Personal Diary Android App with Mood Tracking and PIN Lock

Most diary and journaling apps either lock your words behind a cloud account you do not control or offer no lock at all — your private entries sit in plain local storage that anyone holding your phone can open. Moodline, this personal diary Android app, takes the opposite approach: a local-first diary where every entry is sealed in an AES-256 encrypted vault that opens only with your 4-digit PIN or fingerprint. Each entry also captures your mood on a simple 5-point scale, so over weeks the app builds a genuine picture of your emotional rhythm through 7-day and 14-day trend charts. Full-text search across titles, bodies and tags answers questions like "when did I last feel this way", while photo attachments, writing streaks and one-tap JSON export make the habit stick. Nothing leaves the phone unless you export it — there is no account, no server and no subscription in the base build.

Using Moodline end to end feels like any polished diary app, except every byte of your writing is encrypted before it touches storage. On first launch you choose a 4-digit PIN; the app derives a 256-bit key from it with PBKDF2 (100,000 iterations) and creates an empty AES-256-GCM vault in the app sandbox. Each unlock attempt re-derives the key — five wrong PINs in a row destroy the vault as a safeguard against brute force. You write entries with a title, body, mood, tags and optional photos, and saving re-encrypts the vault atomically. The Insights screen works from decrypted entries in memory only, computing your 7-day average, writing streak and the 14-day mood-trend chart, while search runs a full-text pass in memory — about half a millisecond for 500 entries in the concept demo. Export writes readable JSON or text, or an encrypted backup for a new phone.

1. First launch: you choose a 4-digit PIN; the app derives a 256-bit key with PBKDF2 (100,000 iterations) and creates the AES-256-GCM vault in the app sandbox.
2. Unlock with the PIN or fingerprint — each attempt re-derives the key; five failures in a row wipe the vault (design safeguard).
3. Write entries with title, body, 5-point mood, comma-separated tags and optional photos; every save re-encrypts the vault.
4. The home screen shows recent entries with tag filters, full-text search and the 7-day mood strip.
5. The Insights screen computes the writing streak, totals, average mood and the 14-day trend chart from in-memory entries.
6. Export to JSON or plain text, take an encrypted backup, and let the daily reminder keep the habit alive.

Tech stack:

• Kotlin
• Android Jetpack · Room · ViewModel
• BiometricPrompt API
• SQLCipher · AES-256-GCM
• MPAndroidChart
• WorkManager (reminders)
• Material 3 (Material You)
• Git

Moodline is built around four capabilities that together make it a strong final-year project: real on-device security, structured mood tracking, fast local search, and honest data portability. The security story is the deepest — a PIN-derived AES-256-GCM vault with biometric unlock through Android's BiometricPrompt and an auto-wipe safeguard after repeated failed attempts, which gives you genuine cryptography to explain in the viva instead of a lock screen drawn over plain text. Mood tracking is deliberately simple — one tap on a 5-point scale per entry — because the value is in the aggregate: the 7-day strip on the home screen and the 14-day trend chart turn scattered entries into a visible emotional pattern. Search is a measured engineering detail: a single full-text pass over titles, bodies and tags completes in about 0.5 ms across 500 entries in the concept demo. And export treats your words as yours — readable JSON, plain text, or an encrypted backup file.

• [PIN + biometric app lock] A 4-digit PIN derives a 256-bit key with PBKDF2 (100,000 iterations) that opens the AES-256-GCM vault; fingerprint unlock via BiometricPrompt where the device has a sensor. Five wrong PINs in a row wipe the vault (design safeguard, attempt count configurable).
• [Mood tracking with trend charts] One-tap 5-point mood on every entry, a 7-day mood strip on the home screen and a 14-day trend chart with per-day mood dots on the Insights screen.
• [Full-text search + tags] One query scans titles, bodies, tags and mood labels across all entries — measured at approximately 0.5 ms for 500 entries in the concept demo — with per-query timing shown under the search box.
• [Photo entries] Attach photos to any entry; images stay inside the app's private sandbox, encrypted with the rest of the vault.
• [JSON / text export + encrypted backup] One-tap export to readable JSON or plain text, plus an AES-256 encrypted backup file you can restore on a new phone.
• [Daily reminder + writing streaks] A notification-channel reminder nudges the evening writing habit; the streak counts consecutive days with at least one entry.
• [Entry editor] Title, body, mood picker, comma-separated tags and photo attach/clear in one screen; saving re-encrypts the vault atomically.

• Complete Kotlin source code for the Android app
• Room database schema and encrypted-vault design notes
• Debug APK for demo and testing
• Project report PDF (background, architecture, encryption design, methodology)
• PPT presentation for final review
• Viva Q&A preparation document (encryption, biometric auth, Android storage, charts)
• Setup and build instructions