EDIT: 6/6/2014 – Due to this update, it might be necessary to clear your browser cache before trying to log into ProtonMail, otherwise the mailbox decryption step may fail.
Effective immediately, we have removed the captcha from the ProtonMail login systems. Frankly, it was not very effective at deterring brute force attacks and makes the login process more cumbersome.
We have implemented a far more effective password brute force protection system now which will shortly be complemented with 2 factor authentication.
We want to stress that this is NOT a substitute for using strong passwords. The best way to avoid brute force attack is still to use a strong passphrase that has a good mix of upper/lowercase, numbers, and special characters (!@#$%^&* etc….).