YEONFEEL Keyboard

한국어

YEONFEEL Keyboard is an Android input method (IME) developed with a focus on Korean input quality; English input is supported as well. It was written from scratch in collaboration with Claude, for phones whose stock keyboards handle Korean poorly. The design goal is a keyboard that works entirely offline: the keyboard app requests no INTERNET permission, so it cannot itself send input, clipboard content, or usage data over the network. The keyboard's release APK is about 2.4 MB. If you are looking for a Samsung Keyboard alternative for other Android phones, welcome aboard.

The keyboard can also translate as you type (see Translation). That is the one place where typed text leaves the keyboard app by design: text typed into the translate panel is handed to the translation engine you choose, which runs in a separate app on the phone. ML Kit, the one engine that needs a network permission to download its language models, lives in a separate, optional add-on app, so the keyboard itself stays offline.

Input

Six layouts are included: 두벌식 (standard), 단모음 (10-key short vowel), 천지인, 나랏글, 나랏글 중앙, and English QWERTY/Dvorak. The 3x4 layouts follow the national-standard arrangements, with long-press digit input and compact symbol pages.

The composition automata live in hangul/ as pure Kotlin with no Android dependencies and are covered by JVM unit tests, including edge cases and key rollover during fast typing.

Smaller input options include an auto-replacement of the common misspelling 됬 with 됐, double-tap shift for caps lock, spacebar language switching, configurable multi-tap timing, and backspace undo for auto-corrections. Holding down ㅋ repeats it until the key is released.

Two humor options ship as well, both off by default and disabled in password fields. One retypes ㅋㅋㅋ bursts as ㅋㅋㅎㅋ so the typist reads as a younger texter; the other reproduces the ㅋㅋㅋㅋㄱㅋㅋ typos of the feature-phone 천지인 generation.

Correction (experimental, off by default)

Two correction features can be enabled in the 실험실 menu. Both run on the device.

• Touch correction builds a per-key Gaussian model of where the user actually taps, and re-scores ambiguous touches near key boundaries.
• Word correction proposes replacements using keyboard-adjacency edit distance over a 28,000-word frequency lexicon. A bloom filter of 660,000 known words prevents real words from being replaced.

Translation

The translate button in the toolbar opens a translate panel. The button is part of the default toolbar; if you saved a custom toolbar order earlier, add it from the toolbar edit panel. Type in the source language, and about 350 ms after the last keystroke the translation is written into the app's text field as composing text, replacing the previous translation. Enter confirms it; on an empty panel, Enter performs the field's normal action. Tapping the language chip swaps the pair. The source text is limited to 2,000 characters. The translate button is hidden in fields the app marks as password fields and in fields where the app asks the keyboard not to learn from input (for example, incognito mode).

Supported languages are Korean, English, Japanese, Simplified Chinese, Spanish, French, German, and Vietnamese; the default is Korean to English. Whether a pair actually works depends on the engine.

The engine is chosen in Settings → Input → Translation, which also shows whether each engine works on the phone and offers a test translation:

• System translation (default): Android 12+ TranslationManager. It works only where the phone maker ships a system translation service, which is mainly Pixel. Language packs are installed from system settings; the Translation screen links to them.
• ML Kit: works on most phones, but needs the ML Kit add-on app described below.
• Gemini Nano: Google's on-device model through the AICore system app, available only on the recent flagships that support it. If the model is not on the phone yet, the first translation asks AICore to download it; AICore decides which networks it uses. AICore only serves the app in the foreground, and a keyboard is not that app, so AICore may refuse requests from inside the keyboard. When that happens, the keyboard remembers it for the installed AICore version and stops trying; an AICore update, or "Try Gemini Nano in the keyboard again" in Settings → Input → Translation, clears it. A successful test translation in settings does not prove that Gemini Nano works inside the keyboard.

Every engine is meant to translate on the phone, and the keyboard itself sends typed text to no server. It hands the text only to the engine you chose: the phone maker's system translation service, Google's AICore system app, or this project's signed ML Kit add-on. The first two are closed system apps outside this project, governed by their makers' terms; whether the system translation service keeps text on the device depends on the phone maker.

Behavior on real phones, in particular Gemini Nano inside a keyboard, is not yet verified; that checklist is tracked in #22.

ML Kit add-on

The ML Kit engine ships as a separate app, dev.badalab.yeonfeel.translate.mlkit (module translate-mlkit/). Only people who choose the ML Kit engine need it. It is kept out of the keyboard for two reasons:

• ML Kit's native translation engine is 11–17 MB per CPU architecture. The add-on is currently built as one universal APK for all four architectures, about 63.8 MB.
• ML Kit downloads a language model (about 30 MB per language) the first time a language is used, so the add-on requests INTERNET and ACCESS_NETWORK_STATE. From the keyboard, models are downloaded only on Wi-Fi; the test translation in Settings → Input → Translation may also download over mobile data. Translation itself runs on the device, and the add-on's own code never sends typed text anywhere. Google's ML Kit libraries in the add-on send usage statistics to Google; what those contain is up to Google's closed-source libraries.

To install the add-on, download its APK from the project's GitHub releases page when a release includes it, or build it from source (see Building and running). When the add-on is missing, Settings → Input → Translation links to the releases page. Opening the add-on shows the downloaded language models and lets you delete them.

The keyboard and the add-on talk over a small Messenger IPC protocol (translate-protocol/). The add-on's translation service is protected by the signature permission dev.badalab.yeonfeel.permission.TRANSLATE, so only an app signed with the same key can use it. In the other direction, the keyboard checks that the installed add-on is signed with the keyboard's own certificate before sending any text; an add-on that fails the check is treated as not installed. The keyboard's and the add-on's own code never logs source or translated text.

This means both apps must be signed with the same key. Because both declare the same permission, Android will not install a keyboard and an add-on signed with different keys together; whichever is installed second fails. If you build from source, build and install both apps from your own build, and do not mix them with official release APKs.

Data handling

• Clipboard history is encrypted at rest with an Android Keystore key (AES-256-GCM). The key is hardware-backed where the device supports it.
• Password fields disable key preview, touch-data collection, and all text-transforming options.
• Touch-correction samples are stored per key with no ordering and no timestamps beyond day granularity, and are deleted after 7 days. Typed text cannot be reconstructed from the stored file.
• Translation is off in password fields and in fields that ask for no personalized learning. Typed text goes only to the chosen engine (see Translation), and the keyboard's and the add-on's own code never logs it.
• Besides the project's own translate-protocol module, the keyboard's runtime dependencies are a few AndroidX libraries and Google's ML Kit GenAI library for Gemini Nano, which brings Google Play services, ML Kit common, Firebase components, Kotlin coroutines, Guava, and Google's usage-reporting library (datatransport) along. These Google libraries start only once Gemini Nano is first used. The usage-reporting components are present in the APK and may queue events in the keyboard's private storage, but the keyboard has no network permission, so nothing they collect is uploaded. The network permissions those libraries would add are stripped from the keyboard's manifest, and the keyboard's assemble, bundle, and check tasks include a Gradle check, verify<Variant>NoNetworkPermission, that fails if the merged manifest requests INTERNET or ACCESS_NETWORK_STATE.

Customization

• Light, dark, and four high-contrast themes; three key-text sizes.
• Adjustable keyboard height, margins, and long-press delay through a drag-to-adjust overlay.
• Split keyboard for landscape and large screens, one-handed mode, and an editable toolbar.
• An optional terminal tool row (Esc, Tab, Ctrl, Alt, arrow keys) for SSH clients.
• An emoji panel with 1,082 emojis, skin-tone memory, and 초성 search, plus a kaomoji panel grouped by mood.

Building and running

./gradlew test assembleDebug          # unit tests + debug APKs (keyboard and ML Kit add-on)
adb install app/build/outputs/apk/debug/app-debug.apk
adb install translate-mlkit/build/outputs/apk/debug/translate-mlkit-debug.apk   # optional

Use the project's Gradle wrapper (Gradle 8.13); newer standalone Gradle versions are incompatible with the AGP version in use. Gradle needs the Android SDK: set ANDROID_HOME, or put sdk.dir in a local.properties file at the repository root. After installing, open the YEONFEEL Keyboard app and follow the prompts to enable the keyboard. The add-on only needs to be installed if you want the ML Kit engine. minSdk is 23.

./gradlew assembleRelease builds app-release.apk (about 2.4 MB) and translate-mlkit-release.apk (about 63.8 MB). When the YEONFEEL_RELEASE_STORE_FILE Gradle property is set (for example in ~/.gradle/gradle.properties, together with YEONFEEL_RELEASE_STORE_PASSWORD, YEONFEEL_RELEASE_KEY_ALIAS, and YEONFEEL_RELEASE_KEY_PASSWORD), both are signed with that release keystore; otherwise both use the debug key. Either way, the two APKs from one build share a key.

Project layout

app/src/main/java/dev/badalab/yeonfeel/
├── hangul/       # Composition automata and word corrector (pure Kotlin, JVM-tested)
├── ime/          # InputMethodService, rendering, layouts, touch model, translate panel
├── translate/    # Translation engines (System, Gemini Nano, ML Kit add-on client)
├── clipboard/    # Keystore-encrypted clipboard history
├── settings/     # Settings screens (View-based UI kit)
└── debug/        # Touch-sample store
translate-protocol/ # IPC protocol shared by the keyboard and the add-on (JVM-tested)
translate-mlkit/    # ML Kit translation add-on app
scripts/            # Generators for emoji data and the correction lexicon

License

App code is licensed under the Apache License 2.0.

Third-party assets are listed in THIRD_PARTY_NOTICES.md: Lucide icons (ISC), Google's ML Kit GenAI library in the keyboard and ML Kit Translation in the add-on (Google's terms), and the Korean frequency data derived from FrequencyWords (OpenSubtitles 2018). The derived files app/src/main/assets/ko_freq.txt and ko_known.bloom remain under CC BY-SA 4.0, separately from the app code.