CDDC 2025

Reassemble The Pieces

Network | Qualifier

Extract H.264 video payloads from RTP packets in a .pcap file using rtp_h264_extractor.lua.

Extracted H.264 Video

└─$CDDC2025{5tr3@m1ng_53rv1ce35_u53_th3_RT5P_pr0t0c01}

Break the ECDSA

Cryptography | Qualifier

The overall solution is:

1. Using given signatures to compute the private key using ECDSA Nonce Reuse Attack

2. Brute-force candidate 12-word BIP-39 mnemonics from the given dictionary, derive their corresponding private keys using bip32utils and compare them against the recovered key

from ecdsa.numbertheory import inverse_mod
from itertools import product
import mnemonic
import bip32utils

# Provided constants
r  = 81210355722750344493541519494641458710145722871994877785183554697310523407018
h1 = 45643200378651069483892104393394606812504455659831083323743202489147422538955
h2 = 74831345439009646272332597737070016777412939113737083148228963710487431876647
s1 = 110764343964105699917226529930289538481215574456544978805357332521308340464732
s2 = 90138993253633063487274662700800979929978777245182171200537527514756442604713

# SECP256k1 curve order
N = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141

# Step 1: Recover the private key
numerator = (s2 * h1 - s1 * h2) % N
denominator = inverse_mod(r * ((s1 - s2) % N), N)
private_key = hex((numerator * denominator) % N)

print(f"[+] Recovered Private Key:\n{private_key}")
# 0x9f9068a0cc25f39b9c5fba5bb88d75bc5e4503a8406101a3195dc395194ea690

# Step 2: Brute-force message
words = [
    "v1ew", "5tr0ng", "app1e", "fr13nd", "7hank5", "sch00l", "c0d3", "kn0w1edg3", "g00d",
    "b4d", "dr34m", "r1v3r", "mou7a1n", "f0r3st", "5kyb1u3", "p14n3t", "c4r", "b1k3", "b00k",
    "5tor3", "l4ugh", "sm1l3", "v1ct0ry", "p0w3r", "3n3rgy", "7r4v3l", "p3ac3", "f1gh7", "h0p3",
    "4n7", "1i0nkin9", "7ig3r", "p4d0c0in", "0c34n", "w4v3", "5h0r3", "c4v3", "m00n", "5un",
    "5tar", "7r33", "fl0w3r", "5e3d", "gr455", "b3", "4ir", "c10ud", "r41n", "5n0w", "5t0rm"
]

template = "BIP-39 SECP256k1 {} 5eed r4nd0m {} g00d 5olve c0ffe {} pe4nut 5mart"

print("[*] Brute-forcing possible message...")

m = mnemonic.Mnemonic('english')
found = False
for w1, w2, w3 in product(words, repeat=3):
    msg = template.format(w1, w2, w3)
    seed = m.to_seed(msg, passphrase="")
    h = bip32utils.BIP32Key.fromEntropy(seed).PrivateKey().hex()
    if h == private_key[2:]:
        print(w1, w2, w3)
        found = True
        break

if not found:
    print("[-] No match found.")

└─$CDDC2025{p4d0c0in_b3_5tr0ng_(private key in hex)}

DR0N3CAM

Network | Pre-CTF

Using a certain protocol, a drone was able to stream images through the network.

This challenge revolves around the Real-Time Streaming Protocol (RTSP), a network protocol commonly used to manage real-time streaming media applications and our goal is to analyze captured drone data and extract the hidden flag.

Given Files:

• aaaaaa

• abcdefg-01.cap

• DRONE.apk

The DRONE.apk file appeared to be the drone controller application responsible for streaming video from the drone. Running it on an Android emulator confirmed its role in managing the live video output.

DRONE.apk

The file aaaaaa was identified as a sample of streamed data containing video frames. Running binwalk on this file revealed multiple embedded JPEG images, which represent the individual video frames streamed from the drone.

└─$ binwalk aaaaaa

DECIMAL       HEXADECIMAL     DESCRIPTION
--------------------------------------------------------------------------------
20            0x14            JPEG image data, JFIF standard 1.01
51            0x33            TIFF image data, big-endian, ...
5684          0x1634          JPEG image data, JFIF standard 1.01
5715          0x1653          TIFF image data, big-endian, ...
...

Next, I analyzed the network capture file abcdefg-01.cap. Inspecting the packets revealed a protocol labeled "data" carrying payloads of interest. Extracting the data fields from these packets yielded several binary chunks.

Using binwalk on these extracted binaries confirmed that they contained image frames matching those captured during the streaming session.

By reviewing these extracted images, I successfully found the flag hidden within the video frames.

some_random_frame.jpeg

└─$ CDDC2025{YOU_JUST_HACKED_CAM_DRONE_GOOD}