Yesterday, I did some extensive research on what to get! I was reading plenty of posts/blogs, about how to start and what to get. For beginners it’s recommended to start small, like a Raspberry Pi, Pi Zero, or similar devices, and if you like the hobby, then you expand and go big.

While this is good advice, I ignored it because I want to run more services, for example I want play around with a Kubernetes setup. I already have a Raspberry, but I want something bigger and more powerful, obviously a fully fledged server rack with plenty of space to expand would be the dream lol, but my small room in London doesn’t have the capacity and it would be way too loud.

What did I get

So I decided to go for something small, but still powerfull enough, and I have ordered a refurbished Lenovo ThinkCentre M720q Tiny Core i5-8400T 1.7 with 32 GB RAM and a 512 GB SSD for £300.00, which I believe is a good deal? Those are specs, copied from BackMarket:

Let the tinkering begin

Check out this repository crack-rsa-challenge, all those files are from 2022, I kept them unchanged, you might find a little bit of German in there as well lol.

From what I remember my approach was:

• Research how the algorithm works
• Encrypt my own message, and try to reverse engineer it (manually)
• Write some code that does the encryption and also crack it
• Confirm that everything works with some examples I found online
• Solve the challenge with the new script

If you check the notes from the README.md, I was doing some tests with Bob and Alice, I guess that was the stuff I found online, based on that I have written some functions (see functions.py) that helped me calculating the greatest common divisor gcd(m,n), the extended gcd egcd(a,b), the modular inverse modinv(a,m), and prime factors prime_factors(n). I’m glad I had added some links explaining all the calculations, because I cannot remember at all! And rsa.py puts everything together, My class RSA does all the computation and spits out the results. The remaining py files were just little helper files for testing.

$ python ./rsa.py
### SOLUTION ###
(1, 289396593721086953, -2)
RSA(p=370248451, q=6643838879, e=17, n=2459871053643326429, phi_n=2459871046629239100, d=289396593721086953)
The lucky number is the following decrypted_plain_text:
DECRYPT: encrypted=35632720329943226 -> decrypted_plain_text=1512720


### TEST THE OTHER WAY AROUND ###
Let's test the reverse, we take the lucky number encrypt it and check if the encrypted value is the same
ENCRYPT: plain_text=1512720 -> encrypted=35632720329943226
Encrypted value: 35632720329943226
RSA(p=370248451, q=6643838879, e=17, n=2459871053643326429, phi_n=2459871046629239100, d=289396593721086953)
SUCCESS

I noticed compute_p_q() from my RSA class lacks an implementation, I cannot remember why, but I have used my function prime_factors(n) to get p and q, or just the following script which wraps that function.

python primeFactorDecomposition.py 2459871053643326429
[370248451, 6643838879]

It’s your turn now and all you need is Google, good luck! :computer: :closed_lock_with_key: :mag:

:lock: The Challenge

For next week I have something more on the fun side I hope, as you will have to crack the RSA encryption :stuck_out_tongue: I have encrypted my lucky number using the public key below

modulus : 2459871053643326429
e       : 17

The encrypted value is

35632720329943226

silly me however picked a rather small key size for our current hardware capabilities, and should be able to be cracked within seconds. All you need is to understand how RSA works! (i guess if you know already what my lucky number is you get a free pass on this)

:key: Answer

I hope you had as much fun as I had cracking the algorithm! Check out the next post for my solution.

The Goal

Starting point is a beautiful Retroflag GPi Case 2 + Compute Module 4 + 64GB SD card. The goal is to assemble everything, install Recalbox, install some games and play!

Let’s start

• Found a tutorial how to set it up, looks easy, following the tutorial
• Got the Raspberry Pi Imager, installed Recalbox on the SD card, easy
• Downloaded the GPi Case 2 patch, installed it (that patch allows you to use the built-in screen instead of HDMI)
• The Youtube comments are a bit confusing, some say you don’t need the patch, what is it? (Spoiler: not needed)
• Put in the SD card, not that easy, it can move slightly
• Put in the CM4, difficult?? It took a bit until I had it in place, without it falling out… because no screws are needed, you just plug it in, but there is not much space for your fingers :S
• Started it, it’s on, but nothing is happening, black screen
• Tried again with a HDMI cable instead, bootloader is trying to boot, looks like the logs are repeating, it’s stuck bro
• Apparently there is nothing to start, because “no sd card detected”
• WHYYY ??? Is the SD card properly in place? It was a bit wobbly? I checked - all good. Is it broken? Can’t be, Windows can read from it and write to it, no issues. Maybe the slot is broken?

Of course it’s not working

• Gooooooogle time, again, yay
• Turns out there are many different variations of the Compute Module 4
• Long story short, the light version version can only use the SD card, which I don’t have. I have the CM4 8GB RAM with 32GB storage (eMMC)
• Why is the YT video not mentioning which variant it uses?? Am I the only fool who has run into this??
• Let’s continue, the new goal is to flash Recalbox onto the chip
• Found instructions of how to flash the eMMC
• Installed rpiboot, took a while until I had figured out the right parameters and how to mount the chip as USB
  • Windows: Flashing with rpiboot, download rpiboot_setup.exe
  • Linux: https://github.com/raspberrypi/usbboot#building
• As admin run ./rpiboot.exe -v [-d path to mass-gadget-64 etc.]
• Plug in the micro USB-B cable and connect with laptop, start the thing and rpiboot will do the magic
• Now File Explorer shows an unknown/empty volume D, which means I can use the Raspi Imager to install Recalbox again, noice
• 5 hours later *in French accent*

• At the end when it verifies the image, it fails with “SD Card broken bla bla blaa”
• Nooooooooooo

• So basically it’s saying, the onboard storage (eMMC) is broken, have I broken the chip while I was trying to place it inside the case?? Somehow I don’t believe it
• Played around with PARTDISK, cleaning, formatting, creating new partitions bla bla
• Managed to create a working volume that I can access with the File Explorer
• Tried to install Recalbox again with the Raspi Imager -> same error at the end
• After a lot of googling and frustration I got an idea, I just force the image on the chip ( ͡° ͜ʖ ͡°)
• Downloaded the latest Recalbox image manually, and called in Rufus, let’s go!!
• Installed the image with Rufus
• That seemed to work quite well, after 2 minutes it was already installed, almost… *sigh* it’s stuck at 99.9%
• I cancelled after a while, as nothing was happening - mission failed - again
• Regardless, tried to boot up the module -> black screen
• Again with HDMI, I cannot believe it… Recalbox is starting up
• I just cannot use any buttons to control it
• Plugged in my Microsoft controller -> doesn’t work
• Great, I have bruteforced the Recalbox image onto the chip, but I cannot use it… :S

Did you really think it would be that easy?

• Let’s mount the eMMC chip again and see what else I can do
• Windows (or is it rpiboot??) has problems to mount the chip as volume, so I can’t really access it, I also can’t install anything else on it, since I need an accessible volume
• Shite.. I might haved bricked the whole thing :cry:
• Let’s try the same bullshit again, just from a Linux machine
• I just needed to mount it manually and repeat the whole process
• Recalbox is installed yet again :cool:
• But I need to install the patch, which is a batch file, only used by Windows :S (or do I?)
• Tried starting without patch, as I use the HDMI anyway, well it’s starting up but I STILL DONT HAVE ANY CONTROLS (I’m going crazy)
• Let’s try on Windows again, after several attempts mounting the chip with rpiboot (which sometimes randomly doesn’t work?), I gained access
• I finally have access to the chip again, and could install some roms, *wohooo*, but before I can do anything with it I need to make the system usable
• Installed patch
• Started without HDMI, black screen
• Started with HDMI, black screen
• Uninstalled patch
• Started with HDMI, it WORKS!!
• Alright, I need to make the controls somehow work, let’s get a terminal and see what we can do
• Plugged in a keyboard, tried to open a terminal with some shortcuts, that should work according to Recalbox docs, but only a Recalbox-screen pops up, no terminal in sight

What a journey, like a rollercoaster lol, but I’m making slowly progress! Up until here (16.12.2024) I have spend many hours troubleshooting and jumping from one issue to the next one. So far, we have installed Recalbox, it’s booting, but controls are missing. Next mission: get access to the terminal, I guess?

Back into the last round

It’s January 2025, after a litte christmas break I’m back from Germany and ready to continue with the fun. I didn’t manage to get acccess to the terminal, nor did anything else work, so what do we do if nothing works? Yeah let’s just try again :)

• Run Git Bash as admin (yes I need my linux commands even on Windows)
• cd C:\Apps\ Raspberry Pi and ./rpiboot.exe -v
  • without -d /path/to/massgadget/ as it would only mount the volume SHARE!
• Now we have mounted REACALBOX (D:\) and SHARE (E:\)
• Run Raspi Imager (v1.8.5)
  • Select “Raspi 4 / CM4 / bla bla”
  • Select Recalbox as OS
  • Select the mounted eMMC (RPi-MSD-0001)
• The image version is Recalbox 9.2.3-Pulstar 64bits for Raspi 4 / 400 / CM4 / GPiCase2 / PiBoyDMG (released 2024-08-01)
• Started the device with HDMI -> it WORKS
• Started the device without HDMI -> it WORKS
• Niccce

It’s working

At this point I don’t even know what exactly has worked and what not? I might find out whenever I’m brave enough to reinstall everything, but for now, I will only put some more games on it and play! If this finds you, may you have an easier journey than me lol.

What have I learned?

One word, patience.

Resources

Additional resources that I have used to troubleshoot

• https://thepihut.com/products/raspberry-pi-compute-module-4?variant=39486530519235
• https://datasheets.raspberrypi.com/cm4io/cm4io-datasheet.pdf

• https://www.reddit.com/r/retroflag_gpi/comments/rr9p5k/gpi_case_2_black_screen_at_boot/
• https://www.cytron.io/tutorial/installing-retropie-on-cm4-emmc-with-retroflag-gpi-case-2-swift-performance-actionclear-all-cache_wpnoncef3e213e2f7
• https://www.jeffgeerling.com/blog/2020/how-flash-raspberry-pi-os-compute-module-4-emmc-usbboot
• Retroflag GPi Case 2 w/CM4 - Part 1: Initial Setup Guide (@TeamRetrogue)
• Note to myself, I installed rpiboot here C:\Apps\Raspberry Pi\