Post

Reverse Engineering Training

Posted Updated
By 8 min read

Inroduction

Cyber Talents - training reverse engineering ahram canadian competition.

Description

x64 stripped linux binary file given file called training with description of “ training, keep training.” Reverse it to get the flag

File

For downloading , Visit training .

Solution

The file is a x64 stripped linux binary At running, it reads from stdin and just prints back what I write So let’s load it to IDA I also used HexRays decompiler to get the pseudo-code of the functions For the main function we have

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44

__int64 __fastcall main(__int64 a1, char **a2, char **a3)
{
  _QWORD *v3; // rax
  char v4; // bl
  __int64 v5; // rax
  char v7; // [rsp+Fh] [rbp-A1h]
  char v8; // [rsp+10h] [rbp-A0h]
  char v9; // [rsp+30h] [rbp-80h]
  char v10; // [rsp+50h] [rbp-60h]
  char v11; // [rsp+70h] [rbp-40h]
  unsigned __int64 v12; // [rsp+98h] [rbp-18h]

  v12 = __readfsqword(0x28u);
  std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(&v8, a2, a3);
  sub_130A();
  while ( 1 )
  {
    v3 = std::operator>><char,std::char_traits<char>,std::allocator<char>>(&std::cin, &v8);
    if ( !std::basic_ios<char,std::char_traits<char>>::operator bool(v3 + *(*v3 - 24LL)) )
      break;
    std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(&v9, &v8);
    sub_13DB(&v10, &v9);
    v4 = 0;
    if ( sub_1A6C(&v10, &unk_2046A0) )
    {
      std::allocator<char>::allocator(&v7);
      v4 = 1;
      std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(&v11, "correct", &v7);
    }
    else
    {
      std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(&v11, &v8);
    }
    v5 = std::operator<<<char,std::char_traits<char>,std::allocator<char>>(&std::cout, &v11);
    std::ostream::operator<<(v5, &std::endl<char,std::char_traits<char>>);
    std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string(&v11);
    if ( v4 )
      std::allocator<char>::~allocator(&v7);
    std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string(&v10);
    std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string(&v9);
  }
  std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string(&v8);
  return 0LL;
}

Here we have a C++ program, so we have other functions to allocate memory and copy data For now we can understand

1

std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string

to be a data copy mechanism that copies the data from the second parameter to the first one And

1

std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string

to be a memory free mechanism First let me explain the code line by line

  1. First it defines the different used variables and their types
  2. Assigns v12 to be a 8-byte (qword) address at offset 0x28 from the register segment FS (not important for us)
  3. a2 and a3 are the parameters of the main function so this should be a default operation (not important for us)
  4. Executes the function sub_130A
  5. Initiates an infinity loop (can be broken from inside)
  6. Copies the address of pointer v8 to the address of the stdin (now any input data will be at pointer v8)
  7. Checks if the previous operation returned properly if not it will break (not important for us)
  8. Copies the address of v8 to the address of v9 (now any input data will be at pointer v9)
  9. Executes sub_13DB with two pointers v10 and v9 (our input data)
  10. Assigns v4 to be 0
  11. Executes sub_1A6C with two pointer &v10 and unknown pointer at 0x2046A0 and check for the return
  12. If returned a non-zero value it allocates some memory and gives its address to pointer v7 and assigns v4 to 1
  13. Copies the string correct to memory of v11
  14. If returned zero, it copies data from v8 (our input data) to pointer v11
  15. Copies address of v11 (correct or our input data) to be the address of stdout (prints &v11)
  16. Prints new line
  17. Free memory of v11
  18. Free memory of v7 if v4 is non-zero (or when the previous condition is true)
  19. Free memory of v9, v10, v8, and return

So simply it will read our input, make some check on it, if it passed it will print ‘correct’ if not it will print our input For the checking function sub_1A6C we have

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

_BOOL8 __fastcall sub_1A6C(__int64 a1, __int64 a2)
{
  __int64 v2; // rbx
  __int64 v3; // r12
  __int64 v4; // rbx
  __int64 v5; // rax
  _BOOL8 result; // rax

  v2 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(a1);
  result = 0;
  if ( v2 == std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(a2) )
  {
    v3 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(a1);
    v4 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::data(a2);
    v5 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::data(a1);
    if ( !sub_18BF(v5, v4, v3) )
      result = 1;
  }
  return result;
}

And for sub_18BF

1
2
3
4
5
6
7
8
9
10

int __fastcall sub_18BF(const void *a1, const void *a2, size_t a3)
{
  int result; // eax

  if ( a3 )
    result = memcmp(a1, a2, a3);
  else
    result = 0;
  return result;
}

Which seems to be a simple comparison function that makes sure the data and the size of the two pointers are the same So now we need to know the data at the pointer unk_2046A0, but when I tried to dump it it was not initialised Pointer unk_2046A0 will be initialised at the runtime by some function To know where it will be filled with data in ida you can jump to its x-refernces (right click–>jump to xref) To find that it will be initialised by the function sub_178E at which we have

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

unsigned __int64 __fastcall sub_178E(int a1, int a2)
{
  char v3; // [rsp+17h] [rbp-19h]
  unsigned __int64 v4; // [rsp+18h] [rbp-18h]

  v4 = __readfsqword(0x28u);
  if ( a1 == 1 && a2 == 0xFFFF )
  {
    std::ios_base::Init::Init(&unk_204680);
    __cxa_atexit(&std::ios_base::Init::~Init, &unk_204680, &off_204008);
    std::allocator<char>::allocator(&v3);
    std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(
      &unk_2046A0,
      "IQHR}nxio_vtvk_aapbijsr_vnxwbbmm{",
      &v3);
    std::allocator<char>::~allocator(&v3);
    __cxa_atexit(
      &std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::~basic_string,
      &unk_2046A0,
      &off_204008);
    sub_19B4(&unk_204260);
    __cxa_atexit(sub_261E, &unk_204260, &off_204008);
  }
  return __readfsqword(0x28u) ^ v4;
}

From the code we know that it will copy the string IQHR}nxio_vtvk_aapbijsr_vnxwbbmm{ to our unknown pointer Now I am pretty sure that the function sub_13DB is the encryption function that takes two pointers &v10 and &v9 (our input) and it will encrypt our input and copy the result to &v10 to be checked again by sub_1A6C Now for sub_13DB we have

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51

__int64 __fastcall sub_13DB(__int64 a1, __int64 a2)
{
  int v2; // ebx
  char *v3; // rax
  signed int v4; // eax
  char *v5; // rax
  char *v6; // rax
  __int64 v7; // rbx
  int i; // [rsp+14h] [rbp-1Ch]
  int v10; // [rsp+18h] [rbp-18h]

  for ( i = 0; *std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i); ++i )
  {
    v2 = *std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i);
    v10 = v2 + *sub_1A4C(&unk_204260, i);
    v3 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i);
    if ( sub_1929(*v3) )
    {
      v4 = 122;
    }
    else
    {
      v5 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i);
      if ( sub_194C(*v5) )
        v4 = 90;
      else
        v4 = *std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i);
    }
    while ( v10 > v4 )
      v10 -= 26;
    if ( *std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i) == 123 )
    {
      LOBYTE(v7) = 125;
    }
    else if ( *std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i) == 125 )
    {
      LOBYTE(v7) = 123;
    }
    else
    {
      v6 = std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i);
      if ( sub_18FA(*v6) )
        LOBYTE(v7) = v10;
      else
        v7 = *std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i);
    }
    *std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i) = v7;
  }
  std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::basic_string(a1, a2);
  return a1;
}

You can understand :

1

std::__cxx11::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator[](a2, i)

to be just (&a2+i) which is a2[i]

For sub_1A4C, sub_1929, sub_194C, sub_18FA, they are just small checking functions

Also we have this pointer unk_204260 which is also not initialised, jump to its x-references to find out the function that will fill it This function is sub_130A which is hard to be reversed so we will debug (source code debugging) it to get the data at this pointer I set a break point at the line after the line it used in I used ida x64 remote linux debugger server on ubuntu x64 Dumped the data from it with GetManyBytes idapython api function The dumped data was

1

['\x03\x00\x00\x00\x05\x00\x00\x00\x07\x00\x00\x00\x0b\x00\x00\x00\r\x00\x00\x00\x11\x00\x00\x00\x13\x00\x00\x00\x17\x00\x00\x00\x1d\x00\x00\x00\x1f\x00\x00\x00%\x00\x00\x00)\x00\x00\x00+\x00\x00\x00/\x00\x00\x005\x00\x00\x00;\x00\x00\x00=\x00\x00\x00C\x00\x00\x00G\x00\x00\x00I\x00\x00\x00O\x00\x00\x00S\x00\x00\x00Y\x00\x00\x00a\x00\x00\x00e\x00\x00\x00g\x00\x00\x00k\x00\x00\x00m\x00\x00\x00q\x00\x00\x00\x7f\x00\x00\x00\x83\x00\x00\x00\x89\x00\x00\x00\x8b\x00\x00\x00\x95\x00\x00\x00\x97\x00\x00\x00\x9d\x00\x00\x00\xa3\x00\x00\x00\xa7\x00\x00\x00\xad\x00\x00\x00\xb3\x00\x00\x00\xb5\x00\x00\x00\xbf\x00\x00\x00\xc1\x00\x00\x00\xc5\x00\x00\x00\xc7\x00\x00\x00\xd3\x00\x00\x00\xdf\x00\x00\x00\xe3\x00\x00\x00\xe5\x00\x00\x00\xe9\x00\x00\x00\xef\x00\x00\x00\xf1\x00\x00\x00\xfb\x00\x00\x00']

the fact that these bytes will be casted to be int32 and every int32 is 4 bytes, also we know that the bytes are in little-endian format so a bytes array like '\x03\x00\x00\x00' is just 0x03 and so on So for now we have our key array to be

1

[0x03,0x05,0x07,0x0b,0x0d,0x11,0x13,0x17,0x1D,0x1F,0x25,0x29,0x2B,0x2F,0x35,0x3B,0x3D,0x43,0x47,0x49,0x4F,0x53,0x59,0x61,0x65,0x67,0x6B,0x6D,0x71,0x7F,0x83,0x89,0x8B,0x95,0x97,0x9D,0x0A3,0xA7,0xAD,0xB3,0xB5,0xbf,0xc1,0xc5,0xc7,0xd3,0xdf,0xe3,0xe5,0xe9,0xef,0xf1,0xfb]

So for this encryption function we have the output which is IQHR}nxio_vtvk_aapbijsr_vnxwbbmm{ and the key This is a kind of a rotation encryption function so I assumed that I will got the right flag if just passed the output as input again and so on to get the write flag Also I rewrited it in python so as we can decrypt our string I made this script

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35

key = [0x03,0x05,0x07,0x0b,0x0d,0x11,0x13,0x17,0x1D,0x1F,0x25,0x29,0x2B,0x2F,0x35,0x3B,0x3D,0x43,0x47,0x49,0x4F,0x53,0x59,0x61,0x65,0x67,0x6B,0x6D,0x71,0x7F,0x83,0x89,0x8B,0x95,0x97,0x9D,0x0A3,0xA7,0xAD,0xB3,0xB5,0xbf,0xc1,0xc5,0xc7,0xd3,0xdf,0xe3,0xe5,0xe9,0xef,0xf1,0xfb]
def enc(inp):
    inp = list(inp)
    for i in range(len(inp)):
        current_char = ord(inp[i])
        v17 = ord(inp[i]) + key[i]
        if current_char > 96 and current_char <= 122:
            v6 = 122
        else:
            if (current_char > 96 and current_char <= 122 or current_char > 64 and current_char <= 90) and not (current_char > 96 and current_char <= 122):
                v6 = 90
            else:
                v6 = current_char
        while v17 > v6: v17 -= 26
        if current_char == 123:
            v12 = 125
        else:
            if current_char == 125:
                v12 = 123
            else:
                if current_char > 96 and current_char <= 122 or current_char > 64 and current_char <= 90:
                    v12 = v17
                else:
                    v12 = current_char
        inp[i] = chr(v12)
    return ''.join(inp)


flag="IQHR}nxio_vtvk_aapbijsr_vnxwbbmm{"
while True:
    if "FLAG" in flag:
        print(flag)
        break
    flag = enc(flag)

` Flag is Captured ` » {well_keep_training_yourself}

Reverse Engineering, CT-CTF
linux binary CT-Reverse Engineering C++
This post is licensed under CC BY 4.0 by the author.