Security

I'm in no way a security expert, which makes it scary when trying to create a secure system for dealing with (personal) emails.

These are the things we thought about, or that still pose problems:

• MailServer to MailServer uses TLS with ECDHE for perfect forwarding secrecy.

• Everything depends at the moment on a single private RSA key (4096 bits). It is used for the following:

  • logging into the mailserver to read emails
  • logging into the mailserver to send emails
  • decrypting emails

• Once a private key is exposed, the following can happen

  • we can write emails on behalf of somebody
  • we can decrypt the catalog-key inside each message-header, which results in:
  • we can decrypt the blob-keys inside each message-catalog, which results in:
  • we can decrypt each blob of each message

• Exposing a public key by itself means you need to know the actual email address. The only unencrypted email address reference is a SHA256 sum.

Could we?

• Private key should have a pass-phrase. Can we force this?
• Can we store the private key securely in for instance the OSX keychain for example?
• Can we decrease the attack surface? When a private key is compromised, we might be able to send/receive emails (provided we also know the actual email address for this private key), but can we somehow make sure we can't decrypt the emails?
• Could every email be encrypted with a different key (like DH?). How would this work?