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CTF对抗-kctf2022 秋季赛 第十题 两袖清风 wp

推荐 原创

周杰伦

文章分类 Android安全 CTF对抗 IOS安全 阅读数 : 657 阅读时长 : 9分钟

先看一看ida的f5

看见了1300行的f5，忍不住想关掉ida。

 

但是面对困难不要怕，经过一小会分析，我们恢复了一点细节。

 



作者在程序中插入了很多的 antidebug 技巧。同时字符串一般都用维吉尼亚进行了加密。

 


 

完成程序初始化之后，我们可以看到程序开始了校验逻辑。

 


 

将第一步份校验逻辑用python写下，可以发现那个模数n可以在factordb上查询到一个小因子p。

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# flag = input('Please enter your key:')
# print('Start cheking your key ...')
 
if len (flag) < 89 :
     print ( 'no!' )
     return
 
n = 0x4F62187B5F6590C6CFF0FBDBBEBDAF60AA861BD2F66F8F7FFD57A66AE50DB7D2FFFFFFFFFFFFFFFFFFFFF
# factored from factordb
p = 193
q = n / / p
assert isPrime(q)
assert n = = p * q
 
base = 0xB20446102D1C343D0575674CA28EBC0419BCFE4D75682C2AC81C9502454650BDDAEF6968AF269B54C182
_pow = int (flag[: 85 ], 16 )
powered = hex ( pow (base, _pow, n))[ 2 : ]
if len (powered) % 2 = = 1 : powered = powered[: - 1 ]
encrypted = bytes.fromhex(powered)

乍一看像是 RSA ，实际上是一个离散对数问题，flag的第一部分是长度为85的十六进制数据。

 

$ base^{flagpartone} \mod n = C $

 

那么作者在这里隐藏了离散对数问题吗？根据我们对n的另一个因子q的分析，q-1不为一个光滑数，所以没有办法在多项式时间求解该离散对数问题，也就是说我们知道了C无法求得flagpartone。

 

继续往下分析。

 


 

作者将获得的 C 进行flag的第二部分，flag的85-88字节，必须为ABAB，ABBA的形式的，连接C作为魔改AES的密钥。密文为一段硬编码的 C2 。该AES使用CBC模式，iv是b'ABCDEF0123456789'。

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part2 = flag[ 85 : 89 ]
codes = bytes.fromhex(vigenere(bytes.fromhex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x}da" ).decode())
encrypted = part2.encode() + encrypted # vigenere(encrypted, "Y?j0?")
decrypted = aes_decrypt_cbc(codes, encrypted, b 'ABCDEF0123456789' )

之后解密的内容我们记为 Code1 。也就是“decrypted”。

 


 

程序将解密后内容存入一个可读可写可执行的内存区域，最后使用ZwQueueApcThread创建一个
异步程序调用，这里把它当成call就行了。值得注意的是，解密后的内容在拷贝的过程会出现\0截断问题。

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#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <assert.h>
/ / AES
typedef struct {
     unsigned char rk[ 15 * 4 * 4 ]; / / round key
     unsigned char iv[ 16 ];
} AES_CONTEXT;
 
const unsigned char Sbox[ 256 ] = {
/ /  0    1    2    3    4    5    6    7    8    9    a    b    c    d    e    f
     0x63 , 0x7c , 0x77 , 0x7b , 0xf2 , 0x6b , 0x6f , 0xc5 , 0x30 , 0x01 , 0x67 , 0x2b , 0xfe , 0xd7 , 0xab , 0x76 , / / 0
     0xca , 0x82 , 0xc9 , 0x7d , 0xfa , 0x59 , 0x47 , 0xf0 , 0xad , 0xd4 , 0xa2 , 0xaf , 0x9c , 0xa4 , 0x72 , 0xc0 , / / 1
     0xb7 , 0xfd , 0x93 , 0x26 , 0x36 , 0x3f , 0xf7 , 0xcc , 0x34 , 0xa5 , 0xe5 , 0xf1 , 0x71 , 0xd8 , 0x31 , 0x15 , / / 2
     0x04 , 0xc7 , 0x23 , 0xc3 , 0x18 , 0x96 , 0x05 , 0x9a , 0x07 , 0x12 , 0x80 , 0xe2 , 0xeb , 0x27 , 0xb2 , 0x75 , / / 3
     0x09 , 0x83 , 0x2c , 0x1a , 0x1b , 0x6e , 0x5a , 0xa0 , 0x52 , 0x3b , 0xd6 , 0xb3 , 0x29 , 0xe3 , 0x2f , 0x84 , / / 4
     0x53 , 0xd1 , 0x00 , 0xed , 0x20 , 0xfc , 0xb1 , 0x5b , 0x6a , 0xcb , 0xbe , 0x39 , 0x4a , 0x4c , 0x58 , 0xcf , / / 5
     0xd0 , 0xef , 0xaa , 0xfb , 0x43 , 0x4d , 0x33 , 0x85 , 0x45 , 0xf9 , 0x02 , 0x7f , 0x50 , 0x3c , 0x9f , 0xa8 , / / 6
     0x51 , 0xa3 , 0x40 , 0x8f , 0x92 , 0x9d , 0x38 , 0xf5 , 0xbc , 0xb6 , 0xda , 0x21 , 0x10 , 0xff , 0xf3 , 0xd2 , / / 7
     0xcd , 0x0c , 0x13 , 0xec , 0x5f , 0x97 , 0x44 , 0x17 , 0xc4 , 0xa7 , 0x7e , 0x3d , 0x64 , 0x5d , 0x19 , 0x73 , / / 8
     0x60 , 0x81 , 0x4f , 0xdc , 0x22 , 0x2a , 0x90 , 0x88 , 0x46 , 0xee , 0xb8 , 0x14 , 0xde , 0x5e , 0x0b , 0xdb , / / 9
     0xe0 , 0x32 , 0x3a , 0x0a , 0x49 , 0x06 , 0x24 , 0x5c , 0xc2 , 0xd3 , 0xac , 0x62 , 0x91 , 0x95 , 0xe4 , 0x79 , / / a
     0xe7 , 0xc8 , 0x37 , 0x6d , 0x8d , 0xd5 , 0x4e , 0xa9 , 0x6c , 0x56 , 0xf4 , 0xea , 0x65 , 0x7a , 0xae , 0x08 , / / b
     0xba , 0x78 , 0x25 , 0x2e , 0x1c , 0xa6 , 0xb4 , 0xc6 , 0xe8 , 0xdd , 0x74 , 0x1f , 0x4b , 0xbd , 0x8b , 0x8a , / / c
     0x70 , 0x3e , 0xb5 , 0x66 , 0x48 , 0x03 , 0xf6 , 0x0e , 0x61 , 0x35 , 0x57 , 0xb9 , 0x86 , 0xc1 , 0x1d , 0x9e , / / d
     0xe1 , 0xf8 , 0x98 , 0x11 , 0x69 , 0xd9 , 0x8e , 0x94 , 0x9b , 0x1e , 0x87 , 0xe9 , 0xce , 0x55 , 0x28 , 0xdf , / / e
     0x8c , 0xa1 , 0x89 , 0x0d , 0xbf , 0xe6 , 0x42 , 0x68 , 0x41 , 0x99 , 0x2d , 0x0f , 0xb0 , 0x54 , 0xbb , 0x16  / / f
};
 
const unsigned char InvSbox[ 256 ] = {
/ /  0    1    2    3    4    5    6    7    8    9    a    b    c    d    e    f
     0x52 , 0x09 , 0x6a , 0xd5 , 0x30 , 0x36 , 0xa5 , 0x38 , 0xbf , 0x40 , 0xa3 , 0x9e , 0x81 , 0xf3 , 0xd7 , 0xfb , / / 0
     0x7c , 0xe3 , 0x39 , 0x82 , 0x9b , 0x2f , 0xff , 0x87 , 0x34 , 0x8e , 0x43 , 0x44 , 0xc4 , 0xde , 0xe9 , 0xcb , / / 1
     0x54 , 0x7b , 0x94 , 0x32 , 0xa6 , 0xc2 , 0x23 , 0x3d , 0xee , 0x4c , 0x95 , 0x0b , 0x42 , 0xfa , 0xc3 , 0x4e , / / 2
     0x08 , 0x2e , 0xa1 , 0x66 , 0x28 , 0xd9 , 0x24 , 0xb2 , 0x76 , 0x5b , 0xa2 , 0x49 , 0x6d , 0x8b , 0xd1 , 0x25 , / / 3
     0x72 , 0xf8 , 0xf6 , 0x64 , 0x86 , 0x68 , 0x98 , 0x16 , 0xd4 , 0xa4 , 0x5c , 0xcc , 0x5d , 0x65 , 0xb6 , 0x92 , / / 4
     0x6c , 0x70 , 0x48 , 0x50 , 0xfd , 0xed , 0xb9 , 0xda , 0x5e , 0x15 , 0x46 , 0x57 , 0xa7 , 0x8d , 0x9d , 0x84 , / / 5
     0x90 , 0xd8 , 0xab , 0x00 , 0x8c , 0xbc , 0xd3 , 0x0a , 0xf7 , 0xe4 , 0x58 , 0x05 , 0xb8 , 0xb3 , 0x45 , 0x06 , / / 6
     0xd0 , 0x2c , 0x1e , 0x8f , 0xca , 0x3f , 0x0f , 0x02 , 0xc1 , 0xaf , 0xbd , 0x03 , 0x01 , 0x13 , 0x8a , 0x6b , / / 7
     0x3a , 0x91 , 0x11 , 0x41 , 0x4f , 0x67 , 0xdc , 0xea , 0x97 , 0xf2 , 0xcf , 0xce , 0xf0 , 0xb4 , 0xe6 , 0x73 , / / 8
     0x96 , 0xac , 0x74 , 0x22 , 0xe7 , 0xad , 0x35 , 0x85 , 0xe2 , 0xf9 , 0x37 , 0xe8 , 0x1c , 0x75 , 0xdf , 0x6e , / / 9
     0x47 , 0xf1 , 0x1a , 0x71 , 0x1d , 0x29 , 0xc5 , 0x89 , 0x6f , 0xb7 , 0x62 , 0x0e , 0xaa , 0x18 , 0xbe , 0x1b , / / a
     0xfc , 0x56 , 0x3e , 0x4b , 0xc6 , 0xd2 , 0x79 , 0x20 , 0x9a , 0xdb , 0xc0 , 0xfe , 0x78 , 0xcd , 0x5a , 0xf4 , / / b
     0x1f , 0xdd , 0xa8 , 0x33 , 0x88 , 0x07 , 0xc7 , 0x31 , 0xb1 , 0x12 , 0x10 , 0x59 , 0x27 , 0x80 , 0xec , 0x5f , / / c
     0x60 , 0x51 , 0x7f , 0xa9 , 0x19 , 0xb5 , 0x4a , 0x0d , 0x2d , 0xe5 , 0x7a , 0x9f , 0x93 , 0xc9 , 0x9c , 0xef , / / d
     0xa0 , 0xe0 , 0x3b , 0x4d , 0xae , 0x2a , 0xf5 , 0xb0 , 0xc8 , 0xeb , 0xbb , 0x3c , 0x83 , 0x53 , 0x99 , 0x61 , / / e
     0x17 , 0x2b , 0x04 , 0x7e , 0xba , 0x77 , 0xd6 , 0x26 , 0xe1 , 0x69 , 0x14 , 0x63 , 0x55 , 0x21 , 0x0c , 0x7d  / / f
};
 
void KeyExpansion(AES_CONTEXT * ctx, const unsigned char * key) {
     unsigned char t[ 4 ];
     unsigned char tmp;
 
     memcpy(ctx - >rk, key, 32 );
 
     for ( int i = 8 ; i < 15 * 4 ; i + + ) {
         memcpy(t, ctx - >rk + 4 * (i - 1 ), 4 );
         if (i % 8 ) {
             if (i % 8 = = 4 ) {
                 t[ 0 ] = Sbox[t[ 0 ]];
                 t[ 1 ] = Sbox[t[ 1 ]];
                 t[ 2 ] = Sbox[t[ 2 ]];
                 t[ 3 ] = Sbox[t[ 3 ]];
             }
         } else {
             tmp = Sbox[t[ 1 ]];
             t[ 1 ] = Sbox[t[ 2 ]];
             t[ 2 ] = Sbox[t[ 3 ]];
             t[ 3 ] = Sbox[t[ 0 ]];
             unsigned char rc = 1 ;
             for ( int j = 0 ; j < i / 8 - 1 ; j + + )
                 rc = ( 0x1B * (rc >> 7 )) ^ ( 2 * rc);
             t[ 0 ] = rc ^ tmp;
         }
         ctx - >rk[i * 4 ] = ctx - >rk[i * 4 - 32 ] ^ t[ 0 ];
         ctx - >rk[i * 4 + 1 ] = ctx - >rk[i * 4 - 31 ] ^ t[ 1 ];
         ctx - >rk[i * 4 + 2 ] = ctx - >rk[i * 4 - 30 ] ^ t[ 2 ];
         ctx - >rk[i * 4 + 3 ] = ctx - >rk[i * 4 - 29 ] ^ t[ 3 ];
     }
}
 
unsigned char FFmul(unsigned char a, unsigned char b) {
     unsigned char bw[ 4 ];
     unsigned char res = 0 ;
     int i;
     bw[ 0 ] = b;
     for (i = 1 ; i < 4 ; i + + ) {
         bw[i] = bw[i - 1 ] << 1 ;
         if (bw[i - 1 ] & 0x80 ) {
             bw[i] ^ = 0x1b ;
         }
     }
     for (i = 0 ; i < 4 ; i + + ) {
         if ((a >> i) & 0x01 ) {
             res ^ = bw[i];
         }
     }
     return res;
}
 
void SubBytes(AES_CONTEXT * ctx, unsigned char * state) {
     for ( int i = 0 ; i < 16 ; i + + )
         state[i] = Sbox[state[i]];
}
 
void ShiftRows(AES_CONTEXT * ctx, unsigned char * state) {
     unsigned char tmp[ 4 ];
 
     * (unsigned int * ) tmp = * (unsigned int * ) (state + 4 );
     state[ 7 ] = tmp[ 0 ];
     state[ 4 ] = tmp[ 1 ];
     state[ 5 ] = tmp[ 2 ];
     state[ 6 ] = tmp[ 3 ];
 
     * (unsigned int * ) tmp = * (unsigned int * ) (state + 8 );
     state[ 10 ] = tmp[ 0 ];
     state[ 11 ] = tmp[ 1 ];
     state[ 8 ] = tmp[ 2 ];
     state[ 9 ] = tmp[ 3 ];
 
     * (unsigned int * ) tmp = * (unsigned int * ) (state + 12 );
     state[ 13 ] = tmp[ 0 ];
     state[ 14 ] = tmp[ 1 ];
     state[ 15 ] = tmp[ 2 ];
     state[ 12 ] = tmp[ 3 ];
}
 
void MixColumns(AES_CONTEXT * ctx, unsigned char * state) {
     unsigned char tmp[ 16 ];
     memset(tmp, 0. , 16 );
 
     for ( int i = 0 ; i < 4 ; i + + ) {
         tmp[i] = FFmul( 2 , state[ 0 + i]) ^ FFmul( 3 , state[ 4 + i]) ^ FFmul( 1 , state[ 8 + i]) ^ FFmul( 1 , state[ 12 + i]);
         tmp[i + 4 ] = FFmul( 1 , state[ 0 + i]) ^ FFmul( 2 , state[ 4 + i]) ^ FFmul( 3 , state[ 8 + i]) ^ FFmul( 1 , state[ 12 + i]);
         tmp[i + 8 ] = FFmul( 1 , state[ 0 + i]) ^ FFmul( 1 , state[ 4 + i]) ^ FFmul( 2 , state[ 8 + i]) ^ FFmul( 3 , state[ 12 + i]);
         tmp[i + 12 ] = FFmul( 3 , state[ 0 + i]) ^ FFmul( 1 , state[ 4 + i]) ^ FFmul( 1 , state[ 8 + i]) ^ FFmul( 2 , state[ 12 + i]);
     }
     memcpy(state, tmp, 16 );
}
 
void AddRoundKey(AES_CONTEXT * ctx, unsigned char * state, unsigned char * k) {
     for ( int i = 0 ; i < 4 ; i + + ) {
         for ( int j = 0 ; j < 4 ; j + + ) {
             state[i * 4 + j] ^ = k[j * 4 + i];
         }
     }
}
 
void InvSubBytes(AES_CONTEXT * ctx, unsigned char * state) {
     for ( int i = 0 ; i < 16 ; i + + )
         state[i] = InvSbox[state[i]];
}
 
void InvShiftRows(AES_CONTEXT * ctx, unsigned char * state) {
     unsigned char tmp[ 4 ];
 
     tmp[ 0 ] = state[ 7 ];
     tmp[ 1 ] = state[ 4 ];
     tmp[ 2 ] = state[ 5 ];
     tmp[ 3 ] = state[ 6 ];
     * (unsigned int * ) (state + 4 ) = * (unsigned int * ) tmp;
 
     tmp[ 0 ] = state[ 10 ];
     tmp[ 1 ] = state[ 11 ];
     tmp[ 2 ] = state[ 8 ];
     tmp[ 3 ] = state[ 9 ];
     * (unsigned int * ) (state + 8 ) = * (unsigned int * ) tmp;
 
     tmp[ 0 ] = state[ 13 ];
     tmp[ 1 ] = state[ 14 ];
     tmp[ 2 ] = state[ 15 ];
     tmp[ 3 ] = state[ 12 ];
     * (unsigned int * ) (state + 12 ) = * (unsigned int * ) tmp;
}
 
void InvMixColumns(AES_CONTEXT * ctx, unsigned char * state) {
     unsigned char tmp[ 16 ];
     memset(tmp, 0. , 16 );
 
     for ( int i = 0 ; i < 4 ; i + + ) {
         tmp[i] = FFmul( 0x0e , state[ 0 + i]) ^ FFmul( 0x0b , state[ 4 + i]) ^ FFmul( 0x0d , state[ 8 + i]) ^ FFmul( 0x09 , state[ 12 + i]);
         tmp[i + 4 ] = FFmul( 0x09 , state[ 0 + i]) ^ FFmul( 0x0e , state[ 4 + i]) ^ FFmul( 0x0b , state[ 8 + i]) ^ FFmul( 0x0d , state[ 12 + i]);
         tmp[i + 8 ] = FFmul( 0x0d , state[ 0 + i]) ^ FFmul( 0x09 , state[ 4 + i]) ^ FFmul( 0x0e , state[ 8 + i]) ^ FFmul( 0x0b , state[ 12 + i]);
         tmp[i + 12 ] = FFmul( 0x0b , state[ 0 + i]) ^ FFmul( 0x0d , state[ 4 + i]) ^ FFmul( 0x09 , state[ 8 + i]) ^ FFmul( 0x0e , state[ 12 + i]);
     }
     memcpy(state, tmp, 16 );
}
 
void aes_init(AES_CONTEXT * ctx, const unsigned char * key, const void * iv) {
     / / memcpy(ctx - >Sbox, sBox, 256 );
     / / memcpy(ctx - >InvSbox, invsBox, 256 );
     KeyExpansion(ctx, key);
     if (iv) memcpy(ctx - >iv, iv, 16 );
}
 
unsigned char * aes_decrypt_block(AES_CONTEXT * ctx, unsigned char * input ) {
     unsigned char state[ 16 ];
     int i, r, c;
 
     for ( int i = 0 ; i < 4 ; i + + ) {
         for ( int j = 0 ; j < 4 ; j + + ) {
             state[i * 4 + j] = input [j * 4 + i];
         }
     }
 
     AddRoundKey(ctx, state, ctx - >rk + 14 * 16 );
 
     for ( int i = 13 ; i > 0 ; i - - ) {
         InvSubBytes(ctx, state);
         InvShiftRows(ctx, state);
         AddRoundKey(ctx, state, ctx - >rk + 16 * i);
         InvMixColumns(ctx, state);
     }
 
     InvSubBytes(ctx, state);
     InvShiftRows(ctx, state);
     AddRoundKey(ctx, state, ctx - >rk);
 
     for ( int i = 0 ; i < 4 ; i + + ) {
         for ( int j = 0 ; j < 4 ; j + + ) {
             input [i * 4 + j] = state[j * 4 + i];
         }
     }
 
     return input ;
}
 
 
void aes_decrypt_ecb(const void * key, void * data, int data_len) {
     assert (data_len % 16 = = 0 );
     AES_CONTEXT ctx;
     aes_init(&ctx, (const unsigned char * ) key, NULL);
     unsigned char * _data = (unsigned char * ) data;
     for ( int i = 0 ; i < data_len / 16 ; i + + ) {
         aes_decrypt_block(&ctx, _data);
         _data + = 16 ;
     }
}
 
void aes_decrypt_cbc(const void * key, const void * iv, void * data, int data_len) {
     assert (data_len % 16 = = 0 );
     AES_CONTEXT ctx;
     aes_init(&ctx, (const unsigned char * ) key, iv);
     unsigned char * _data = (unsigned char * ) data;
     unsigned char buf[ 16 ];
     for ( int i = 0 ; i < data_len / 16 ; i + + ) {
         memcpy(buf, _data, 16 );
         aes_decrypt_block(&ctx, _data);
         for ( int j = 0 ; j < 16 ; j + + ) _data[j] ^ = ctx.iv[j];
         _data + = 16 ;
         memcpy(ctx.iv, buf, 16 );
     }
}
 
int main() {
     unsigned char data[] =
         "\x94\xC7\xA9\x05\xC7\xDC\x74\xB9\x28\x9E\x58\x9C\x98\xDA\x3A\xBD"
         "\xD3\x56\x72\xB3\x8C\x7F\xA3\x06\x00\x19\xE7\x0C\x85\xB8\xE7\x9D"
         "\x09\x87\xF8\xF3\x7F\x61\x2A\x05\xB5\x27\x20\x99\x35\x9C\x51\x27"
         "\xCC\x55\x73\x34\x73\x0F\xC3\x35\xC9\xE2\x7B\x1B\xBD\x93\x7A\x2C"
         "\x47\x2D\xA0\xBD\xBA\x72\x77\xD2\x8A\xDB\x35\xBE\x38\xC1\x63\xBA"
         "\x72\xCD\x6A\x63\x91\x5D\xC0\x4B\x8C\xD7\x2B\x0E\x16\x64\xB3\xFD" ;
 
     unsigned char key[] =  "\x44\x44\x45\x45\xaf\x20\x5d\xb8\xe5\x0e\xb8\x95\x2b\x78\x22\xcc\x7a\x3a\xeb\x55\x0c\x0a\xc9\xb7\xfc\xf1\x59\xc0\xe8\xc7\x29\xf4" ;
     unsigned char iv[] = "ABCDEF0123456789" ;
 
     #define BLOCK_SIZE 4
     unsigned char tmp[ 16 * BLOCK_SIZE];
 
     for ( int i = 0 ; i < 100 ; i + + ) {
         memcpy(tmp, data, 16 * BLOCK_SIZE);
         key[ 0 ] = 0x30 + i / 10 ;
         key[ 1 ] = 0x30 + i / 10 ;
         key[ 2 ] = 0x30 + i % 10 ;
         key[ 3 ] = 0x30 + i % 10 ;
 
         aes_decrypt_cbc(key, iv, tmp, 16 * BLOCK_SIZE);
         if (tmp[ 0 ] = = 0xC3 ) break ;
     }
 
     printf( "%s\n" , tmp[ 0 ] = = 0xc3 ? "Yes." : "No." );
 
     for ( int i = 0 ; i < 2 ; i + + ) {
         for ( int j = 0 ; j < 16 ; j + + ) {
             printf( "%02x " , key[i * 16 + j]);
         }
         putchar( '\n' );
     }
     putchar( '\n' );
 
     for ( int i = 0 ; i < BLOCK_SIZE; i + + ) {
         for ( int j = 0 ; j < 16 ; j + + ) {
             printf( "%02x " , tmp[i * 16 + j]);
         }
         putchar( '\n' );
     }
 
     return 0 ;
}

仅仅魔改了密钥扩展，该AES为修改密钥扩展的14轮AES。与常见的AES-128不同，这个AES使用256bit(32字节)密钥，加密块却是128bit。在98k成员的有限的知识中应该不存在算法缺点。

 

如果我们假设这里能顺利运行，我们的问题就变成了需要猜测明文可执行指令，和密文，求出密钥。显然这个场景的是不可解问题。约等于AES的已知明文攻击。在没有实现缺陷的情况下，不可求出密钥。

 

所以，我们认为，以上两步中我们可以控制输入的flag的第一和第二部分控制AES解密结果，只要确保第一个字节为 0xc3 也就是 ret 指令就能让程序完美运行。平均256次穷举就能获得一个通过上述所有检查的输入。

 

该步骤最多可产生 16 ^ 85 / 256 种解。

 


 

然后程序使用UUID的方式编码shellcode。

 


 

经过分析，程序会根据输入的flag条件，在程序main函数中存在正确或者失败的字符串的内存中写入good或者bad。所以我们可以根据第三部分来控制main函数中的输出，我们想覆盖main函数中字符串的位置为220。根据分析我们需要控制 ‘kctf’ + 数据长度单字节 + 数据 为20字节。

 

并且满足以下约束条件。

1
2
3
4
5
6
7
8
9
10
11
12
13
flag:
a - z: 1
0 - 9 ,A - Z: 0
other: 2
maybe, 3
debug: xx
type [A[i] * 10 + B[i]] ! = 1
A[i + 1 ] - A[i] = = 1 || A[i + 1 ] - A[i] = = - 1 || B[i + 1 ] - B[i] = = 1 || B[i + 1 ] - B[i] = = - 1
B[ 14 ] = = 0 || B[ 14 ] = = 9 || A[ 14 ] = = 0 || A[ 14 ] = = 9
B[ 14 ] ! = B[ 0 ] || A[ 14 ] ! = A[ 0 ]
B[ 0 ] = = 0
type [A[i] * 10 + B[i] + 100 ] ! = 1
B[ 14 ] < = 9

A数组代表输入hex中的的高位组成的数组，B数组代表输入hex中的的低位组成的数组。

 

这里宽松的约束条件也能产生很多组解。

 

笔者这里给出九组解。

1
2
3
[ '1111110123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCEF' + '9911' + '0F' + f '201{i}' * 7 + '01' for i in range ( 1 , 10 )]
1111110123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCEF99110F101110111011101110111011101101
1111110123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCEF99110F102110211021102110211021102102




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