firmware

Coldcard Mk4 firmware with SLIP-19 coinjoin remote signing

Unofficial build. Not made, endorsed or supported by Coinkite.

This is Coldcard firmware 5.6.0 plus the SLIP-19 ownership proofs and HSM policy rules a Coldcard needs to sign WabiSabi coinjoin rounds unattended. The same code is open as pull request #685 against Coinkite's firmware repository. Until it is merged and released by Coinkite, this build is how the feature can be tried on a device.

Mk4 only. The header declares Mk4/Mk5 compatibility. The Q disables the classic HSM command set, and the Mk3 line ended before the policy rules this depends on existed.

Files

file	what
firmware-5.6.0-slip19-NODEBUG-mk4.dfu	the firmware image, as loaded from SD card
SHA256SUMS.txt	SHA-256 of that image
SHA256SUMS.txt.sig	detached GPG signature over SHA256SUMS.txt

Verify before you install

Put all three files in the same directory and run the commands there.

Check the hash.

Linux:

sha256sum -c SHA256SUMS.txt

macOS (no sha256sum by default; shasum reads the same file):

shasum -a 256 -c SHA256SUMS.txt

Windows, in PowerShell:

$want = (Get-Content SHA256SUMS.txt).Split(' ')[0]
$got = (Get-FileHash firmware-5.6.0-slip19-NODEBUG-mk4.dfu -Algorithm SHA256).Hash
if ($got -eq $want) { "OK" } else { "MISMATCH" }

Either way the answer is one word. Anything other than OK means stop.

Check the signature over the hash file. Same command everywhere, once GPG is installed — most Linux distributions ship it, macOS gets it from brew install gnupg, Windows from Gpg4win:

gpg --verify SHA256SUMS.txt.sig SHA256SUMS.txt

The signing key is not in this repository, deliberately. Get it from the primary source, the builder's own site:

curl -sLo signing-key.asc "https://kravens.nl/Kevin%20Ravensberg_0x01272020_public.asc"
gpg --import signing-key.asc

Expected fingerprint:

2536 F69E 9C37 2566 2B6C  146B DCF1 7F7A 0127 2020

Cross-check that against an independent copy on a keyserver, which is run by neither the builder nor this site:

gpg --keyserver hkps://keyserver.ubuntu.com --recv-keys 2536F69E9C3725662B6C146BDCF17F7A01272020

The two copies are byte-identical today. If they ever stop being, believe neither and ask.

GPG will say the key is not certified with a trusted signature. That is expected and not a failure: nobody has signed this key for you. The fingerprint is the thing to check.

Verifying the hash without checking the signature only tells you the file downloaded intact, not who built it. Checking a signature against a key this page hands you tells you only that whoever wrote this page also made the signature — which is why the key comes from somewhere else.

Install on the Mk4

Verify the file first. Then, from microSD:

1. Copy firmware-5.6.0-slip19-NODEBUG-mk4.dfu to a FAT32 microSD card. Keep the .dfu extension — the device only lists those.
2. Insert the card, then Advanced/Tools → Upgrade Firmware → From MicroSD, and pick the file.
3. The device checks the header, copies the image to internal flash, and reboots into its bootloader to finish. This takes a few minutes. Leave it powered until it comes back on its own.
4. It boots with a "Danger: Custom Firmware" screen. Expected — see the warnings below.
5. Confirm with Advanced/Tools → Upgrade Firmware → Show Version. This image reports version 5.6.0, built 2026-08-02.

Over USB instead, if you already have USB enabled:

pip install ckcc-protocol
ckcc upgrade firmware-5.6.0-slip19-NODEBUG-mk4.dfu

To go back to Coinkite's official firmware, install their .dfu the same way. This build does not raise the device's OTP downgrade floor, so it does not stand in the way of that.

Enable USB and HSM mode

Two settings on the device, once. Both need a real seed loaded.

	where	set it to
1	Settings → Hardware On/Off → USB Port	Default On
2	Advanced/Tools → Spending Policy → HSM Mode	Enable

USB is on unless somebody turned it off. HSM Mode is off from the factory — leave it off and the coinjoin USB commands are refused, which is the correct default for a device nobody is watching.

Then, each time the host starts a session:

3. The host uploads its policy. The device shows what that policy allows, ending in "Press OK to enable HSM mode." — read it, then press OK.
4. For a policy it has not seen before, a second screen shows the policy hash and asks you to press one specific digit it names. The digit changes every time, so a host cannot script its way past this screen.
5. Compare that hash with the one your host is showing before you walk away. Matching hashes mean the device is enforcing the policy the host sent, not one it was talked into.

The device now signs within that policy without asking again. Power-cycle it to leave HSM mode.

What is in it

Base: Coinkite firmware c849c4e0, which is 5.6.0 including the July 2026 entropy hotfix.

Added, from branch feature/slip19-coinjoin at commit 5d0dd982:

• slp9 USB command returning SLIP-19 ownership proofs (P2WPKH and P2TR), with a slip19_paths HSM policy whitelist gating which paths may be proven unattended.
• Five HSM policy rules bounding what unattended signing may do: max_txn, max_txn_per_period, max_sats_leaving, max_fee_per_kvbyte and min_inputs.
• Two safety fixes: devmode test commands (EVAL/EXEC/XKEY) no longer bypass HSM mode on a debug build, and the master seed copy is blanked after the one HMAC that needs it.
• Two screen changes so an unattended device reads honestly: an indicator while a proof is being signed, and a busy line that expires instead of claiming to be working after the host goes quiet.

Firmware header, as built:

version_string   5.6.0
timestamp        2026-08-02 02:48:55 UTC
pubkey_num       0          (shared developer key, not Coinkite's factory key)
install_flags    0x0        (FWHIF_HIGH_WATER not set)
firmware_length  1011712
hw_compat        0x28       (Mk4, Mk5)

The build is not bit-reproducible: the signing timestamp is part of the header, so rebuilding the same source produces a different hash. To rebuild the payload yourself, check out 5d0dd982 from https://github.com/kravens/firmware and build stm32/ for Mk4.

Read this before installing

• The device warns at every boot, and you should expect it to. This image is signed with the shared developer key (pubkey_num 0), not Coinkite's factory key, so an Mk4 running it shows a "Danger: Custom Firmware" screen on each start. That warning is the indicator that matters. If a device running this build ever stops showing it, stop and find out why.
• The green genuine light stays green. Observed on a retail Mk4 with this exact image. Do not read that as "Coinkite signed this" — on this build they did not. Whatever the light attests to, it is not the identity of the key in the firmware header, so the boot warning above is the only thing distinguishing an official image from this one.
• Use a device that does not hold significant funds. This code has not been reviewed by Coinkite. It has been tested — on a retail Mk4, in the simulator, and in coinjoin rounds that confirmed on regtest and mainnet — but tested is not audited.
• FWHIF_HIGH_WATER is not set in the header, so installing this does not raise the device's permanent OTP downgrade floor.
• Signing speed is the practical limit today. An Mk4 needs roughly 2.7 ms per PSBT byte, so a ~14 KB round signs in about 40 s while a ~40 KB round takes 100 s or more and can miss a coordinator's signing phase. Small rounds work; large ones need a coordinator with a longer signing phase.

Why the debug build is not here

A debug build of the same source exists and was used for testing, but it is not published. Its serial REPL and dev_helper keypad injection are enabled at boot and have no HSM concept, so a debug build is not safe to leave connected to a host — which is exactly what unattended coinjoin signing does. Debug builds belong on test units.

Licence

Coldcard firmware is copyright Coinkite Inc., licensed MIT with Commons Clause v1.0. The Commons Clause withholds one right: selling. This build is not sold, and neither is anything derived from it here. See COPYING-CC in the upstream repository for the full text.

"Coldcard" and "Coinkite" are their trademarks. This build is not affiliated with them.