Pi. Ku.

A safe haven for your unsaid and unsent letters.

Pi. Ku. is a zero-knowledge, end-to-end encrypted letter-writing app for the words you've carried too long. Write a letter, seal it in an envelope, send it through a secure link, or lock it away in a time-capsule vault all without the server ever seeing a single word.

Live — writepiku.app
Philosophy — writepiku.app/know-piku
Video Demo — YouTube
Source — git.ramvignesh.dev/me/pi-ku

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What It Does

Authors write letters on a canvas-based editor with image support, seal them into encrypted envelopes, and choose what happens next:

Action	What happens
Keep	Stored in your private drawer, encrypted at rest
Send	Generates a zero-knowledge share link, without needing an account to open the letter
Vault	Locked with time (time-capsule): even you can't open it until the unlock date. The server emails you on the date when it's ready
Burn	Release the unsaid words into the wind, ritualistically

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Architecture

Zero-Knowledge & End-to-End Encryption

The server is intentionally blind. It stores ciphertext it can never decrypt.

graph TD
    subgraph Browser ["Client / Browser (Secure Context)"]
        Input["User: Password & Email"] --> PBKDF2["PBKDF2 (600k iterations)"]
        PBKDF2 --> MasterKey["Master Key (CryptoKey, non-extractable)"]
        PBKDF2 --> AuthHash["Auth Hash (hex string)"]

        MasterKey --> IndexedDB[("IndexedDB")]

        DEK["Per-Letter DEK (AES-GCM-256)"]
        Letter["Plaintext Letter"] -->|"Encrypt with DEK"| EncBlob["Encrypted Payload"]
        DEK -->|"Wrapped by Master Key"| WrappedDEK["Wrapped DEK"]
    end

    subgraph Server ["Django Backend"]
        AuthHash -->|"TLS"| Auth["Auth Middleware"]
        Auth -->|"Re-hash & verify"| DBUsers[("User DB")]

        EncBlob -->|"Upload"| Storage["Storage API"]
        Storage -->|"Stream"| R2[("Cloudflare R2")]

        WrappedDEK -->|"Store"| DBLetters[("Letter Metadata DB")]
    end

    subgraph Guest ["Guest Reader"]
        URL["Share URL (#key=DEK)"] -->|"Extract DEK"| GuestDEK["DEK"]
        GuestDEK -->|"Decrypt"| Decrypted["Decrypted Letter"]
    end

    URL -.->|"# stripped by browser"| Server

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Key points:

• Password never leaves the browser in plaintext. A master key and auth hash are derived locally via PBKDF2 (600k iterations). Only the auth hash is sent over TLS, and the server re-hashes it before storage.
• Envelope encryption: each letter gets its own Data Encryption Key (AES-GCM-256), wrapped by the master key. Compromising one letter's key doesn't expose the rest. Future password rotation only requires re-wrapping DEKs, not re-encrypting all content (same pattern as Bitwarden and Standard Notes).
• Zero-knowledge sharing: the decryption key lives in the URL fragment (#key=...). Browsers strip fragments before sending requests, so the key never touches the server.
• Master key persistence: the non-extractable CryptoKey is stored in IndexedDB across sessions so users don't re-enter their password on every tab.

Canvas-Based Letter Editor

The editor is built on Fabric.js, it's not just a text editor but a full canvas supporting image placement, multiple font families, color styling, and layout serialization to JSON. The canvas is responsive across mobile and desktop, with dynamic height adjustment, so letters look the same on both viewports.

Backend Services

• APScheduler background job polls for vault letters whose unlock time has passed and triggers email notifications. The scheduler is guarded to run only on the main server process (not during migrations, tests, or duplicate workers).
• S3-compatible object storage (Cloudflare R2) for encrypted image blobs: edge-served, decoupled, scalable. Django's default file serving is not used.
• JWT auth with httpOnly cookies for refresh tokens. CORS is a strict allowlist. SSL is enforced even in local dev.
• Structured JSON logging in production, piped to Grafana (Loki + Alloy) for observability.

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Tech Stack

Frontend

Framework	React 19, Vite
State	Zustand
Crypto	Web Crypto API (PBKDF2, AES-GCM-256)
Canvas	Fabric.js
Styling	Tailwind CSS v4, DaisyUI, Phosphor Icons
Animation	Motion (Framer Motion), Lenis (smooth scroll)
Testing	Vitest (unit), Playwright (E2E)

Backend

Framework	Django 6, Django REST Framework
Auth	SimpleJWT (httpOnly cookie refresh)
Scheduler	APScheduler + django-apscheduler
Storage	django-storages + boto3 (S3 / Cloudflare R2)
Logging	structlog + django-structlog
Server	Gunicorn
DB	PostgreSQL (w/ SQLite fallback)

Infrastructure

Containers	Docker, Docker Compose
CI/CD	GitHub Actions / Gitea Runner
Linting	Biome (frontend), Ruff (backend)
Git Hooks	Lefthook (pre-commit checks: lint, format, type-check)
Package Managers	Bun (frontend), uv (backend)

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Testing

The project uses a layered testing strategy:

• Unit tests: Vitest (frontend) and Django's test runner (backend) with freezegun for time-dependent logic. Frontend coverage threshold at 80%.
• Integration tests: MSW (Mock Service Worker) for API mocking in the frontend test suite.
• E2E tests: Playwright against the full stack (Chromium + Firefox). Tests cover the complete auth flow, letter CRUD, encryption/decryption, and guest sharing. A dedicated docker-compose.e2e.yml runs isolated Postgres and Mailpit containers on separate ports.
• CI pipeline: four parallel jobs on every push/PR: env setup, frontend checks (lint + types + unit), backend checks (lint + Django tests), and full E2E.

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Running Locally

Prerequisites

• Node.js or Bun (frontend)
• Python 3.14+ with uv (backend)
• Docker / Podman (for Postgres & Mailpit containers: optional, see fallbacks below)

Quick Start (with Docker)

# Clone
git clone https://git.ramvignesh.dev/me/pi-ku.git && cd pi-ku

# Setup env and install deps
cp .env.example .env
./scripts/setup.sh

# Start everything (containers + migrations + dev servers)
./scripts/start.sh

Without Docker

If you don't have Docker, both the database and email services have local fallbacks:

1. Database: set DB_ENGINE=sqlite (or comment it out) in .env to use a local SQLite file. For PostgreSQL, configure DB_NAME, DB_USER, DB_PASSWORD, DB_HOST, DB_PORT and set DB_ENGINE=postgresql.

2. Email: if EMAIL_HOST is not configured, the server falls back to printing emails to the console.

3. Run manually:

   # Backend
   cd backend
   uv run manage.py migrate
   uv run manage.py serve
   
   # Frontend (in another terminal)
   cd frontend
   bun run dev

Running E2E Tests

./scripts/run-e2e.sh

This spins up an isolated test stack (docker-compose.e2e.yml), runs Playwright, and tears everything down. Supports --ui mode for interactive debugging.

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Self-Hosting & Deployment

The project is fully containerised with production-ready Dockerfiles for both frontend and backend:

• Frontend: multi-stage build: Bun installs deps → Vite builds the static bundle → served via Nginx Alpine with SPA routing.
• Backend: uv installs deps → runs migrations on startup → serves via Gunicorn.

Environment Variables

Copy .env.example and configure for your environment. Key production variables:

Variable	Purpose
SECRET_KEY	Django secret key
ALLOWED_HOSTS	Comma-separated hostnames
CORS_ALLOWED_ORIGINS	Frontend origin(s)
DB_ENGINE, DB_*	PostgreSQL connection
S3_*	S3-compatible storage (Cloudflare R2, AWS, MinIO)
EMAIL_HOST, EMAIL_*	SMTP for activation & vault unlock emails
SSL_ENABLED	Enable HTTPS in dev (true generates local certs)

Observability

The backend outputs structured JSON logs in production. These are designed to be ingested by Grafana Loki (via Alloy) for request tracing and monitoring.

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Project Structure

pi-ku/
├── backend/           # Django REST API
│   ├── config/        # Settings, URLs, logging, WSGI
│   ├── letters/       # Letter CRUD, models, serializers, vault scheduler
│   ├── users/         # Custom User model, auth views, email utils
│   ├── scripts/       # Custom management commands (SSL-aware dev server)
│   └── templates/     # Email templates (activation, vault unlock)
├── frontend/          # React SPA (Vite)
│   ├── src/
│   │   ├── api/       # Axios client with JWT interceptor
│   │   ├── components/# Editor, Drawer, Reader, Envelope, UI primitives
│   │   ├── hooks/     # useAuth, useLetters
│   │   ├── store/     # Zustand (auth tokens, crypto keys)
│   │   ├── utils/     # Crypto, keystore (IndexedDB), letter logic
│   │   └── pages/     # Home, Login, Register, Drawer, Editor, Reader, About
│   └── e2e/           # Playwright specs and test utils
├── scripts/           # setup.sh, start.sh, run-e2e.sh
├── certs/             # Local SSL certificates (generated)
├── docker-compose.yml # Dev services (Postgres, Mailpit)
└── docker-compose.e2e.yml # Isolated E2E test services

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"Sometimes the wrong train takes you to the right station." — Saajan Fernandes, The Lunchbox