Bitcoin MRE Wallet

Warning

No guarantees are provided about the safety of this app. I tried my best to ensure the PRNG is properly seeded and I have debugged and tested it as much as possible, but bugs do exists and could happen.

I do not take any responsibility from loss of funds or misuse of the app.

The app is open source and you can recompile it from scratch (please do, don't trust, verify).

Keep in mind that this is a hobby project and I strongly recommend (if you can afford it) using better hardware wallets (there are plenty out there: ColdCard, BlockStream Jade, Foundation Passport, Keystone Pro, Trezor Model T, Ledgers, etc.).

Compiled apps

Mainnet: https://github.com/BitcoinComfy/BitcoinMRE/blob/main/arm/Mainnet.vxp?raw=true

Testnet: https://github.com/BitcoinComfy/BitcoinMRE/blob/main/arm/Testnet.vxp?raw=true

Why?

Because it can run on cheap mass-market devices / feature-phones that do not require any assembling / firmware flashing or particular skills to install and run.

A Nokia 5310 2020 is sold for 15/20$, it has a keyboard, a screen, a SD card reader, a camera and it can run C code.

Tested devices

• Nokia 5310 2020: WORKING
• Nokia 110: NOT WORKING (limited ram, it can only run 10kb ELFs...)

I recommend trying with a 320x240 Nokia Series S30+ with SD card support (there are several of them).

Nokia 215, 216, 220, 222, 225, 230 and 3310 (2017) SHOULD work. Please help me testing them.

Other MRE phones that potentially could work if they have enough ram:

Nokia:

• Nokia 200
• Nokia 320
• Nokia 130
• Nokia 105
• Nokia 150
• Nokia 106

Symphony:

• Symphony D67
• Symphony D64i
• Symphony s100
• Symphony s100i
• Symphony FT60i
• Symphony FT540
• Symphony SL10
• Symphony d52j
• other Symphony brand non-Android toches phone

Winmax:

• Winmax w678
• Winmax w102

Gphone:

• some models

Other:

• Micromax x445
• Zonda ZM340TH
• Alcatel 3003G
• Alcatel 3041d
• Western D32
• Explay tv240
• Fly e154
• Fly e157
• Phillips Xenium x1560
• Lenovo MA309
• Alcatel OT-2012D
• Texet TM-512R
• Maxvi (Nokia 225 clone/class-A)

How to run it

Copy the vpx application into the SD Card and run it from the File Manager.

Use the arrow keys to navigate (up, down, left->Back, right->Next/OK/Enter). When selecting the files (e.g. the PSBT) with the integrated file explorer, use the left action button to select the file (cannot change OS internals/behavior).

Entropy generation / randomness

I spent quite some time to ensure the entropy/randomness is decent.

Here is the source of entropy used:

• vm_che private key generation (Mediatek MRE crypto engine, no idea how safe it is)
• sha256 of 255 unicode keyboard inputs
• sha256 of operative system ticks
• sha256 of remaining space on phone and SDcard
• sha256 of microphone recording (it records less than a second, hash the result and delete the file) -> working on a real device, not working on the Win emulator
• sha256 of Photo folder (so take a bunch of pictures before starting the app)
• sha256 of a bunch of files on the SD card
• sha256 of imsi
• sha256 of imei
• sha256 of operator code

Once the seed is generated, chacha-drbg is used as CSPRNG.

How do I save the XPUB?

Open, recover or create a wallet, choose the derivation scheme, the optional passphrase and then Display Account Extended Public Key (QR). Then scan the QR with Specter wallet.

If you don't want to use Camera / QR, you can export it using the SD Card.

Open, recover or create a wallet, choose the derivation scheme, the optional passphrase and then "Save Public Info". It will create a file on the SD card with all the XPUBs that you can import on Specter for example.

Public keys will be saved in btc_mre_wallet/xpub/ (if using derivations) or btc_mre_wallet/pub/ (if using a single keypar).

Private keys will be saved in btc_mre_wallet/xpriv/ (if using derivations) or btc_mre_wallet/priv/ (if using a single keypar).

Do not save private keys on the SD Card unless you have a really good necessity. Wipe the SD Card properly after.

How do I create a PSBT?

I use a full Bitcoin Core node + Specter.

Once you created for example the Unsigned Transaction from Specter wallet, you can scan it from the MRE app using: "Sign PSBT (QR)"

If you don't want to use Camera / QR, you can export it to file, and move the PSBT inside the SD Card folder called: btc_mre_wallet/psbt/

Then you can sign it from the MRE app using "Sign PSBT (file)".

Which PSBT format does it supports?

Raw, hex encoded and base64 encoded.

I have signed PSBT, how do I broadcast it?

You can scan the QR using for example Specter wallet and broadcast it.

If you don't want to use Camera / QR, you can export it using the SD Card.

First of all perform a Clean Exit (it will wipe BSS/Heap) and remove the battery for 1 minute to ensure data on the RAM is gone.

When you sign a PSBT, a base64 encoded PSBT is generated with the appended ".signed.psbt" extension.

To broadcast I use either Specter wallet either the Bitcoin Core command line (works better, especially if the PSBT needs an utxoupdatepsbt).

Fire up the daemon:

bitcoind --testnet

Then use the command line:

bitcoin-cli --testnet utxoupdatepsbt <base64_psbt>

bitcoin-cli --testnet finalizepsbt <utxoupdatepsbt_base64_psbt>

bitcoin-cli --testnet sendrawtransaction <finalizepsbt_raw_tx>

Done.

Wallet format

It's "proprietary". It uses some rudimental PBKDF2-style derivation + AES + redudancy/reed-solomon for error correction, so a bunch of random bit-flips will not make it unrecoverable.

Limitations

No taproot support.

Camera preview works only once per app start. The second time the camera still works and captures frames but there is no preview (cannot do anything about it, the preview is started by MRE kernel/PlutoMMI and API do not provide control over it).

Many other smaller limitations.

Graphical glitches.

Word wrap is ugly.

Some actions don't have a graphical warning (e.g. wrong wallet password will just go back to text input, until you type the correct pass).

What I could do next (or not)

Broadcasting transactions via SMS (cool).

Fix word wrap.

Maybe switch to micropython+embit (easier to maintain, more features, etc.).

Cleanup the spaghetti-code mess (not a good coder, sorry, I'm a security researcher, I hack devices and crack apps for fun and profit). The idea was to "finish it ASAP", ignore the fancy stuff, and see if people find it useful.

Apps and libraries used

• https://github.com/UstadMobile/ustadmobile-mre
• https://github.com/Immediate-Mode-UI/Nuklear
• https://github.com/micro-bitcoin/uBitcoin
• https://github.com/trezor/trezor-firmware/tree/master/crypto
• https://github.com/kokke/tiny-AES-c
• https://github.com/jannson/reedsolomon-c
• https://sourceforge.net/projects/zbar/files/zbar/0.10/

Building the app and using the Windows simulator for debugging

Read below (copy pasted from from https://github.com/UstadMobile/ustadmobile-mre )

Use the C-flag -DTRY_TESTNET=1 to compile for Testnet.

The final executable will be placed in arm/Default.vxp

Why don't you add obj files to gitignore?

Cause compiling it's not trivial, and I want to allow everyone to inspect the app.

Default.axf / BitcoinMRE.axf is the compiled ELF file (easier to inspect).

Default.axf / BitcoinMRE.axf / Mainnet.vxp / Testnet.vxp is the Mediatek packed ELF after bundling resources.

Quick notes of an MRE app (from UstadMobile)

An MRE app can have access to file system, play media, draw 2D graphics with a 2D engine, has keyboard, touch and pen access, play audio, record, have access to Network with http, tcp socket support, have 5 levels of logging, parse xmls and has its own simulator. Mediatek has today ended support for MRE and even its tools and discussions making it quite hard to develop for this platform.

The file extension for an MRE App is .vxp and it is a binary executable file that runs on an phone that has a Mediatek chip and supports MRE applications.

During development, an MRE application gets built for x86 based processors that can run on the MRE simulator on Windows. In order to get the same project built for an actual MRE application, there is another step where we "make" the application that gives a vxp file that can be executed on the Mediatek MRE capable phone.

An MRE application structured is based around events. At the start of the application you attach event handlers to system, key and pen events. It seems you cannot change the handlers again once they are assigned.

Development Setup

You will need a Windows machine, Visual Studio 2008 is recommended and works the best (thats how old the documentation is) and have an ARM compiler (ADS1.2, RVCT3.1, GCC).

The next step is to Download Mediatek MRE's SDK from here (since Mediatek has removed any reference, discussion or tools relating to MRE) and install it. This SDK will make sure the necessary include header files for MRE SDK are available to be included in your project. This will also install a Wizard tool that will make you a demo application. The SDK also comes with a simulator and a Visual Studio 2008 toolbar that gets installed on to Visual Studio.

Please install MRE_SDK into this path: C:\tools\MRE_SDK_V3

Visual Studio Steps##

Get Visual Studio2008 / Visual C++ 2008 Express. Might work with newer versions but I haven't tried those.

Tools > Options> Projects and Solutions > VC++ Directories

$(VCInstallDir)include

C:\tools\MRE_SDK_V3\include

C:\tools\MRE_SDK_V3\include\service

 Library files:

$(VCInstallDir)lib

$(WindowsSdkDir)\lib

C:\tools\MRE_SDK_V3\lib

 

Or edit the file in:

C:\Use