Abraxus's Blog

Bamboo Fox Better Than Assembly Write Up

Details:

Points: 500

Jeopardy style CTF

Category: Reversing

Write up:

This challenge yet again had very little detail, only providing the .ll file and the title "Better than ASM". Not knowing what type of file .ll was my first step was using file, this however was not the most useful as I got the following:

$ file task.ll 
task.ll: ASCII text

Simply running the file also did not seem to work so I turned to Google, a simple search for .ll file however was not the most useful since the only information that turned up was that it was a List and Label preview file for Combit List & Label, this seemed highly unlikely so I searched .ll file assembly since the title referenced assembly and the contents of the file looked similar to assembly. This lead to results coming up for LLVM Static Compiler, finally something useful.

After some more research I had two paths to choose from, converting the file to assembly since I was more comfortable with that or compiling the file so I had something to run. I chose to compile it since I was planning on throwing it into IDA anyways.

After some research I found out that I could compile the file using clang with the following command:

clang task.ll -mllvm -W -g -W1,-pie -o task.out

I then ran the file which resulted in the following output:

$ ./task.out               
Only the chosen one will know what the flag is!
Are you the chosen one?
flag:

Entering a random value caused the program to print out a rather mean message and exit:

flag: t

😠😡😠😡😠😡 You are not the chosen one! 😡😠😡😠😡😠

At this point I threw the compiled program into IDA so that I could see psuedocode for the program and debug through it.

The psuedocode produced by ida for the main function was:

int __cdecl main(int argc, const char **argv, const char **envp)
{
  char v4; // [rsp+14h] [rbp-94h]
  char v5; // [rsp+34h] [rbp-74h]
  size_t v6; // [rsp+40h] [rbp-68h]
  int j; // [rsp+58h] [rbp-50h]
  int i; // [rsp+5Ch] [rbp-4Ch]
  char s[68]; // [rsp+60h] [rbp-48h] BYREF
  int v10; // [rsp+A4h] [rbp-4h]

  v10 = 0;
  printf("Only the chosen one will know what the flag is!\n");
  printf("Are you the chosen one?\n");
  printf("flag: ");
  __isoc99_scanf("%64s", s);
  v6 = strlen(s);
  if ( v6 == strlen(&what) )
  {
    if ( (unsigned int)check(s) )
    {
      for ( i = 0; i < strlen(s); ++i )
      {
        v5 = s[i];
        s[i] = secret[i % strlen(secret)] ^ v5;
      }
    }
    else
    {
      for ( j = 0; j < strlen(s); ++j )
      {
        v4 = flag[j];
        s[j] = secret[j % strlen(secret)] ^ v4;
      }
    }
    printf(format, s);
    v10 = 0;
  }
  else
  {
    printf(asc_40205A);
    v10 = 1;
  }
  return v10;
}

As you can see this matches with what the program outputted when it was run. The program reads in a string (max 64 chars), if it is the same length as what it then checks the string and if it passes it xor's the string with secret, if not it xor's some string (flag) with secret.

I then went through and got the values for the different variables:

what = b"\x17/'\x17\x1DJy\x03,\x11\x1E&\x0AexjONacA-&\x01LANH'.&\x12>#'Z\x0FO\x0B%:(&HI\x0CJylL'\x1EmtdC\x00\x00\x00\x00\x00\x00\x00\x00"

secret = b'B\x0A|_\x22\x06\x1Bg7#\x5CF\x0A)\x090Q8_{Y\x13\x18\x0DP\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

flag = b'\x1DU#hJ7.8\x06\x16\x03rUO=[bg9JmtGt`7U\x0BnNjD\x01\x03\x120\x19;OVIaM\x00\x08,qu<g\x1D;K\x00}Y\x00\x00\x00\x00\x00\x00\x00\x00'

format = b'\x0A\xF0\x9F\x98\x82\xF0\x9F\x91\x8C\xF0\x9F\x98\x82\xF0\x9F\x91\x8C\xF0\x9F\x98\x82\xF0\x9F\x91\x8C flag{%s} \xF0\x9F\x91\x8C\xF0\x9F\x98\x82\xF0\x9F\x91\x8C\xF0\x9F\x98\x82\xF0\x9F\x91\x8C\xF0\x9F\x98\x82\x0A\x0A\x00\x00\x00'

asc_40205A = b'\x0A\xF0\x9F\x98\xA0\xF0\x9F\x98\xA1\xF0\x9F\x98\xA0\xF0\x9F\x98\xA1\xF0\x9F\x98\xA0\xF0\x9F\x98\xA1 You are not the chosen one! \xF0\x9F\x98\xA1\xF0\x9F\x98\xA0\xF0\x9F\x98\xA1\xF0\x9F\x98\xA0\xF0\x9F\x98\xA1\xF0\x9F\x98\xA0\x0A\x0A\x00'

asc_40205A was obviously the string that was printed when I put in a random value, however I noticed that the format string included the flag component of the flag meaning that I would not simply be able to solve the flag by assuming the first 5 characters and the last character since the wrapper was added later.

Now that I had all the values I was also able to test the different components, my first step was the see what the length of what was, this ended up being 56 meaning the code we would need to put in needed to be that long, the next thing I wanted to figure out was what would be printed if check returned false. To test that I first wrote some c++ code and then remembered that c++ is not always the most fun for quick scripting so I wrote the following python code instead:

newStr = ""
for i in range(0, 56):
    newStr += chr(secret[i % len(secret)] ^ flag[i])
print(newStr)

Once ran this code output the following value:

___7h15_15_4_f4k3_f14g_y0u_w1ll_f41l_1f_y0u_subm17_17___

I hadn't expected this to work so this allowed me to confirm that the actual flag depended on the correct string being xor'd with secret leading me to look into the check function.

__int64 __fastcall check(__int64 a1)
{
  int v2; // [rsp+1Ch] [rbp-1Ch]
  int i; // [rsp+28h] [rbp-10h]
  unsigned int v4; // [rsp+2Ch] [rbp-Ch]

  v4 = 1;
  for ( i = 0; i < strlen(what); ++i )
  {
    v2 = *(char *)(a1 + i);
    v4 = (unsigned __int8)v4 & ((*(char *)(a1 + (i + 1) % strlen(what)) ^ v2) == what[i]);
  }
  return v4;
}

This code goes through what and checks to see if each ith character in what is equal to the ith character in the input string xor'd with the ith + 1 character in the input string with the mod allowing the string to wrap back around to the start.

I sadly went down a rabbithole here for a bit of trying to brute force combinations since I realized that if I was able to solve for one character I would be able to solve for the entire key. Then I realized I could just generate the 90 (127 - 30, writable characters) keys possible since the rest of the key would be based on the first character.

From here I created a script that iterated through the possible writeable characters and then went through each value in what and appended the ith value xor'd with the ith value in what to generate the ith + 1 value since A ^ B = C &#x3C;==> A ^ C = B. The script then went through and xor'd each ith value with the ith value of secret to generate a list of potential flags. I also went ahead and removed all of the end of line characters from the strings.

what = b"\x17/'\x17\x1DJy\x03,\x11\x1E&\x0AexjONacA-&\x01LANH'.&\x12>#'Z\x0FO\x0B%:(&HI\x0CJylL'\x1EmtdC\x00\x00\x00\x00\x00\x00\x00\x00"
secret = b'B\x0A|_\x22\x06\x1Bg7#\x5CF\x0A)\x090Q8_{Y\x13\x18\x0DP\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

what = what.rstrip(b'\0')
secret = secret.rstrip(b'\0')

for i in range(30, 127):
    flag = []
    flag.append(i)

    for j in range(len(what)):
        flag.append(flag[j]^what[j])

    newFlag = ""
    for j in range(len(what)):
        newFlag += chr(flag[j]^secret[j%len(secret)])

    print(newFlag)

This create a list of potential flags in which I spotted:

7h15_f14g_15_v3ry_v3ry_l0ng_4nd_1_h0p3_th3r3_4r3_n0_7yp0

However having the script print out so much junk was annoying me a bit so I went and added a sanity check to the print, only printing values that were between 30 and 127 on the ascii table:

what = b"\x17/'\x17\x1DJy\x03,\x11\x1E&\x0AexjONacA-&\x01LANH'.&\x12>#'Z\x0FO\x0B%:(&HI\x0CJylL'\x1EmtdC\x00\x00\x00\x00\x00\x00\x00\x00"
secret = b'B\x0A|_\x22\x06\x1Bg7#\x5CF\x0A)\x090Q8_{Y\x13\x18\x0DP\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

what = what.rstrip(b'\0')
secret = secret.rstrip(b'\0')

for i in range(30, 127):
    flag = []
    flag.append(i)

    for j in range(len(what)):
        flag.append(flag[j]^what[j])

    newFlag = ""
    for j in range(len(what)):
        char = chr(flag[j]^secret[j%len(secret)])
        if ord(char) not in range(30, 127):
            continue
        else:
            newFlag += char

    print(newFlag)