Two-way encryption: I need to store passwords that can be retrieved

Question

I am creating an application that will store passwords, which the user can retrieve and see. The passwords are for a hardware device, so checking against hashes are out of

Answer 1

I'd only suggest public key encryption if you want the ability to set a user's password without their interaction (this can be handy for resets and shared passwords).

Public key

1. The OpenSSL extension, specifically openssl_public_encrypt and openssl_private_decrypt
2. This would be straight RSA assuming your passwords will fit in key size - padding, otherwise you need a symmetric layer
3. Store both keys for each user, the private key's passphrase is their application password

Symmetric

1. The Mcrypt extension
2. AES-256 is probably a safe bet, but this could be a SO question in itself
3. You don't - this would be their application password

Both

4. Yes - users would have to enter their application password every time, but storing it in the session would raise other issues

5.

• If someone steals the application data, it's as secure as the symmetric cipher (for the public key scheme, it's used to protect the private key with the passphrase.)
• Your application should definitely be only accessible over SSL, preferably using client certificates.
• Consider adding a second factor for authentication which would only be used once per session, like a token sent via SMS.