0x00 分析

给了三个文件，一个程序，两个.enc结尾的，应该是加密了

后面要爆破时间戳

主打一个写代码

0x01 解

无壳，直接上IDA

main函数直接给出，一步步分析

接收输入然后根据输入的命令执行不同操作

具体操作可以查看printHelp函数

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"Usage: FinalEncrypt [OPTIONS]...\n"
"Encrypts/Decrypts files using FinalEncrypt\n"
"\n"
" -h                        Prints help screen\n"
" -e [filename]             Encrypts a specified file (must be passed with -r)\n"
" -d [filename]             Decrypts a specified file (must be passed with -k)\n"
" -k [512 bit Key in hex]   Specifies the key used for decryption\n"
" -r                        Generates a random key\n"
"\n"
"Examples - \n"
"FinalEncrypt -r -e testfile.txt\n"
"FinalEncrypt -d testfile.txt.enc -k D84165183346405877C13F497EB32B34410D5653A8141B07E307044F780EB733A62F1D087C126537"
"BBEAB769EA56E15FCAC19E4E215A4E1A28D3FC46362FC80D\n");

根据help，可以知道程序用于加密和解密文件，加密流程可以在main函数中找到

就是以时间为种子，取随机数，然后用此随机数去进行加密

进去fileEncrtpt函数会发现使用了ChaChaEncrypt函数，不过这里不需要深究，只要搞出时间戳出来就能解密了

尝试使用文件的修改时间的时间戳

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import os
print(hex(int(os.path.getmtime('./Encryption.exe.enc'))))
print(hex(int(os.path.getmtime('./flag.md.enc'))))
#0x663b35c6
#0x663b3627

动调

记得设置参数

断在time位置，单步运行，修改srand的传入值

获取两次的key

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exe:507CD82354B1A821ED46A45FAF06D53D0F941C24E085D511F244410B2666056462E126287107616B308DDB193B62036C89C7112C8E713828BC8E8C5079ED221A
md:CB0C24457D0BE9695EFDD94C533DE2036E0E6E706C1B06471DBE334DA3195C32F46C8078C7311D068270A10EAE446D0553FA1F628CE4336A39DA852730625324

使用key对两个文件进行解密

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./FinalEncrypt -d Encryption.exe.enc -k 507CD82354B1A821ED46A45FAF06D53D0F941C24E085D511F244410B2666056462E126287107616B308DDB193B62036C89C7112C8E713828BC8E8C5079ED221A

./FinalEncrypt -d flag.md.enc -k CB0C24457D0BE9695EFDD94C533DE2036E0E6E706C1B06471DBE334DA3195C32F46C8078C7311D068270A10EAE446D0553FA1F628CE4336A39DA852730625324

解密出来一个exe和一个md

md内容如下

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e07816e1dba1da61536634bef2c3b6346d533cc3b6b834e3beb634c80264143c34e36400bb4daa6902ff643414e3b8344dff6634b8b66db6bbc33834143461ab147e04

又是加密

exe就是一个加密器，用于加密

接下来就是看exe了

无壳

又是用到时间戳生成随机数

加密就是根据输入的flag字符在v8中找值

本质上是一个代换

那么重点就是搞出v8的内容，这里直接dump下生成表的过程然后进行爆破

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#include <stdio.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>
using namespace std;

unsigned char __ROR1__(unsigned char value, int count) {
    return ((value >> count) | (value << (8 - count))) & 0xFF;
}
void custom_algorithm(unsigned int v3, unsigned char *v8) {
    unsigned int time_seed; // eax
    char Str[8]; // [rsp+20h] [rbp-60h] BYREF
    char v6[56]; // [rsp+28h] [rbp-58h] BYREF
    int v7; // [rsp+60h] [rbp-20h]
    int v9; // [rsp+174h] [rbp+F4h]
    int v10; // [rsp+178h] [rbp+F8h]
    int v11; // [rsp+17Ch] [rbp+FCh]
    int v12; // [rsp+180h] [rbp+100h]
    int v13; // [rsp+184h] [rbp+104h]
    int v14; // [rsp+188h] [rbp+108h]
    int v15; // [rsp+18Ch] [rbp+10Ch]
    int v16; // [rsp+190h] [rbp+110h]
    char v17; // [rsp+195h] [rbp+115h]
    char v18; // [rsp+196h] [rbp+116h]
    char v19; // [rsp+197h] [rbp+117h]
    int i; // [rsp+198h] [rbp+118h]
    unsigned __int8 v21; // [rsp+19Ch] [rbp+11Ch]
    char v22; // [rsp+19Dh] [rbp+11Dh]
    char v23; // [rsp+19Eh] [rbp+11Eh]
    char v24; // [rsp+19Fh] [rbp+11Fh]

    memset(v8, 0, 0x100);
    srand(v3);
    do
      v19 = rand();
    while ( !v19 );
    memset(v8, 0, 0x100);
    v8[0] = v19;
    v22 = 1;
    v21 = 1;
    do
    {
      if ( v22 < 0 )
        v24 = 27;
      else
        v24 = 0;
      v22 ^= v24 ^ (2 * v22);
      v18 = (2 * v21) ^ (4 * ((2 * v21) ^ v21)) ^ v21;
      v17 = v18 ^ (16 * v18);
      if ( v17 < 0 )
        v23 = 9;
      else
        v23 = 0;
      v21 = v23 ^ v17;
      v16 = (unsigned __int8)(v23 ^ v17);
      v16 = __ROR1__(v23 ^ v17, 7);
      v15 = v16 ^ (unsigned __int8)(v21 ^ v8[0]);
      v14 = v21;
      v14 = __ROR1__(v21, 6);
      v13 = v15 ^ v14;
      v12 = v21;
      v12 = __ROR1__(v21, 5);
      v11 = v13 ^ v12;
      v10 = v21;
      v10 = __ROR1__(v21, 4);
      v9 = v11 ^ v10;
      v8[v22] = v11 ^ v10;
    }
    while ( v22 != 1 );
}

bool match_v8_with_b(const unsigned char *v8, const char *b, unsigned char *indices) {
    size_t b_len = strlen(b);
    bool all_found = true;
    size_t idx = 0;

    for (size_t i = 0; i < b_len; i += 2) {
        char hex_str[3] = {b[i], b[i + 1], '\0'};
        unsigned char byte = (unsigned char)strtol(hex_str, NULL, 16);
        bool found = false;

        for (int j = 0; j < 128; j++) {
            if (v8[j] == byte) {
                indices[idx++] = j; // Store index of v8 where match is found
                found = true;
                break;
            }
        }
        if (!found) {
            all_found = false;
            break;
        }
    }
    indices[idx] = 0xFF; // Terminate the indices array
    return all_found;
}

int main(){
    const char *b = "e07816e1dba1da61536634bef2c3b6346d533cc3b6b834e3beb634c80264143c34e36400bb4daa6902ff643414e3b8344dff6634b8b66db6bbc33834143461ab147e04";
    unsigned char v8[256];
    unsigned char indices[128]; // Large enough to store all indices needed
    unsigned int start_time =1715153796; // time stamp of FileEncrypt
    unsigned int end_time = 1715156519; // time stamp of flag.md.enc

    for (unsigned int v3 = start_time; v3 <= end_time; v3++) {
        custom_algorithm(v3, v8);
        if (match_v8_with_b(v8, b, indices)) {
            printf("Match found for seed %u\n", v3);
            printf("Matching indices in v8: ");
            for (int i = 0; indices[i] != 128; i++) {
                printf("%c", indices[i]);
            }
            printf("\n");
        }
    }
    return 0;
}
// DASCTF{7ou_h@ve_5o1ved_4he_fina1_4ncrypti0n_a4d_y0u_de5erv3_a_7lag}