ProtonKit

A native macOS Proton Mail client built with pure Swift and SwiftUI. No Proton Bridge required, no paid subscription needed.

ProtonKit directly interfaces with the Proton REST API, implementing SRP-6a authentication (Proton's non-standard variant), PGP encryption/decryption, and token-based key hierarchy from scratch in Swift.

Features

• SRP-6a Authentication - Full Proton SRP variant (little-endian, 4xSHA512 expandHash, bcrypt password hashing)
• TOTP 2FA support
• PGP Encryption/Decryption - Email encryption via ObjectivePGP with per-key passphrase mapping, supports both RSA and Curve25519 (ECDH) keys
• Token-based Key Hierarchy - User key -> address key via encrypted token
• Multi-Account - Outlook-style sidebar with per-account folder trees, independent sessions
• macOS Notifications - Background polling with native notifications, click-to-navigate, dock badge
• Session Persistence - macOS Keychain with namespace isolation per account, automatic restore on launch
• Three-Pane Layout - NavigationSplitView (folders / message list / message detail)
• Infinite Scroll - Paginated message loading (50 per page)
• HTML Rendering - WKWebView with sanitization, auto-height, link interception
• Message Actions - Trash, star, archive, move to folder, mark spam/not spam, permanent delete, empty Trash/Spam, block sender, mark all as read — via context menu, swipe, hover quick actions, toolbar, or keyboard
• Labels - Proton's dual folder/label system: colored labels in the sidebar with counts, apply/remove from the context menu, colored chips in the reading pane; custom folders use their API colors and nest with indentation
• Batch Select - Cmd+Click / Shift+Click multi-select with batch trash, archive, and mark read/unread
• Double-Click to Open - Double-click a message to open it in a separate window
• Thread View - Detail view lists the other messages in the same conversation
• Attachment Download - Download and decrypt PGP-encrypted attachments with save dialog
• Attachment Upload - PGP-encrypted attachment upload with per-recipient session key management (RSA + ECDH)
• Inline Images - cid: inline images rendered via a custom WKURLSchemeHandler with on-demand decryption
• Privacy Hardening - Viewer JavaScript disabled; remote images blocked by default (WKContentRuleList) with per-message "Load Images"; HTML sanitization
• Dark Mode - Adaptive rendering for both the message viewer and the compose editor
• Search - Keyword search across messages via Proton API
• Keyboard Shortcuts - Cmd+R reply, Shift+Cmd+R reply all, Shift+Cmd+F forward, Shift+Cmd+N refresh, Cmd+E archive, Delete trash, Cmd+Shift+U toggle read/unread, Cmd+D save draft
• Compose Windows - Reply / Reply All / Forward / New in separate windows with Bcc, From-address selector, signature, and discard confirmation
• Send Controls - Undo send (0–20s delay), schedule send, message expiration, plain-text mode — mirroring the official client
• Reading Tools - Print, view raw headers, QuickLook attachment preview, PGP signature verification with a lock badge
• mailto: Handler - Registers as a system mail client; "Set as Default Email Client" in Settings
• Save & Edit Drafts - Save drafts (Cmd+D), edit and send from Drafts folder with attachment preservation
• Event Sync Engine - Incremental event polling applies creates/updates/deletes/counts to the UI, recovers from stale event IDs, and drives notifications
• Offline & Cache - Accounts restore offline from disk snapshots; folder lists and message bodies (still PGP-encrypted at rest) cached for instant startup and offline reading
• Settings - Poll interval, notification toggle, remote-image policy, signature (Cmd+,)
• Error Feedback - Failed operations surface as toasts instead of silently diverging local state

Screenshots

Coming soon

Requirements

• macOS 14+ (Sonoma)
• Apple Silicon (arm64)
• Xcode Command Line Tools (xcode-select --install)

Build & Run

git clone https://github.com/JulianHunag/ProtonKit.git
cd ProtonKit

# Debug build + package
bash scripts/build-app.sh
open /Applications/ProtonKit.app

For release DMG:

bash scripts/build-dmg.sh
# Output: build/ProtonKit.dmg

Since the app is ad-hoc signed (no Apple Developer certificate), you may need:

xattr -cr /Applications/ProtonKit.app

Tech Stack

Component	Choice	Notes
Language	Swift 5.9+	Pure Swift, no ObjC
UI	SwiftUI (macOS 14+)	NavigationSplitView
PGP	ObjectivePGP 0.99.4	BSD, SPM via xcframework
SRP BigNum	BigInt 5.3+	MIT, pure Swift
Crypto	CryptoKit	X25519 ECDH, built-in
Build	SPM + bash scripts	No Xcode GUI required
Signing	Ad-hoc	No Apple Developer cert

Architecture

ProtonKit/
├── Sources/ProtonCore/          # Core library (SRP, API, crypto, session)
│   ├── ProtonSRP/               # SRP-6a authentication (Proton variant)
│   ├── ProtonAPI/               # REST API client (actor-based)
│   ├── ProtonCrypto/            # PGP encryption/decryption, attachment crypto, key passphrase
│   ├── Models/                  # API response models
│   └── Session/                 # Multi-account session management + Keychain
├── Sources/ProtonKit/           # SwiftUI application layer
│   ├── App/                     # App entry, ContentView
│   ├── Services/                # Notification polling service
│   └── Views/                   # Auth, Sidebar, MessageList, MessageDetail, Compose
└── scripts/                     # Build & packaging scripts

Key design decisions:

• Each account has its own ProtonClient (actor) and MessageDecryptor - fully isolated
• Keychain credentials namespaced by account UID ({uid}.accessToken)
• Attachment encryption uses self-managed session keys with per-recipient key packets (RSA + ECDH)
• SRP implementation ported from go-srp and hydroxide

References

• ProtonMail/go-srp - SRP reference implementation (Go)
• emersion/hydroxide - Open-source Proton Mail Bridge (Go)
• ObjectivePGP - PGP library
• Proton API behavior reverse-engineered from hydroxide source + network inspection

License

MIT