How the @@ Protocol Works

flowchart TB
    subgraph SEND["📤 SEND"]
        A["1. Compose email to
user@@domain.com"]
        B["2. Fetch recipient's
RSA-4096 public key"]
        C["3. Build .eml file
(RFC 822: headers + body + attachments)"]
        D["4. Compress with gzip level 9"]
        E["5. Hybrid encryption:
AES-256-GCM (data) + RSA-4096 OAEP (AES key)
🔐 Authenticated encryption"]
        F["6. ⚡ ENVELOPE detachment
eHead = encrypted AES key
eBody = nonce + tag + ciphertext"]
        G["7. Store eBody on server
(SFTP external or local storage)
Referenced by hashed token"]
        H["8. Generate download token
(256-bit, stored as SHA-256 hash)"]
        I["9. Encrypt token with
recipient's public RSA key"]
        J["10. Send regular email:
📎 eHead.bin attachment
🔑 Encrypted token in body"]
    end

    subgraph TRANSIT["📡 IN TRANSIT"]
        K["eHead travels via SMTP
through mail servers"]
        L["eBody NEVER travels
⬅ stays on server ⬅"]
    end

    subgraph RECEIVE["📥 RECEIVE"]
        M["11. Recipient opens email
saves eHead attachment"]
        N["12. Decrypts token with
private RSA key
🔑 Session key wallet (enter once)"]
        O["13. Download page:
Token via POST/session
(no URL leak, hashed in DB)"]
        P{"PIN required?"}
        Q["Enter 6-digit PIN
🛡️ Progressive delay:
1s→2s→4s→8s→lockout"]
        R{"2nd-channel (PWA)
enabled by sender?"}
        S["📱 Authorize via PWA
on mobile phone"]
        T["5-minute download window
🛡️ Rate-limited: 5 att/5min"]
        U["14. Download eBody.bin"]
    end

    subgraph DECRYPT["🔓 FINAL DECRYPTION"]
        V["15. Upload 2 files:
eHead.bin + eBody.bin
🔑 Key from session wallet"]
        W["16. Parse envelope:
eHead → extract AES key
eBody → extract nonce+tag+ciphertext"]
        X["17. Hybrid decrypt:
RSA-4096 decrypts AES key →
AES-256-GCM authenticates + decrypts"]
        Y["18. Decompress (gzip)"]
        Z["✅ Original .eml file
Openable in any email client"]
    end

    A --> B --> C --> D --> E --> F
    F --> G
    F --> H
    G --> J
    H --> I --> J
    J --> K --> M
    L -.-|"eBody never leaves the server"| U
    M --> N --> O --> P
    P -->|"Yes"| Q --> R
    P -->|"No"| R
    R -->|"Yes"| S --> T --> U
    R -->|"No"| U
    U --> V --> W --> X --> Y --> Z

    style F fill:#ffc107,color:#333,stroke:#e0a800
    style L fill:#dc3545,color:#fff,stroke:#c82333
    style S fill:#ff6b6b,color:#fff,stroke:#e55a5a
    style Z fill:#28a745,color:#fff,stroke:#1e7e34