Ciphermail

Features

Encrypt Emails: Secure your emails with OpenPGP encryption
Decrypt Emails: Read encrypted emails directly in Gmail
Sign Messages: Digitally sign your emails for authenticity
Verify Signatures: Automatically verify signatures on received messages
Key Server Lookup: Search for recipient public keys on HKP servers
Key Management: Generate, export, import, and rotate your PGP keys
Secure Storage: Your private keys are encrypted with AES-256-GCM

Security

RSA-4096 bit key generation
AES-256-GCM encryption for local key storage
PBKDF2 with 310,000 iterations for key derivation
Brute-force protection (5 attempts = 30 min lockout)
Passphrase strength validation
Automatic clipboard clearing
Session timeout

The extension is open sourced.