HIP7CTF_Writeups/twisted.md

# Twisted

`twisted` is a reverse engineering challenge where we must recover a flag from a provided binary and an encrypted output string.

## Information Gathering

We start by analyzing the file type of the `twisted` binary:

```bash
$ file twisted
twisted: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, ... stripped
```

The binary is stripped, meaning it lacks debugging symbols like function names. Connecting to the challenge server gives us the encrypted flag:

```
Here is your twisted flag: 34d133c640536c58ffcebb864a836aaf3bc432c3606b331df2d981a472bd6e80
```

## Reverse Engineering

We open the binary in Ghidra to analyze its logic. Since the binary is stripped, we first locate the entry point (`entry`), which calls `__libc_start_main`. The first argument to `__libc_start_main` is the address of the `main` function. Following this path leads us to the function at `0x40190a`, which we rename to `main`.

### Main Function (`0x40190a`)

The decompiled code for `main` reveals the expected arguments and basic validation:

```c
undefined8 main(int param_1,long param_2)

{
  size_t sVar1;
  
  if (param_1 < 2) {
    printf("Usage: %s <flag>\n",*(undefined8 *)(param_2 + 8)); // Usage string at 0x49e081
    return 1;
  }
  sVar1 = strlen(*(char **)(param_2 + 8));
  if (sVar1 == 32) {
    FUN_004017b5(*(undefined8 *)(param_2 + 8));
    return 0;
  }
  printf("Error: Flag must be exactly %d characters long.\n",32); // Error string at 0x49e098
  return 1;
}
```

From this, we learn that the input flag must be exactly **32 characters** long. If the length is correct, it calls `FUN_004017b5`.

### Transformation Function (`0x4017b5`)

We analyze `FUN_004017b5`, which contains the core encryption logic. It performs two distinct operations on the input string.

```c
void FUN_004017b5(long param_1)

{
  long lVar1;
  int local_84; // Counter for Loop 1
  int local_80; // Counter for Loop 2
  int local_7c; // Counter for Loop 3
  byte local_70 [32]; // Shuffled Buffer
  byte local_50 [32]; // Final XOR Buffer
  byte local_30 [32]; // Input Copy
  
  // ... Setup and copying input to local_30 ...

  // --- STEP 1: Permutation ---
  local_84 = 0;
  while (local_84 < 32) {
    // Load byte from Permutation Table at 0x49e020
    // Use it as an index into the input string
    local_70[local_84] = local_30[(int)(uint)(byte)(&DAT_0049e020)[local_84]];
    local_84 = local_84 + 1;
  }

  // --- STEP 2: XOR Encryption ---
  local_80 = 0;
  while (local_80 < 32) {
    // XOR the shuffled byte with a Key byte from 0x49e040
    local_50[local_80] = local_70[local_80] ^ (&DAT_0049e040)[local_80];
    local_80 = local_80 + 1;
  }

  // --- Print Result ---
  printf("Here is your twisted flag: "); // String at 0x49e060
  local_7c = 0;
  while (local_7c < 32) {
    printf("%02x",(ulong)local_50[local_7c]);
    local_7c = local_7c + 1;
  }
  // ...
}
```

The algorithm is:
1.  **Permutation**: Use the array at `0x49e020` to reorder the input characters.
    `shuffled[i] = input[PERM[i]]`
2.  **XOR**: XOR the reordered characters with the array at `0x49e040`.
    `encrypted[i] = shuffled[i] ^ KEY[i]`

### Data Extraction

We inspect the memory at the identified addresses to retrieve the Permutation Table and XOR Key.

**Permutation Table (`0x49e020`):**
Values: `3, 0, 1, 2, 7, 4, 5, 6, 10, 11, 8, 9, 15, 12, 13, 14, 19, 16, 17, 18, 22, 23, 20, 21, 25, 26, 27, 24, 31, 28, 29, 30`

**XOR Key (`0x49e040`):**
Values (Hex): `55, AA, 55, AA, 12, 34, 56, 78, 9A, BC, DE, F0, 0F, F0, 0F, F0, 55, AA, 55, AA, 12, 34, 56, 78, 9A, BC, DE, F0, 0F, F0, 0F, F0`

## Solution

To decrypt the flag `34d133c6...`, we reverse the operations:
1.  **Reverse XOR**: `shuffled[i] = encrypted[i] ^ KEY[i]`
2.  **Reverse Permutation**: `input[PERM[i]] = shuffled[i]`

### Solver Script

```python
import sys

# Extracted from 0x49e020
PERM = [
    3, 0, 1, 2, 7, 4, 5, 6, 
    10, 11, 8, 9, 15, 12, 13, 14, 
    19, 16, 17, 18, 22, 23, 20, 21, 
    25, 26, 27, 24, 31, 28, 29, 30
]

# Extracted from 0x49e040
KEY = [
    0x55, 0xAA, 0x55, 0xAA, 0x12, 0x34, 0x56, 0x78,
    0x9A, 0xBC, 0xDE, 0xF0, 0x0F, 0xF0, 0x0F, 0xF0,
    0x55, 0xAA, 0x55, 0xAA, 0x12, 0x34, 0x56, 0x78,
    0x9A, 0xBC, 0xDE, 0xF0, 0x0F, 0xF0, 0x0F, 0xF0
]

def solve(hex_string):
    encrypted_bytes = bytes.fromhex(hex_string)
    
    if len(encrypted_bytes) != 32:
        print("Error: Length mismatch")
        return

    # 1. Reverse XOR
    shuffled = [0] * 32
    for i in range(32):
        shuffled[i] = encrypted_bytes[i] ^ KEY[i]

    # 2. Reverse Permutation
    original = [0] * 32
    for i in range(32):
        target_idx = PERM[i]
        original[target_idx] = shuffled[i]

    print("Flag: " + "".join(chr(b) for b in original))

if __name__ == "__main__":
    solve("34d133c640536c58ffcebb864a836aaf3bc432c3606b331df2d981a472bd6e80")
```

Running the script gives us the flag:
`{flag: Reverse_Engineer_The_Map}`