[LitCTF 2023]enbase64

WriteUp 1年前 (2023) admin

62 0 0

这是一个将flag换base64表之后加密的代码。

附件下载

https://wwvc.lanzouj.com/iwZuF1bmzehg

查壳

无壳，32位。

[LitCTF 2023]enbase64

分析

丢入ida32，找到main函数F5。

int __cdecl main(int argc, const char **argv, const char **envp)
{
    char Source[61]; // [esp+1Fh] [ebp-81Dh] BYREF
    char v5[4]; // [esp+5Ch] [ebp-7E0h] BYREF
    char Str1[1000]; // [esp+60h] [ebp-7DCh] BYREF
    char Str[1012]; // [esp+448h] [ebp-3F4h] BYREF

    __main();
    memset(Str, 0, 1000);
    memset(Str1, 0, sizeof(Str1));
    *(_DWORD *)Source = *(_DWORD *)"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    strcpy(v5, "9+/");
    qmemcpy(&Source[1], &aAbcdefghijklmn[-(Source - &Source[1])], 4 * (((Source - &Source[1] + 65) & 0xFFFFFFFC) >> 2));
    puts("Please input flag:");
    gets(Str);
    if ( strlen(Str) == 33 )
{
    base64(Source, Str, Str1);
    basecheck(Str1);
}
    return 0;
}

第12和第13没有用，Source是base64表，也就是ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/，Str是输入的flag，先经过base64这个函数加密，我们先点入base64函数里面看看。

[LitCTF 2023]enbase64

发现basechange(Source)，怀疑将base64表换了，我们继续跟进basechange函数。

[LitCTF 2023]enbase64

将Source的表换成Destination，可以直接跑出来换后的表，即Destination值。

[LitCTF 2023]enbase64

脚本

#include<stdio.h>
#include<string.h>
int main()
{
  char *result; // eax
  char Destination[65]; // [esp+13h] [ebp-155h] BYREF
  int v3[65]; // [esp+54h] [ebp-114h] BYREF
  int j; // [esp+158h] [ebp-10h]
  int i; // [esp+15Ch] [ebp-Ch]
  char Source[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  memset(v3, 0, sizeof(v3));
  v3[0] = 16;
  v3[1] = 34;
  v3[2] = 56;
  v3[3] = 7;
  v3[4] = 46;
  v3[5] = 2;
  v3[6] = 10;
  v3[7] = 44;
  v3[8] = 20;
  v3[9] = 41;
  v3[10] = 59;
  v3[11] = 31;
  v3[12] = 51;
  v3[13] = 60;
  v3[14] = 61;
  v3[15] = 26;
  v3[16] = 5;
  v3[17] = 40;
  v3[18] = 21;
  v3[19] = 38;
  v3[20] = 4;
  v3[21] = 54;
  v3[22] = 52;
  v3[23] = 47;
  v3[24] = 3;
  v3[25] = 11;
  v3[26] = 58;
  v3[27] = 48;
  v3[28] = 32;
  v3[29] = 15;
  v3[30] = 49;
  v3[31] = 14;
  v3[32] = 37;
  v3[34] = 55;
  v3[35] = 53;
  v3[36] = 24;
  v3[37] = 35;
  v3[38] = 18;
  v3[39] = 25;
  v3[40] = 33;
  v3[41] = 43;
  v3[42] = 50;
  v3[43] = 39;
  v3[44] = 12;
  v3[45] = 19;
  v3[46] = 13;
  v3[47] = 42;
  v3[48] = 9;
  v3[49] = 17;
  v3[50] = 28;
  v3[51] = 30;
  v3[52] = 23;
  v3[53] = 36;
  v3[54] = 1;
  v3[55] = 22;
  v3[56] = 57;
  v3[57] = 63;
  v3[58] = 8;
  v3[59] = 27;
  v3[60] = 6;
  v3[61] = 62;
  v3[62] = 45;
  v3[63] = 29;
  result = strcpy(Destination, Source);
  for ( i = 0; i <= 47; ++i )
  {
    for ( j = 0; j <= 63; ++j )
      Source[j] = Destination[v3[j]];
    result = strcpy(Destination, Source);
  }
  printf("%s",Destination);
  return 0;
}

换后的表

[LitCTF 2023]enbase64

gJ1BRjQie/FIWhEslq7GxbnL26M4+HXUtcpmVTKaydOP38of5v90ZSwrkYzCAuND

接下来就好办了，把之前写过的base64解密代码的base64表替换一下，找到加密后的值。

也就是basecheck函数。

[LitCTF 2023]enbase64

双击跟进，找到加密后的值。

[LitCTF 2023]enbase64

GQTZlSqQXZ/ghxxwhju3hbuZ4wufWjujWrhYe7Rce7ju

现在换的表和加密后的值都找到了，可以得到flag值了，直接上脚本。

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char base64CharsArr[] = "gJ1BRjQie/FIWhEslq7GxbnL26M4+HXUtcpmVTKaydOP38of5v90ZSwrkYzCAuND";

void base64decode(char str[]) {
    int length = strlen(str);
    int padding = 0;

    // 计算填充字符数量
    if (str[length - 1] == '=') {
        padding++;
        if (str[length - 2] == '=')
            padding++;
    }

    // 计算解码后的字符数量
    int decodedLength = (length * 3) / 4 - padding;

    // 分配存储解码结果的内存
    char* decodedStr = (char*)malloc(decodedLength + 1);

    int outIndex = 0;
    for (int i = 0; i < length; i += 4) {
        char char1 = -1, char2 = -1, char3 = -1, char4 = -1;

        // 查找每个字符在Base64字符集中的索引
        for (int j = 0; j < 64; j++) {
            if (base64CharsArr[j] == str[i]) {
                char1 = j;
                break;
            }
        }

        for (int j = 0; j < 64; j++) {
            if (base64CharsArr[j] == str[i + 1]) {
                char2 = j;
                break;
            }
        }

        for (int j = 0; j < 64; j++) {
            if (base64CharsArr[j] == str[i + 2]) {
                 char3 = j;
                break;
            }
        }

        for (int j = 0; j < 64; j++) {
            if (base64CharsArr[j] == str[i + 3]) {
                 char4 = j;
                break;
            }
        }

        // 解码并存储结果
        decodedStr[outIndex++] = (char1 << 2) | ((char2 & 0x30) >> 4);
        if (char3 != -1)
            decodedStr[outIndex++] = ((char2 & 0xf) << 4) | ((char3 & 0x3c) >> 2);
        if (char4 != -1)
            decodedStr[outIndex++] = ((char3 & 0x3) << 6) | char4;
    }

    // 添加字符串结束符
    decodedStr[decodedLength] = '';

    printf("Decoded string: %sn", decodedStr);

    // 释放内存
    free(decodedStr);
}

int main() {
    char str[100]="GQTZlSqQXZ/ghxxwhju3hbuZ4wufWjujWrhYe7Rce7ju"; // Example base64 encoded string
    base64decode(str);
    return 0;
}