We built post-quantum encrypted email that works with Gmail. Here's how.

We built post-quantum encrypted email that works with Gmail. Here's how. Most encrypted email services have a glaring problem: their privacy guarantees only apply when both people use the same service. Email a Proton user from Gmail and you get standard TLS, not end-to-end encryption. That's the gap we built Aster Mail to close. The cryptography stack For Aster-to-Aster messages, we use a Signal-inspired protocol: X3DH for key agreement, Double Ratchet for forward secrecy, and ML-KEM-768 (CRYSTALS-Kyber) for post-quantum key encapsulation. This means every message gets a fresh encryption key, past messages stay protected if future keys are compromised, and the key exchange itself is resistant to quantum attacks. For external recipients (Gmail, Outlook, anyone), we use RSA-4096 PGP. Not post-quantum, but the best practical option for interoperability with the broader email ecosystem today. The rest of the stack: Argon2id (128MB, 3 iterations) for key derivation, AES-256-GCM for symmetri