Cheap mass produced keyboard compatible with any opensource firmware?

[consumerkey]

Do you know a cheap mass produced keyboard compatible with any opensource firmware?
I can't afford ~$200+ (full size wonderful mech) with an expensive delivery from US/EU with their high labor costs (included to a product price) + customs fees.
I want a ~$20 ugly (not necessarily mech) which could be bought locally - but then somehow I can install an opensource firmware to it.

Here's a list of companies with <$50 keyboards available in my sh1thole country. Is there any match?

0-9
3Cott , 3Q
A-C
A4Tech , Abkoncore , AGESTAR , ASUS , Atom , Baseus , BTC , Canyon , CBR , Chicony , Corsair , COUGAR , CROWN MICRO
D-H
Defender , DELL , Delux , DGMedia , Dialog , ExeGate , Gembird , Genesis , Genius , GIGABYTE , Golden Media , HAMA , HARPER , HIPER , HP
I-M
Intro , Invin , Jet.A , Kreolz , KREZ , KS-is , Logitech , Logitech G , Lenovo , Microsoft , Mad Catz , Manhattan , MARVO , Modecom , MSI , Mystery
N-R
NAKATOMI , NATIONAL , OKLICK , Palmexx , Perfeo , Qcyber , Qumo , Razer , Redragon , Rapoo , Red Square , Rii , Ritmix , RSQ
S-T
Samsung , Satechi , Simple , SmartBuy , SONNEN , SVEN , Thermaltake , Thomson , Trust , Tt eSPORTS by Thermaltake
V-Z
Vontar , WolfKing , Xiaomi , XtremeMac , Xtrikeme , Zalman

Closest thing I was able to find, is that modded Gigabyte Force K83 - https://www.reddit.com/r/MechanicalKeyboards/comments/alz1qs/the_cheapest_full_size_qmk_keyboard_with_cherry/ - someone replaced its' controller with a Teensy++ 2.0 loaded with QMK opensource firmware.

[yui]

well there is hasu's usb to usb converter that exists: https://geekhack.org/index.php?topic=69169.0
Makes any old usb hid compliant keyboard fully programmable. so likely any of your keyboard would work

[consumerkey]

well there is hasu's usb to usb converter that exists

My concern is not to make a generic keyboard programmable - but to make sure a keyboard isn't running even a byte of a closed source code!
For example: you have a full disk encryption, but a proprietary firmware accumulating your keypresses in a hidden memory,
from which your keypresses - including your full disk encryption passphrase - could be retrieved with some debugger tool.
So I need to either flash a firmware like QMK to its' original controller, or to replace its' controller with one which could run something like QMK.

I've already replaced a firmware of a lot of devices, and even UEFI with an opensource coreboot BIOS. Now I'd like to do the same for a keyboard

[suicidal_orange]

You can connect the switch matrix of pretty mucn any keyboard to a Teensy or similar, it's a bit tricky with cheap rubber dome boards as membranes don't like solder.

You can also replace the controller board on Filco and CM Storm keyboards with bpiphany controllers but they aren't cheap boards and the controller is extra...

Or you could pick up an old Cherry board from a time before flash memory was cheap enough to even consider planting in a keyboard in the hope of catching something useful which you somehow manage to get access to.

[yui]

i thought i was a bit "tin hat" before coming to this forum, now i see that i am not even close. if you want to be sure that one one has tempered with the software to include a spy the only way is a custom controller you would try to find an old-ish rubber dome with a through hole controller and replace that controller with an arduino pro micro or teensy and map out the matrix in tmk/qmk but even then you run the risk of having a spy in the boot loader so you need an isp programmer and a compiler to be sure.