vault-door-3

Description

Solution

1. Step 1Understand the scrambling loops
   The checkPassword() method copies characters from the input into a buffer array using index arithmetic from several for-loops (forward copy, reverse copy, interleaving). Read each loop carefully and note which output index receives which input index.
   Learn more

   The scrambling in this challenge is a permutation cipher -- all the original characters are present in the buffer, just rearranged. Each for loop implements a specific reordering: copying a range forward, copying a range backward, or interleaving characters from two halves. Understanding the permutation is a matter of careful index arithmetic.

   A useful mental model: think of the scramble as a function f(i) that maps each output index to a source index. Once you know f for every index, you can construct the reverse mapping f⁻¹ and apply it to the scrambled buffer to recover the original string. This is exactly how transposition ciphers work in classical cryptography.

   In modern security contexts, this kind of analysis appears in:

   • Reverse engineering -- understanding how a proprietary protocol encodes data
   • Malware analysis -- decoding obfuscated strings that have been shuffled to evade detection
   • DRM research -- understanding how content protection schemes rearrange data
   • Fuzzing -- learning the input format by reading the validation logic
2. Step 2Reverse the index permutation in Python
   Model the scramble as a permutation of indices. Apply the same operations to a list of known positions to determine where each scrambled character originally came from. Then read the characters back in the original order.
   python3 -c " # Paste the scrambled buffer from the source here buffer = list('jU5t_a_s1mpl3_an4gr4m_4_u_xxxxxxxx') # Reverse the loop operations to recover original order print(''.join(buffer)) "Copied!
   Learn more

   The key insight for reversing this permutation in Python is to simulate the scramble on a list of indices rather than on the actual characters. If you create a list [0, 1, 2, ..., 31] and apply the exact same loop operations to it that the Java code applies to characters, you end up with a list that tells you "the character at position i in the scrambled buffer originally came from position scrambled[i] in the password." This gives you the reverse mapping for free.

   Python is ideal for this because list slicing and index manipulation are concise. The general pattern:

   • Create an index list: idx = list(range(32))
   • Apply the same swaps/copies the Java code performs
   • Read the scrambled buffer using the resulting index order

   This approach generalizes to any block cipher mode analysis, custom encoding scheme, or obfuscation layer where the transformation is deterministic and reversible. The same technique is used by cryptanalysts to recover plaintexts from transposition ciphers -- the Rail Fence cipher and columnar transposition both use index permutations that can be reversed this way.

Flag

picoCTF{...}

Array-index scrambling is an anagram cipher -- all characters are present, just rearranged. Reversing the index permutation restores the original order.