End-to-End Encryption
SipherMail's end-to-end encryption (E2EE) protocol is called SIPHER-E2EE-V1. It uses RSA-OAEP 4096-bit for key exchange and AES-256-GCM for message encryption — all performed client-side in your browser or mobile app. We never see your unencrypted messages or private keys.
How it works
- Key generation: When you enable E2EE, your browser generates an RSA-OAEP 4096-bit key pair using the Web Crypto API. The private key never leaves your device in unencrypted form.
- Key storage: The private key is encrypted with AES-256-GCM using a key derived from your passphrase (PBKDF2, 310,000 iterations, SHA-256, 256-bit key). The encrypted private key is stored on SipherMail's servers.
- Sending: When you send an E2EE email, the message body is encrypted with a random AES-256-GCM session key. That session key is then encrypted with each recipient's RSA-OAEP public key. The server stores only ciphertext.
- Receiving: Your browser fetches the encrypted email, decrypts the session key using your RSA private key (unlocked with your passphrase), then decrypts the message body with the session key.
Enable E2EE
- Go to Settings → Security → End-to-End Encryption.
- Click Enable E2EE.
- Choose a strong passphrase. This is separate from your login password. Write it down — there is no recovery if lost.
- Your key pair is generated and the encrypted private key is uploaded to the server.
- Share your public key fingerprint with contacts you want to exchange E2EE email with.
⚠️ Passphrase warning
Your E2EE passphrase cannot be recovered if lost. There is no "forgot passphrase" option. If you lose your passphrase, you will permanently lose access to all E2EE-encrypted messages. Store your passphrase in a password manager.
Multi-recipient encryption
E2EE emails can be sent to multiple recipients. Each recipient's public key is used to encrypt a copy of the session key. Recipients without a SipherMail public key receive a standard (non-E2EE) fallback copy of the email.
Mobile parity
E2EE is fully supported on iOS and Android. The same SIPHER-E2EE-V1 protocol runs in the mobile apps. You can read and compose E2EE emails on any device where you've unlocked your private key with your passphrase.
E2EE vs server-side encryption
All mailbox data is encrypted at rest on SipherMail servers (server-side encryption). E2EE goes further: messages are encrypted before they leave your device, so even SipherMail cannot read them. E2EE is optional and must be explicitly enabled.