ECW | LAIN BREAKCORE A Tale of Firmware Emulation and Wasted Hours
A Note on ECW and Fair Play
I need to address something that left a bitter taste after this competition. While the technical challenges at ECW were interesting and well-designed, I was extremely disappointed to discover that several participants engaged in flag hoarding, despite this being explicitly prohibited in the competition rules.
This behavior is not just unsportsmanlike. It fundamentally undermines the spirit of CTF competitions. When people ignore clear rules for competitive advantage, it degrades the entire experience for everyone who came to compete fairly. It’s deeply disheartening to invest time and effort into a competition only to see others gain an edge through rule violations.
I find this behavior deplorable, and frankly, it’s making me reconsider my participation in future CTF events. The technical challenges are engaging, but not when the competition itself is compromised by those who can’t respect basic rules of fair play. I can’t support or enjoy participating in an environment where such conduct is present.
The Setup
So there I was, connecting to yet another CTF challenge via netcat. The UART interface greets me with “Welcome to LAIN BREAKCORE UART v2.0” and the usual “Type ‘help’ for available commands” prompt. Fair enough, let’s see what we’re working with.
The help menu showed a handful of commands: flag for getting the encrypted flag, dump_bin to grab the firmware, settings to see the XOR key, and a couple others that turned out to be mostly flavor text. The interesting stuff came from settings:
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Firmware XOR key: L41N
Key (hex): 4C 34 31 4E
Firmware is XOR-obfuscated. Unxor before reversing!
Alright, straightforward enough. The firmware is XOR’d with “L41N”. And when I ran the flag command, I got this beautiful 32-byte hex string:
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5bdd7eecdeab0b8eca2a19c9bbe919caa410a18a247355925bbc58456568143c
The good news? The flag was deterministic - same result every time I connected. That meant no randomness to deal with, which I appreciated.
Deobfuscating the Firmware
First things first, I needed to get that firmware unXOR’d. It’s a simple repeating XOR with a 4-byte key, so nothing fancy here. I threw together a quick Python script that would read the hex dump, XOR each byte with the corresponding byte from “L41N” (cycling through the key), and spit out the deobfuscated binary.
The result was a 3156-byte firmware image that immediately looked like AVR code. Those telltale 0c 94 instruction sequences at the start? Classic AVR jump instructions. We were definitely looking at embedded firmware here, probably for an ATmega microcontroller.
Into the Ghidra
I loaded the deobfuscated binary into Ghidra, told it we’re dealing with AVR8 architecture, set the base address to 0x0, and let the auto-analysis do its thing. Ghidra chewed through the binary and gave me 35 functions, various interrupt vectors, and some interesting strings scattered throughout:
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== LAIN BREAKCORE FIRMWARE ==
boot: Glitch detected!
== KEYS INJECTED ==
KEY (hex):
IV (hex):
.....W1R3D G1L1TCH....
C0RRUPTION: 0xABADF00D
That “KEY (hex):” and “IV (hex):” caught my eye immediately. This had to be AES, right? The presence of both a key and an IV pretty much screamed AES-CBC mode. I started digging through the firmware data sections looking for those crypto keys.
Around offset 0xa60, I found what looked like crypto material - 32 bytes of what could plausibly be an AES key and IV:
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0xa60: f3 00 07 01 de ad be ef 12 34 55 66 01 10 20 30
0xa70: 99 88 77 66 13 37 ba ad c0 fe 42 42 01 23 34 56
Perfect, I thought. That’s either a 32-byte AES-256 key, or more likely a 16-byte AES-128 key plus a 16-byte IV. Time to start trying combinations.
Down the Rabbit Hole
This is where things got… frustrating. I spent what I’m embarrassed to admit was about three and a half hours trying every possible combination of those bytes with every crypto algorithm I could think of.
I started with simple XOR, figuring maybe the encryption was as simple as the firmware obfuscation. I extracted every plausible 8-byte, 16-byte, and 32-byte sequence from that crypto region and XOR’d them against the encrypted flag. Nothing. I tried XOR’ing with combinations of keys, doing multi-pass XOR operations, even XOR’ing the keys together before using them. Still nothing.
Okay, fine. Time for real crypto. I tried AES-128 in CBC mode with every possible 16-byte key and 16-byte IV combination I could extract from the firmware. Then I tried ECB mode, just in case. No dice. Maybe it was AES-192? AES-256? I worked through those too, trying different chunks of the data as keys.
At some point I wondered if maybe it was a stream cipher instead. ChaCha20, RC4, even Salsa20 got their turn. I tried each one with various key sizes and nonces extracted from the firmware. The encrypted flag remained stubbornly encrypted.
Then I started getting creative - or desperate, depending on how you look at it. Maybe the key was derived from a password using PBKDF2? I tried “L41N”, “LAIN”, various strings from the firmware. I tried MD5 and SHA256 hashing different values and using those as keys. I even tried adding and subtracting the key bytes instead of XOR’ing them, thinking maybe it was some custom arithmetic operation.
Nothing worked. I had the encrypted flag, I had what looked like perfectly reasonable crypto keys in the firmware, I knew it was probably AES based on the strings, but I could not get these two pieces to connect.
The Lightbulb Moment
After hitting my head against this wall for way too long, I took a step back. Those strings in the firmware - “KEY (hex):” and “IV (hex):” - they weren’t just labels in a data structure. They looked like format strings for printf statements. What if the firmware actually prints the keys when it runs?
That would explain everything. The keys I found in the firmware data weren’t the actual AES key and IV - they were raw material that gets processed at runtime to produce the real keys. I wasn’t going to find the answer through static analysis. I needed to run this thing.
Time to break out the emulator. I installed simavr, converted my deobfuscated firmware to Intel HEX format with avr-objcopy, and fired it up:
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simavr -m atmega328p -f 16000000 firmware_decrypted.hex
And there, right in the emulator output, the firmware cheerfully printed:
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== KEYS INJECTED ==
KEY (hex):
0x13 0x37 0xBA 0xAD 0xC0 0xFE 0x42 0x42
0x01 0x23 0x34 0x56 0x78 0xAB 0xCD 0xEF
IV (hex):
0xDE 0xAD 0xBE 0xEF 0x12 0x34 0x55 0x66
0x01 0x10 0x20 0x30 0x99 0x88 0x77 0x66
There they were. The actual runtime-computed AES key and IV. The firmware had been combining and manipulating those static bytes all along to produce these values dynamically.
The Easy Part
With the real keys in hand, decryption was trivial:
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from Crypto.Cipher import AES
from Crypto.Util.Padding import unpad
encrypted_flag = bytes.fromhex('5bdd7eecdeab0b8eca2a19c9bbe919caa410a18a247355925bbc58456568143c')
key = bytes([0x13, 0x37, 0xBA, 0xAD, 0xC0, 0xFE, 0x42, 0x42,
0x01, 0x23, 0x34, 0x56, 0x78, 0xAB, 0xCD, 0xEF])
iv = bytes([0xDE, 0xAD, 0xBE, 0xEF, 0x12, 0x34, 0x55, 0x66,
0x01, 0x10, 0x20, 0x30, 0x99, 0x88, 0x77, 0x66])
cipher = AES.new(key, AES.MODE_CBC, iv)
decrypted = cipher.decrypt(encrypted_flag)
flag = unpad(decrypted, AES.block_size).decode('utf-8')
print(flag) # ECW{LAIN_Br34k_CryPT0}
Done. Challenge solved.
Lessons from the Trenches
Looking back, this challenge taught me something important about the difference between static and dynamic analysis. I got tunnel vision looking at the firmware data, trying to make those static bytes work directly as crypto keys. The challenge author did a nice job of making them look plausible enough that you’d waste time on them.
The real trick was recognizing that firmware doesn’t just sit there - it runs. Those format strings were a hint I should have picked up on earlier, but I was so focused on the static analysis that I missed it. Once I actually emulated the firmware and watched what it did at runtime, the answer was right there waiting for me.
I also learned not to over-complicate things. After trying fifty different exotic crypto combinations, it turned out to be plain old AES-128-CBC all along. Sometimes the simplest answer really is the right one - you just need to look at it from the right angle.
Would I have solved this faster if I’d jumped straight to emulation? Absolutely. But that’s CTFs for you. Sometimes you learn the most from the paths that don’t work out. And hey, I can now say with confidence that I’ve tried pretty much every crypto algorithm known to man against that flag.
The whole thing took me about four hours from start to finish. Three and a half of those were spent on approaches that didn’t work. The last thirty minutes, after I finally thought to use simavr, were smooth sailing. That ratio probably says something about the importance of choosing the right tool for the job.
Technical Notes
For anyone trying to reproduce this or learn from it, here’s what you need:
The firmware deobfuscation is straightforward repeating XOR with the key “L41N” (0x4C 0x34 0x31 0x4E). Any XOR tool will do, or write a simple Python script.
Converting the binary to Intel HEX for simavr: avr-objcopy -I binary -O ihex firmware_decrypted.bin firmware_decrypted.hex
Running simavr: simavr -m atmega328p -f 16000000 firmware_decrypted.hex
The final decryption is AES-128-CBC with the key 1337baadc0fe42420123345678abcdef and IV deadbeef123455660110203099887766.
Closing Thoughts
This was a well-crafted challenge that punished static-only analysis and rewarded anyone who thought to actually run the code. The LAIN BREAKCORE theme was fun, and the glitchy output from the firmware emulator definitely added to the aesthetic.
If I were to rate this challenge, I’d give it a solid 7/10 for difficulty - it’s not trivial, but it’s not insurmountable either. The fun factor was high, even if I did spend most of my time barking up the wrong tree. There’s something satisfying about finally cracking a problem that’s been giving you grief for hours.
For other CTF players working on similar firmware challenges: don’t forget that emulation is a tool in your arsenal. Static analysis with Ghidra or IDA is great, but sometimes you really do need to see what the code does when it runs. And if you see format strings in the binary that suggest output, that’s a pretty good hint that you should be watching that output.
Thanks for reading, and happy hacking.