I am not sure how mint does it, but several ways to store highly secure data is since you asked for it * Boxes have no direct internet connection * Spread

I am not sure how mint does it, but several ways to store highly secure data is since you asked for it * Boxes have no direct internet connection * Spread the Ciphertext, Decryption Keys, and Algorithm across multiple yet different machines (each with unique distributions/operation systems and data storage solution). * Instead of a global salt, use a separate salt per each user (login name / account creation time) * Generate the encryption/decryption keys for storing the credentials using a newhash(login+password) and use it only when the user logs in (this hash is never stored, it's hashed, generate the encryption/decryption keys, decrypt the data, use the data, and destroyed). So this key only awailable for a short period of time when the user logs in. * Use something strong to encrypt like PGP as the base and roll-your-own obscure one on top of it (which is a non-standard algorithm) There will always be holes and weaknesses, it's just making it really really hard, never impossible.