write-up: ESP/ESP_Start/README.md

ESP/ESP_Start/README.md

@@ -1,3 +1,131 @@
-# test
+# ESP Start
 
-hello world
+| Field | Value |
+|-------|-------|
+| Category | ESP |
+| Difficulty | Easy |
+| Points | 50 |
+| Author | Eun0us |
+| CTF | Espilon 2026 |
+
+---
+
+## Description
+
+Welcome to the ESP category.
+
+Flash this firmware onto an ESP32 and connect to the UART console at `115200` baud.
+
+The device outputs something on boot. Decode it.
+
+Format: **ESPILON{flag}**
+
+---
+
+## TL;DR
+
+Flash the provided firmware, read the XOR-encrypted flag and its key from the UART
+boot output, apply XOR, get the flag.
+
+---
+
+## Tools
+
+| Tool | Purpose |
+|------|---------|
+| `esptool.py` | Flash firmware to ESP32 |
+| `screen` / `minicom` | Read UART serial output |
+| Python 3 | XOR decryption |
+
+---
+
+## Solution
+
+### Step 1 — Flash the firmware
+
+Install esptool and flash all three binary files at their correct offsets:
+
+```bash
+pip install esptool
+
+esptool.py --chip esp32 --port /dev/ttyUSB0 --baud 460800 write_flash -z \
+    0x1000  bootloader.bin \
+    0x8000  partition-table.bin \
+    0x10000 hello-espilon.bin
+```
+
+On Linux, add your user to the `dialout` group first if you get a permission error:
+
+```bash
+sudo usermod -a -G dialout $USER
+```
+
+> 📸 `[screenshot: esptool.py flashing — progress bar reaching 100%]`
+
+### Step 2 — Connect to the UART console
+
+Open a serial terminal at 115200 baud:
+
+```bash
+screen /dev/ttyUSB0 115200
+# or
+minicom -D /dev/ttyUSB0 -b 115200
+```
+
+Press the RESET button on the ESP32. The boot output appears:
+
+```text
+=== Hello ESP ===
+System ready.
+
+Encrypted flag: 09 12 19 07 00 0E 07 35 3F 35 7D 3C 38 1E 3D 26 7F 1E 3E 7F 3E 72 34
+XOR Key: 4C 41 49 4E
+```
+
+> 📸 `[screenshot: serial terminal showing the encrypted flag and XOR key on boot]`
+
+### Step 3 — Identify the XOR key
+
+Convert the key bytes `4C 41 49 4E` to ASCII:
+
+```
+0x4C = 'L'
+0x41 = 'A'
+0x49 = 'I'
+0x4E = 'N'
+```
+
+Key = `LAIN` (the protagonist of the series).
+
+### Step 4 — Decrypt the flag
+
+Apply the key cyclically to each byte of the encrypted output:
+
+```python
+enc = bytes([
+    0x09, 0x12, 0x19, 0x07, 0x00, 0x0E, 0x07, 0x35,
+    0x3F, 0x35, 0x7D, 0x3C, 0x38, 0x1E, 0x3D, 0x26,
+    0x7F, 0x1E, 0x3E, 0x7F, 0x3E, 0x72, 0x34
+])
+key = b"LAIN"
+flag = bytes(b ^ key[i % len(key)] for i, b in enumerate(enc))
+print(flag.decode())
+```
+
+Output: `ESPILON{st4rt_th3_w1r3}`
+
+> 📸 `[screenshot: Python decryption script running and printing the flag]`
+
+### Key concepts
+
+- **ESP32 flashing**: `esptool.py` writes bootloader, partition table, and application binary at
+  their respective flash offsets (`0x1000`, `0x8000`, `0x10000`).
+- **UART monitoring**: ESP32 default baud rate is 115200 baud, 8N1, no parity.
+- **XOR cipher**: The key `LAIN` is broadcast in plaintext — trivial for an intro challenge;
+  the real-world lesson is that hardcoded keys in `.rodata` are easily extracted with `strings`.
+
+---
+
+## Flag
+
+`ESPILON{st4rt_th3_w1r3}`