ROTL(d, 16)
    4. c += d
    5. b ^= c
    6. b = ROTL(b, 12)
    7. a += b
    8. d ^= a
    9. d = ROTL(d, 8)
    10. c += d
    11. b ^= c
    12. b = ROTL(b, 7)
  - 第二个代码片段：
    1. dest += v9 （对应 a += b）
    2. v17 = (dest << 16) ^ v17 | (dest >> 16) ^ v17 （对应 d ^= a 和 d = ROTL(d, 16)）
    3. v13 += v17 （对应 c += d）
    4. v9 = (v13 << 12) ^ v9 | (v13 >> 20) ^ v9 （对应 b ^= c 和 b = ROTL(b, 12)）
    5. dest += v9 （对应 a += b）
    6. v17 = (dest << 8) ^ v17 | (dest >> 24) ^ v17 （对应 d ^= a 和 d = ROTL(d, 8)）
    7. v13 += v17 （对应 c += d）
    8. v9 = (v13 << 7) ^ v9 | (v13 >> 25) ^ v9 （对应 b ^= c 和 b = ROTL(b, 7)）

step4 重新化简代码

unsigned __int64 __fastcall chacha20_encrypt(int *a1, int *a2, __int64 a3, __int64 a4, int a5)
{int i; // [rsp+38h] [rbp-98h]int j; // [rsp+3Ch] [rbp-94h]int v11[12]; // [rsp+40h] [rbp-90h] BYREFint v12; // [rsp+70h] [rbp-60h]int v13; // [rsp+74h] [rbp-5Ch]int v14; // [rsp+78h] [rbp-58h]int v15; // [rsp+7Ch] [rbp-54h]char v16[72]; // [rsp+80h] [rbp-50h] BYREFunsigned __int64 v17; // [rsp+C8h] [rbp-8h]v17 = __readfsqword(0x28u);qmemcpy(v11, "expand 32-byte k", 16);v11[4] = *a1;v11[5] = a1[1];v11[6] = a1[2];v11[7] = a1[3];v11[8] = a1[4];v11[9] = a1[5];v11[10] = a1[6];v11[11] = a1[7];v12 = 0;v13 = 0;v14 = *a2;v15 = a2[1];for ( i = 0; i < a5; i += 64 ){chacha20_block(v16, v11);++v12;for ( j = 0; j <= 63 && a5 > i + j; ++j )*(_BYTE *)(i + j + a4) = v16[j] ^ *(_BYTE *)(i + j + a3);}return v17 - __readfsqword(0x28u);
}

1.根据Chacha20加密代码v11是state[16]重定义v11为int state[16]
2.根据Chacha20加密代码v12重命名为block
3.根据Chacha20加密代码a4=>char out[64],a3=>in[64]

unsigned __int64 __fastcall chacha20_encrypt(int *a1, int *a2, char (*in)[64], char (*out)[64], int a5)
{int i; // [rsp+38h] [rbp-98h]int j; // [rsp+3Ch] [rbp-94h]int state[16]; // [rsp+40h] [rbp-90h] BYREF_BYTE block[72]; // [rsp+80h] [rbp-50h] BYREFunsigned __int64 v13; // [rsp+C8h] [rbp-8h]v13 = __readfsqword(0x28u);qmemcpy(state, "expand 32-byte k", 16);state[4] = *a1;state[5] = a1[1];state[6] = a1[2];state[7] = a1[3];state[8] = a1[4];state[9] = a1[5];state[10] = a1[6];state[11] = a1[7];state[12] = 0;state[13] = 0;state[14] = *a2;state[15] = a2[1];for ( i = 0; i < a5; i += 64 ){chacha20_block(block, state);++state[12];for ( j = 0; j <= 63 && a5 > i + j; ++j )(*out)[i + j] = block[j] ^ (*in)[i + j];}return v13 - __readfsqword(0x28u);
}

我们能看到此时ida代码已经趋近与标准代码

try get flag

#include <stdint.h>  // 包含标准整数类型
#include <string.h>  // 包含内存操作函数，如 memcpy
#include <cstdio>    // 包含标准输入输出函数，如 putchar// 宏定义：循环左移（左移操作后，右边溢出的部分重新回到左边）
#define ROTL(a, b) (((a) << (b)) | ((a) >> (32 - (b))))// 宏定义：Chacha20的四分之一轮（Quarter Round）操作
#define QR(a, b, c, d) ( \a += b, d ^= a, d = ROTL(d, 16), \c += d, b ^= c, b = ROTL(b, 12), \a += b, d ^= a, d = ROTL(d, 8), \c += d, b ^= c, b = ROTL(b, 7))// ChaCha20加密算法中的块函数
void chacha20_block(uint32_t output[16], const uint32_t input[16]) {int i;uint32_t x[16];  // 创建一个32位的数组用于存储状态memcpy(x, input, sizeof(x));  // 将输入状态复制到数组x// 进行20轮加密操作，每轮执行四分之一轮操作for (i = 0; i < 10; i++) {// 奇数轮QR(x[0], x[4], x[8], x[12]);QR(x[1], x[5], x[9], x[13]);QR(x[2], x[6], x[10], x[14]);QR(x[3], x[7], x[11], x[15]);// 偶数轮QR(x[0], x[5], x[10], x[15]);QR(x[1], x[6], x[11], x[12]);QR(x[2], x[7], x[8], x[13]);QR(x[3], x[4], x[9], x[14]);}// 将加密结果与原始输入状态相加，输出最终结果for (i = 0; i < 16; ++i) {output[i] = x[i] + input[i];}
}// ChaCha20加密函数
void chacha20_encrypt(uint8_t *out, const uint8_t *in, size_t in_len) {/** 从 chacha20_block(block, state); 断点抽取*/uint32_t state[16] = {0x61707865, 0x3320646E, 0x79622D32, 0x6B206574, 0x74756F59, 0x74532068, 0x676E6572, 0x6E656874,0x68742073, 0x614E2065, 0x6E6F6974, 0x00000000, 0x00000000, 0x00000000, 0x33323130, 0x37363534};uint8_t block[64];  // 存储每次生成的64字节的加密块size_t i, j;while (in_len > 0) {// 生成一个加密块chacha20_block((uint32_t *)block, state);state[12]++;  // 每次加密后递增计数器size_t block_size = (in_len < 64) ? in_len : 64;  // 计算当前块的大小for (i = 0; i < block_size; i++) {out[i] = in[i] ^ block[i];  // 将输入数据与加密块异或得到密文}// 更新剩余输入数据的长度和指针in_len -= block_size;in += block_size;out += block_size;}
}int main() {/** 从 if ( !memcmp(s1, s2, 0x2AuLL) ) 断点抽取*/uint8_t ciphertext[42] = {0x5E, 0x13, 0xAA, 0xD3, 0x87, 0x75, 0x2B, 0x7A, 0x1B, 0x16, 0x04, 0xA3, 0x49, 0x7E, 0x1D, 0xD2,0x6B, 0x5D, 0x58, 0x40, 0x5E, 0x44, 0x63, 0x59, 0x48, 0x51, 0x0D, 0x54, 0x5E, 0x58, 0x55, 0x58,0xAD, 0x82, 0xAF, 0xDC, 0xE7, 0xAB, 0x58, 0x5D, 0xCE, 0xC1};  // 用于存储加密后的密文uint8_t decrypted[64];  // 用于存储解密后的数据// 执行解密操作（加密是对称的，解密过程与加密相同）chacha20_encrypt(decrypted, ciphertext, sizeof(ciphertext));// 解密后的数据应当与原始明文相同for (int i = 0; i < sizeof(decrypted); i++) {putchar(decrypted[i]);  // 输出解密后的字符}return 0;
}

不知道为什么不对,直接暴力解题,尝试抽取chacha20_block(block, state);的block和密文暴力解题

a = [0x38, 0x7F, 0xCB, 0xB4, 0xFC, 0x46, 0x13, 0x4F, 0x22, 0x27, 0x31, 0xC2, 0x2D, 0x53, 0x25, 0xB4, 0x58, 0x6F, 0x75, 0x74, 0x67, 0x20, 0x53, 0x74, 0x71, 0x65, 0x6E, 0x67, 0x73, 0x68, 0x65, 0x6E, 0x9A, 0xE4, 0x9E, 0xB8, 0x86, 0xCF, 0x69, 0x3F, 0xAA, 0xBC, 0x94, 0x90, 0x84, 0xDD, 0xFE, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x2F, 0x31, 0x32, 0x33, 0x33, 0x35, 0x36, 0x37]
b = [0x5E, 0x13, 0xAA, 0xD3, 0x87, 0x75, 0x2B, 0x7A, 0x1B, 0x16, 0x04, 0xA3, 0x49, 0x7E, 0x1D, 0xD2, 0x6B, 0x5D, 0x58, 0x40, 0x5E, 0x44, 0x63, 0x59, 0x48, 0x51, 0x0D, 0x54, 0x5E, 0x58, 0x55, 0x58, 0xAD, 0x82, 0xAF, 0xDC, 0xE7, 0xAB, 0x58, 0x5D, 0xCE, 0xC1]
flag = ''
for i in range(len(b)):flag += chr(a[i] ^ b[i])print(flag)