2020 BJDCTF Re encode

Posted 2021-03-08

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娴嬭瘯鏂囦欢锛?a href="https://www.lanzous.com/i9la55a" target="_blank">https://www.lanzous.com/i9la55a

 

鍑嗗

鑾峰彇淇℃伅锛?/p>

• 32浣嶆枃浠?/li>
• UPX澹?/li>

 

IDA鍒嗘瀽

UPX鑴卞３鍚庯紝IDA鎵撳紑

int sub_804887C()
{
  int v0; // eax
  int result; // eax
  int v2; // ecx
  unsigned int v3; // et1
  unsigned int i; // [esp+Ch] [ebp-FCh]
  unsigned int v5; // [esp+10h] [ebp-F8h]
  unsigned int v6; // [esp+14h] [ebp-F4h]
  int v7; // [esp+1Ah] [ebp-EEh]
  int v8; // [esp+1Eh] [ebp-EAh]
  int v9; // [esp+22h] [ebp-E6h]
  int v10; // [esp+26h] [ebp-E2h]
  __int16 v11; // [esp+2Ah] [ebp-DEh]
  char v12[30]; // [esp+2Ch] [ebp-DCh]
  int v13; // [esp+4Ah] [ebp-BEh]
  int v14; // [esp+4Eh] [ebp-BAh]
  int v15; // [esp+52h] [ebp-B6h]
  int v16; // [esp+56h] [ebp-B2h]
  int v17; // [esp+5Ah] [ebp-AEh]
  int v18; // [esp+5Eh] [ebp-AAh]
  int v19; // [esp+62h] [ebp-A6h]
  int v20; // [esp+66h] [ebp-A2h]
  int v21; // [esp+6Ah] [ebp-9Eh]
  int v22; // [esp+6Eh] [ebp-9Ah]
  int v23; // [esp+72h] [ebp-96h]
  int v24; // [esp+76h] [ebp-92h]
  __int16 v25; // [esp+7Ah] [ebp-8Eh]
  char v26; // [esp+7Ch] [ebp-8Ch]
  unsigned int v27; // [esp+FCh] [ebp-Ch]

  v27 = __readgsdword(0x14u);
  v7 = 鈥?/span>galF鈥?/span>;
  v8 = 鈥?/span>ihT{鈥?/span>;
  v9 = 鈥?/span>_a_s鈥?/span>;
  v10 = 鈥?/span>galF鈥?/span>;
  v11 = 鈥?/span>}鈥?/span>;
  v5 = strlen(&v7);
  v13 = 鈥?/span>8D8E鈥?/span>;
  v14 = 鈥?/span>19DB鈥?/span>;
  v15 = 鈥?/span>A178鈥?/span>;
  v16 = 鈥?/span>65E1鈥?/span>;
  v17 = 鈥?/span>F35F鈥?/span>;
  v18 = 鈥?/span>9884鈥?/span>;
  v19 = 鈥?/span>F286鈥?/span>;
  v20 = 鈥?/span>4169鈥?/span>;
  v21 = 鈥?/span>2FA2鈥?/span>;
  v22 = 鈥?/span>F8BA鈥?/span>;
  v23 = 鈥?/span>A7DE鈥?/span>;
  v24 = 鈥?/span>5DFC鈥?/span>;
  v25 = 鈥?/span>E鈥?/span>;
  printf("Please input your flag:");
  read(0, &v26, 256);
  if ( strlen(&v26) != 21 )
    exit(0);
  v0 = sub_8048AC2((int)&v26);
  strcpy((int)v12, v0);
  v6 = length(v12);
  for ( i = 0; i < v6; ++i )                    // 寮傛垨鎿嶄綔
    v12[i] ^= *((_BYTE *)&v7 + i % v5);
  sub_8048E24(v12, v6, &v7, v5);
  if ( !strcmp(v12, &v13) )
    exit(0);
  printf("right!");
  result = 0;
  v3 = __readgsdword(0x14u);
  v2 = v3 ^ v27;
  if ( v3 != v27 )
    sub_806FA00(v2);
  return result;
}

寰堝鏈煡鍑芥暟锛屾垜浠兘澶熺寽鍑烘槸浠€涔堝嚱鏁帮紝鍦ㄤ唬鐮佷腑宸叉敼銆?/p>

 

浠ｇ爜鍒嗘瀽

鍦ㄤ唬鐮佷腑锛屾湁涓ゅ鍑芥暟鎴戜滑涓嶆竻妤氫綔鐢ㄣ€傜涓€澶?/p>

v0 = sub_8048AC2((int)&v26);

鍑芥暟浼犲叆鍙傛暟鏄垜浠緭鍏ョ殑瀛楃涓诧紝鎵撳紑鍑芥暟鍚?/p>

int __cdecl sub_8048AC2(int a1)
{
  int v2; // [esp+8h] [ebp-20h]
  int v3; // [esp+Ch] [ebp-1Ch]
  int v4; // [esp+10h] [ebp-18h]
  int v5; // [esp+18h] [ebp-10h]
  int v6; // [esp+1Ch] [ebp-Ch]

  v5 = length(a1);
  if ( v5 % 3 )
    v2 = 4 * (v5 / 3 + 1);
  else
    v2 = 4 * (v5 / 3);
  v6 = sub_80597A0(v2 + 1);
  *(_BYTE *)(v2 + v6) = 0;
  v3 = 0;
  v4 = 0;
  while ( v2 - 2 > v3 )
  {
    *(_BYTE *)(v6 + v3) = a0123456789Abcd[(unsigned __int8)(*(_BYTE *)(v4 + a1) >> 2)];
    *(_BYTE *)(v6 + v3 + 1) = a0123456789Abcd[16 * (*(_BYTE *)(v4 + a1) & 3) | (unsigned __int8)(*(_BYTE *)(v4 + 1 + a1) >> 4)];
    *(_BYTE *)(v6 + v3 + 2) = a0123456789Abcd[4 * (*(_BYTE *)(v4 + 1 + a1) & 0xF) | (unsigned __int8)(*(_BYTE *)(v4 + 2 + a1) >> 6)];
    *(_BYTE *)(v6 + v3 + 3) = a0123456789Abcd[*(_BYTE *)(v4 + 2 + a1) & 0x3F];
    v4 += 3;
    v3 += 4;
  }
  if ( v5 % 3 == 1 )
  {
    *(_BYTE *)(v3 - 2 + v6) = 61;
    *(_BYTE *)(v3 - 1 + v6) = 61;
  }
  else if ( v5 % 3 == 2 )
  {
    *(_BYTE *)(v3 - 1 + v6) = 61;
  }
  return v6;
}

.rodata:080BB9A8 a0123456789Abcd db 鈥?/span>0123456789+/abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ鈥?/span>,0

閫氳繃瑙傚療锛岃繖搴旇鏄竴涓彉琛ㄧ殑Base64鍔犲瘑

 

绗簩澶?/p>

  sub_8048E24((int)v10, v4, (int)&v5, v3)

unsigned int __cdecl sub_8048E24(int a1, unsigned int a2, int a3, int a4)
{
  char v4; // ST2B_1
  unsigned int result; // eax
  unsigned int v6; // et1
  int v7; // [esp+1Ch] [ebp-11Ch]
  int v8; // [esp+20h] [ebp-118h]
  unsigned int i; // [esp+24h] [ebp-114h]
  char v10[256]; // [esp+2Ch] [ebp-10Ch]
  unsigned int v11; // [esp+12Ch] [ebp-Ch]

  v11 = __readgsdword(0x14u);
  sub_8048CC2((int)v10, a3, a4);
  LOBYTE(v7) = 0;
  LOBYTE(v8) = 0;
  for ( i = 0; i < a2; ++i )
  {
    v7 = (unsigned __int8)(v7 + 1);
    v8 = (unsigned __int8)(v10[v7] + v8);
    v4 = v10[v7];
    v10[v7] = v10[v8];
    v10[v8] = v4;
    *(_BYTE *)(a1 + i) ^= v10[(unsigned __int8)(v10[v7] + v10[v8])];
  }
  v6 = __readgsdword(0x14u);
  result = v6 ^ v11;
  if ( v6 != v11 )
    sub_806FA00();
  return result;
}

杩欏簲璇ユ槸涓€涓猂C4鍔犲瘑鍑芥暟锛?nbsp;sub_8048CC2((int)v10, a3, a4);鍑芥暟鏄垵濮嬪寲鍑芥暟

 

鎬濊矾

杩欐牱鏁翠釜浠ｇ爜鐨勬€濊矾灏卞緢鏄庢櫚浜嗭紝瀵硅緭鍏ュ瓧绗︿覆杩涜base64鍔犲瘑鍚庯紝鍐嶈繘琛屽紓鎴栨搷浣滐紝鏈€鍚庤繘琛孯C4鍔犲瘑(Key = "Flag{This_a_Flag}")锛屽苟涓?E8D8BD91871A1E56F53F4889682F96142AF2AB8FED7ACFD5E"姣旇緝

 

瑙ｅ瘑

瑙ｅ瘑缃戠珯锛?/p>

http://tool.chacuo.net/cryptrc4

http://ctf.ssleye.com/rc4.html

http://tools.jb51.net/password/rc4_encode

https://gchq.github.io/CyberChef/

寰楀埌瑙ｅ瘑鐨勫瓧绗︿覆锛?3152553081a5938126a3931275b0b1313085c330b356101511f105c

# -*- coding:utf-8 -*-

from base64 import b64decode

key = 鈥?/span>Flag{This_a_Flag}鈥?/span>
decode_byte = 鈥?/span>23152553081a5938126a3931275b0b1313085c330b356101511f105c鈥?/span>

encode_base64 = 鈥樷€?/span>
lists = []

for i in range(len(decode_byte)//2):
    lists.append(int(decode_byte[i*2:(i+1)*2],16))
# for i in range(0,len(decode_byte),2):
#     lists.append(int(decode_byte[i:i+2],16))

print (lists)
for i in range(len(lists)):
    encode_base64 += chr(lists[i] ^ ord(key[i%len(key)]))

t = 鈥?/span>0123456789+/abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ=鈥?/span>
table = 鈥?/span>ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=鈥?/span>
table = str.maketrans(t, table)
flag = b64decode(encode_base64.translate(table))
print(flag)

 

get flag!

BJD{0v0_Y0u_g07_1T!}

 

 

 

E8D8BD91871A1E56F53F4889682F96142AF2AB8FED7ACFD5E