undo

Undoing obfuscated functions in Ghidra.

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undo.zip

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This binary encodes our input data and is compared against a buffer. We can use Ghidra to determine how the flag is encrypted and reverse the encryption to get the flag.

This will be a good introduction to reversing obfuscated functions in Ghidra.

Running checksec on the binary, we notice all security features are enabled:

$ checksec undo
[*] '/home/joybuzzer/Documents/vunrotc/public/reverse-engineering/11-ghidra/undo/src/undo'
    Arch:     i386-32-little
    RELRO:    Full RELRO
    Stack:    Canary found
    NX:       NX enabled
    PIE:      PIE enabled

Static Analysis

Let's check the raw decompilation of main():

void main(undefined param_1)
{
  int in_GS_OFFSET;
  int local_44;
  char local_3d [41];
  int local_14;
  undefined1 *local_10;
  
  local_10 = &param_1;
  local_14 = *(int *)(in_GS_OFFSET + 0x14);
  puts("Enter your password");
  fflush(_stdout);
  fgets(local_3d,0x29,_stdin);
  printf("You entered: %s\n",local_3d);
  fflush(_stdout);
  encode((int)local_3d);
  local_44 = 0;
  do {
    if (0x28 < local_44) {
      win();
LAB_00011382:
      if (local_14 != *(int *)(in_GS_OFFSET + 0x14)) {
        __stack_chk_fail_local();
      }
      return;
    }
    if (enc_key[local_44] != buffer[local_44]) {
      puts("You lose!");
      goto LAB_00011382;
    }
    local_44 = local_44 + 1;
  } while( true );
}

This code is a mess. There is a label in the middle of the function, a do-while loop, and many wonky variables. Let's discuss how we can clean this up:

• First, we'll rename variables to something more meaningful. This immediately makes the code easier to read.

• Second, we'll remove the label by moving the code after the label to the jump.

• Third, we can remove the canary check. We know it's there, and it doesn't affect control flow.

• We notice that param_1 is never used, so we'll remove it. This changes the signature to void main(void).

• Once this is done, we'll remove variables we no longer need and variables that aren't used.

After these changes, we get the following code:

void main(void)
{
  int i;
  char buf[41];
  
  puts("Enter your password");
  fflush(stdout);
  fgets(buf, 0x29, stdin);
  printf("You entered: %s\n",buf);
  fflush(stdout);
  encode((int)buf);
  i = 0;
  do {
    if (0x28 < i) {
      win();
      return;
    }
    if (enc_key[i] != buffer[i]) {
      puts("You lose!");
      return;
    }
    i = i + 1;
  } while( true );
}

This is a lot easier to read. We can simplify this further by switching the do-while loop for a while loop where the condition is checked.

while (i <= 0x28) {
    if (enc_key[i] != buffer[i]) {
      puts("You lose!");
      return;
    }
    i = i + 1;
}
win();

Make sure you understand how we made this conversion. This is a tough thing to analyze and change. Although it's not required to rearrange the loop, it makes the code easier to read.

This means that we're checking the first 0x28 = 40 bytes of enc_key against buffer. If they match, we call win(). If they don't match, we call puts("You lose!") and return.

We have a few things to check out:

• What is enc_key?

• What is buffer?

• What does the encode() function do?

If we look at enc_key and buffer, we notice these variables are colored aqua. This means they are global variables. Local variables are colored yellow. Double-clicking on enc_key and buffer takes us to the location where they are defined.

This is the definition of enc_key:

                             enc_key                                 
        00014080                 undefine   ??
00014080                 undefined1??                      [0]
00014081                 undefined1??                      [1]
00014082                 undefined1??                      [2]
00014083                 undefined1??                      [3]
00014084                 undefined1??                      [4]
00014085                 undefined1??                      [5]
00014086                 undefined1??                      [6]
00014087                 undefined1??                      [7]
00014088                 undefined1??                      [8]
00014089                 undefined1??                      [9]
0001408a                 undefined1??                      [10]
0001408b                 undefined1??                      [11]
0001408c                 undefined1??                      [12]
0001408d                 undefined1??                      [13]
0001408e                 undefined1??                      [14]
0001408f                 undefined1??                      [15]
00014090                 undefined1??                      [16]
00014091                 undefined1??                      [17]
00014092                 undefined1??                      [18]
00014093                 undefined1??                      [19]
00014094                 undefined1??                      [20]
00014095                 undefined1??                      [21]
00014096                 undefined1??                      [22]
00014097                 undefined1??                      [23]
00014098                 undefined1??                      [24]
00014099                 undefined1??                      [25]
0001409a                 undefined1??                      [26]
0001409b                 undefined1??                      [27]
0001409c                 undefined1??                      [28]
0001409d                 undefined1??                      [29]
0001409e                 undefined1??                      [30]
0001409f                 undefined1??                      [31]
000140a0                 undefined1??                      [32]
000140a1                 undefined1??                      [33]
000140a2                 undefined1??                      [34]
000140a3                 undefined1??                      [35]
000140a4                 undefined1??                      [36]
000140a5                 undefined1??                      [37]
000140a6                 undefined1??                      [38]
000140a7                 undefined1??                      [39]

We notice that enc_key is 40 bytes big and starts at 0x14080. The data in this array is undefined.

Why?

We'll notice that the array is defined during encode(). The program has not started running, but since the variable is global, the space is allocated at compile-time.

Now, let's check buffer:

                             buffer                                     
        00014020 c7 65 2b        undefine
                 7d 47 d3 
                 fb 5f 30 
00014020 c7              undefined1C7h                     [0]
00014021 65              undefined165h                     [1]
00014022 2b              undefined12Bh                     [2]
00014023 7d              undefined17Dh                     [3]
00014024 47              undefined147h                     [4]
00014025 d3              undefined1D3h                     [5]
00014026 fb              undefined1FBh                     [6]
00014027 5f              undefined15Fh                     [7]
00014028 30              undefined130h                     [8]
00014029 80              undefined180h                     [9]
0001402a 1d              undefined11Dh                     [10]
0001402b e3              undefined1E3h                     [11]
0001402c 23              undefined123h                     [12]
0001402d 59              undefined159h                     [13]
0001402e 34              undefined134h                     [14]
0001402f db              undefined1DBh                     [15]
00014030 ab              undefined1ABh                     [16]
00014031 48              undefined148h                     [17]
00014032 ed              undefined1EDh                     [18]
00014033 93              undefined193h                     [19]
00014034 49              undefined149h                     [20]
00014035 b0              undefined1B0h                     [21]
00014036 68              undefined168h                     [22]
00014037 30              undefined130h                     [23]
00014038 9e              undefined19Eh                     [24]
00014039 8d              undefined18Dh                     [25]
0001403a 37              undefined137h                     [26]
0001403b 1c              undefined11Ch                     [27]
0001403c 7d              undefined17Dh                     [28]
0001403d 48              undefined148h                     [29]
0001403e 7b              undefined17Bh                     [30]
0001403f 6f              undefined16Fh                     [31]
00014040 34              undefined134h                     [32]
00014041 45              undefined145h                     [33]
00014042 e7              undefined1E7h                     [34]
00014043 ca              undefined1CAh                     [35]
00014044 9f              undefined19Fh                     [36]
00014045 8e              undefined18Eh                     [37]
00014046 50              undefined150h                     [38]
00014047 cb              undefined1CBh                     [39]

On the left side, we see this data is initialized. The second column of data is the value at each byte. We can get Ghidra to copy this as a Python List using Right Click -> Copy Special -> Python List. With no extra effort, here is the value of buffer:

buffer = [ 0xc7, 0x65, 0x2b, 0x7d, 0x47, 0xd3, 0xfb, 0x5f, 0x30, 0x80, 0x1d, 0xe3, 0x23, 0x59, 0x34, 0xdb, 0xab, 0x48, 0xed, 0x93, 0x49, 0xb0, 0x68, 0x30, 0x9e, 0x8d, 0x37, 0x1c, 0x7d, 0x48, 0x7b, 0x6f, 0x34, 0x45, 0xe7, 0xca, 0x9f, 0x8e, 0x50, 0xcb ]

Now that we have our data, we can look at encode():

undefined1 * encode(int param_1)
{
  int local_c;
  
  for (local_c = 0; *(char *)(param_1 + local_c) != '\0'; local_c = local_c + 1) {
    enc_key[local_c] = (*(byte *)(param_1 + local_c) ^ 0x55) + 8;
  }
  return enc_key;
}

From this, we notice three things:

• As it stands, encode() takes an int argument.

• encode() must cast the input to an int, and then cast it again inside the function.

• Ghidra doesn't know the return type.

From this, we can gather that Ghidra got the parameter type wrong. We can help Ghidra by changing the type to what we think it is. It appears that this function is casting to a char* and a byte*. A byte* is simply a signed char*. Let's change the type in Ghidra to char* and see what happens. At the same time, we know that enc_key is a char*, so we'll change the return type too.

char* encode(char *param_1)
{
  int i;
  
  for (i = 0; param_1[i] != '\0'; i = i + 1) {
    enc_key[i] = (param_1[i] ^ 0x55U) + 8;
  }
  return enc_key;
}

This is a lot better. This function takes a string, performs byte-wise operations on each byte, and stores it in enc_key. It performs the following operation:

out = (in ^ 0x55) + 8

Both these operations are undoable, meaning:

in = (out - 8) ^ 0x55

This is our key to solving this problem! If we take the ouput buffer, and perform this operation on each byte, we'll get the correct input.

Writing a Solve Script

First, we must write a decode() function that reverses the input. This will take in the list and return a list with the decoded bytes.

def decode(in_list):
    out_list = []
    for i in in_list:
        out_list.append((i - 8) ^ 0x55)
    return out_list

We'll take our buffer and decode it:

buffer = [ 0xc7, 0x65, 0x2b, 0x7d, 0x47, 0xd3, 0xfb, 0x5f, 0x30, 0x80, 0x1d, 0xe3, 0x23, 0x59, 0x34, 0xdb, 0xab, 0x48, 0xed, 0x93, 0x49, 0xb0, 0x68, 0x30, 0x9e, 0x8d, 0x37, 0x1c, 0x7d, 0x48, 0x7b, 0x6f, 0x34, 0x45, 0xe7, 0xca, 0x9f, 0x8e, 0x50, 0xcb ]
payload = decode(buffer)

Now, we need to convert this list to a string. We can do this with the bytes() function:

payload = bytes(payload)

Finally, we can send this payload to the server:

proc = remote('vunrotc.cole-ellis.com', 11200)
proc.sendline(payload)
proc.interactive()

This gives us the flag:

$ python3 exploit.py
[+] Opening connection to vunrotc.cole-ellis.com on port 11200: Done
[*] Switching to interactive mode
Enter your password
You entered: \xev j\x9e\xa6}-@\x8eN\x04y\x86\xf6\xb0\xde\x14\xfd5}\xc3\xd0zA \x15&2yh\x8a\x97\xc2\xd3\x1d\x96
flag{ghidra_is_awesome}You win! Here you go: 
[*] Got EOF while reading in interactive
$ 
[*] Interrupted
[*] Closed connection to vunrotc.cole-ellis.com port 11200