buu-[WUSTCTF2020]level3

64位GCC

查看字符串，base甩脸上

看一眼main函数

int __cdecl main(int argc, const char **argv, const char **envp)
{char v3; // ST0F_1const char *v4; // raxchar v6; // [rsp+10h] [rbp-40h]unsigned __int64 v7; // [rsp+48h] [rbp-8h]v7 = __readfsqword(0x28u);printf("Try my base64 program?.....\n>", argv, envp);__isoc99_scanf("%20s", &v6);v3 = time(0LL);srand(v3);if ( rand() & 1 ){v4 = (const char *)base64_encode(&v6);puts(v4);puts("Is there something wrong?");}else{puts("Sorry I think it's not prepared yet....");puts("And I get a strange string from my program which is different from the standard base64:");puts("d2G0ZjLwHjS7DmOzZAY0X2lzX3CoZV9zdNOydO9vZl9yZXZlcnGlfD==");puts("What's wrong??");}return 0;
}

输入一个值，在base64_encode()后的值为d2G0ZjLwHjS7DmOzZAY0X2lzX3CoZV9zdNOydO9vZl9yZXZlcnGlfD==
直接解是解不出来的，应该是更改了密码表

main函数头上

果然更改了密码表

__int64 O_OLookAtYou()
{char v0; // ST03_1__int64 result; // raxsigned int i; // [rsp+2h] [rbp-4h]for ( i = 0; i <= 9; ++i ){v0 = base64_table[i];base64_table[i] = base64_table[19 - i];result = 19 - i;base64_table[result] = v0;}return result;
}

import base64
import stringflag = 'd2G0ZjLwHjS7DmOzZAY0X2lzX3CoZV9zdNOydO9vZl9yZXZlcnGlfD=='
base64_table =list("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
v0=''i=0
for i in range(10):v0 = base64_table[i]base64_table[i] = base64_table[19 - i]result = 19 - ibase64_table[result] = v0base64_table=''.join(base64_table)string2 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"print(base64.b64decode(flag.translate(str.maketrans(base64_table,string2))))

flag{Base64_is_the_start_of_reverse}

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