ctf-writeups

yourock

Co-authors: @Rosayxy

Forget Enigma, forget Caesar. The only cipher you need is 2009's infamous password dump.

Attachment contains a encoded text:

charlie junior babygirl soccer qwerty 111111 000000 tigger jessica jasmine joseph 12345 tigger chelsea melissa 123123 mickey computer anthony chelsea brandon brandon matthew soccer bubbles playboy spongebob eminem ashley password ashley

Decompiled encode program via IDA by @Rosayxy:

// a1: result string, printed to encoded.rj
// a2: input string, contains flag
// a3: vector of strings, read from rockyou.txt
// a4: a random number
__int64 __fastcall encode(__int64 a1, __int64 a2, __int64 a3, unsigned __int8 a4)
{
  __int64 v4; // rax
  __int64 v5; // rax
  unsigned __int8 v9; // [rsp+27h] [rbp-19h]
  unsigned __int64 i; // [rsp+28h] [rbp-18h]

  std::vector<std::string>::vector(a1);
  v4 = std::vector<std::string>::operator[](a3, a4);
  std::vector<std::string>::push_back(a1, v4);
  for ( i = 0LL; i < std::string::size(a2); ++i )
  {
    v9 = a4 ^ *(_BYTE *)std::string::operator[](a2, i);
    if ( v9 >= (unsigned __int64)std::vector<std::string>::size(a3) )
      exit(1);
    v5 = std::vector<std::string>::operator[](a3, v9);
    std::vector<std::string>::push_back(a1, v5);
    a4 ^= i ^ v9;
  }
  return a1;
}

Analysis

The encoding algorithm works as follows:

1. Start with a random index a4 (the key) into the rockyou.txt word list
2. Append the word at index a4 to the output
3. For each character in the input flag:
   • Compute v9 = a4 XOR flag_char
   • Append the word at index v9 to the output
   • Update a4 = a4 XOR i XOR v9 where i is the character index

Given the encoded output, we can reverse this process:

1. The first word “charlie” gives us the initial key a4 (its index in rockyou.txt)
2. For each subsequent word, we find its index in rockyou.txt
3. Compute flag_char = index XOR key
4. Update key = key XOR index XOR i for the next iteration

We need a copy of rockyou.txt (the famous password list from 2009).

# from encoded.rj
encoded = "charlie junior babygirl soccer qwerty 111111 000000 tigger jessica jasmine joseph 12345 tigger chelsea melissa 123123 mickey computer anthony chelsea brandon brandon matthew soccer bubbles playboy spongebob eminem ashley password ashley"
# rockyou.txt from web
words = open("rockyou.txt", "r", encoding='latin-1').readlines()
key = words.index("charlie\n")
i = 0
res = ""
for part in encoded.split()[1:]:
    index = words.index(part + "\n")
    ch = index ^ key
    res += chr(ch)
    key = key ^ index ^ i
    i += 1
print(res)

Get flag: corctf{r0cky0u_3nc0d1ng_r0cks}.