slock

Two computers... one-bit. A tiny E2E encrypted messenger, tucked inside your capslock key.

Hold Caps Lock on your Mac. A little light comes on on someone else’s keyboard. Let go, and it goes off.

That’s the whole idea. Caps Lock has spent quite enough time shouting. It can have a small, gentle job now.

slock lives in your menu bar and connects exactly two Macs. Send a tiny hello, invent a secret blink language, or sit at opposite desks and be a little ridiculous. If you both opt in, the same key becomes a walkie-talkie.

The firefly is Dit. Dit has one light and considerable enthusiasm.

Why you might like it

• It’s ambient, not another chat window. No notifications, no unread badge, no typing. Just a light that means someone is thinking of you right now.
• It’s private by construction. Every message is end-to-end encrypted between your two Macs. There are no accounts, no server that knows who you are, and no analytics. The relay only sees ciphertext and routing.
• It’s tiny and open. Two Swift files, no third-party libraries, no Xcode project required. Read the whole thing over a coffee, then build it yourself.

An honest note before you download. slock is an experiment with rough edges. It uses a public MQTT test relay, is not notarized (macOS will ask you to confirm the first launch), and its encryption has no forward secrecy. Keyboard lights also vary by model. Please use it with someone you trust and keep sensitive conversations elsewhere. The security details · What’s been tested

Give it a blink

You’ll need two Macs running macOS 13 or newer, an internet connection, and one willing accomplice.

1. Install. Download slock.app.zip on both Macs, unzip it, move slock.app to Applications, and open it. If macOS blocks the first launch, see Apple’s instructions for opening an app from an unidentified developer.
2. Grant keyboard access. Follow the Permissions Required guide for Accessibility and Input Monitoring. slock explains where to click; macOS handles the approvals.
3. Share a code. On one Mac, open Pairing…, copy your code, and send it to the other person through a channel you trust.
4. Pair. On the other Mac, paste it into Other Mac’s pairing code and choose Send Pairing Request. Back on the first Mac, choose Review Pair Request from …, compare the complete pairing codes with your person, then Accept Pairing. A nickname alone doesn’t verify who’s there.
5. Blink. Wait for the connection, then hold Caps Lock. Your person gets a light. You have successfully sent one very small hello.

Light signals start as soon as they arrive. Short blinks and gaps retain their timing when packets arrive in time; a hold or gap longer than one second resets playback to the earliest opportunity. Hold Option in the menu and choose Test Caps Lock Light to check whether your keyboard’s LED cooperates. Some keyboards are more willing participants than others.

While slock is active, Caps Lock stops capitalizing text. Your keyboard light follows the other person’s key; Dit’s tail shows your own outgoing activity. Pause Slock hands Caps Lock its old job back whenever you like.

A tiny walkie-talkie, too

1. One person chooses Invite Peer to Enable PTT.
2. The other chooses Accept PTT Invitation.
3. Both grant microphone access. Once both menus show PTT Enabled, hold Caps Lock to talk and release it to stop.

Both people must agree, and either can turn it off for both. Only one person talks at a time; if you press together, the apps pick one sender consistently. Excellent practice for saying “over.” Voice is end-to-end encrypted Opus and runs independently of light timing.

Do Not Disturb and other Focus modes pause voice on your Mac, including an ongoing talk. Light signals keep working, and both people’s PTT consent stays enabled. Once Focus is off, release Caps Lock and hold it again to talk; interrupted audio is discarded. The menu shows Audio Paused while Focus is active. If macOS’s Focus status cannot be read, voice stays paused and the menu explains why.

Getting to know Dit

With Reduce Motion enabled, the accepted-pairing tail stays steady green.

A few useful things in the menu:

When either Mac unpairs, the other returns to Unpaired too. If the message is missed, the other Mac catches up when both Macs are online and check in.

Hold Option in the menu to reveal This Mac (your fingerprint), Test Caps Lock Light, Diagnostics…, and the installed version.

Privacy at a glance

• What slock sees. To catch Caps Lock, macOS requires Accessibility and Input Monitoring, which technically expose all key events to the app. slock acts only on Caps Lock and the F18 key it is remapped to, passes everything else through untouched, and never records, stores, or transmits other keystrokes.
• What leaves your Mac. Encrypted key up/down events, your chosen nickname, and (only with mutual push-to-talk consent, while you hold the key) encrypted voice. The relay and any observer can see your public key, timing, and message sizes, but not content.
• What stays on your Mac. Your identity key in a private folder, your recent pairings, and your settings. Nothing is sent to the author. The only other network traffic outside the relay is a version check against GitHub at launch and hourly, plus the update download when you choose to install one.
• What it never does. No accounts, no telemetry, no ads, no contacts access, no clipboard reading. The microphone is used only during a consented, held push-to-talk transmission.

Full details, including the limits of the current relay and encryption, are in SECURITY.md. Found a vulnerability? Please use private reporting.

Small app, real caveats

This is still a prototype. Bug reports are welcome, especially from keyboards that have decided to express themselves in unexpected ways.

• Lights are best-effort. The main target is a MacBook’s built-in keyboard. Some external keyboards cannot control the LED independently. slock reports that instead of turning on system Caps Lock to fake a light.
• The relay is a public testing service. slock uses wss://test.mosquitto.org:8081/mqtt. Availability is not guaranteed; other clients can send spam or interrupt delivery. A production service would need an operated broker with per-user authentication and abuse controls.
• Encryption has limits. Key states, nicknames, and voice are encrypted with X25519, HKDF-SHA256, and ChaCha20-Poly1305. If either Mac’s private identity key is later stolen, recorded traffic can be decrypted: there is no forward secrecy. Keep identity files and backups private.
• Recovery isn’t magic. slock preserves existing keyboard mappings and restores them when capture stops or the app quits. After a crash, the next launch uses a recovery journal. Force-quitting or losing power cannot run immediate cleanup.

Curious how the blinking sausage is made? See the architecture.

tccutil reset ListenEvent com.jonaraphael.CapsLink

Build from source

Install Xcode or the Xcode Command Line Tools, then run:

git clone https://github.com/jonaraphael/slock.git
cd slock
./build.command

The build renders Dit’s vector artwork into an app icon and produces a universal app plus a distributable ZIP in dist/. Signing defaults to ad-hoc. There is no Xcode project and no third-party dependency; the default build needs no internet access.

CAPSLINK_ARCHS=arm64 ./build.command  # one architecture only (or x86_64)
./test.command                        # regression, security, and timing suites

To keep a stable macOS identity across builds, save a code-signing certificate fingerprint from security find-identity -v -p codesigning in the Git-ignored .release-signing/codesign-identity file, or set SLOCK_CODESIGN_IDENTITY for one build. Use an Apple Development identity for local development and a Developer ID Application identity for releases. Certificate signing requests an online secure timestamp and fails if signing fails; it never retries with an ad-hoc signature. Publication requires a Developer ID Application signature. See release signing for setup and migration details.

The tests use temporary identities, fake keyboard commands, and synthetic audio. They never capture your keyboard, use your microphone, or contact the relay. Maintainers can find the tagging and publishing steps in docs/RELEASING.md.

Project guide

slock was previously called CapsLink. Internal preference keys, the identity storage directory, bundle identifier, and wire identifiers keep the old name so existing settings and pairings keep working.

Contributing

Issues and pull requests are welcome. Keep changes small, add a regression test where one fits, and run ./test.command before opening a PR. For anything security-related, please use private reporting first.

License

MIT. See third-party notices for attribution. Dit is happy to be forked, as long as the light stays gentle.