An introduction to mailbox Guard

PGP Backup

Mails that are encrypted by your communication partners with your public key and then transmitted to our servers can only be decrypted with the matching private key and the corresponding password. The same applies if you have activated the encrypted mailbox.

The team at mailbox has no replacement or secondary key. We cannot decrypt your encrypted mails even if you have stored your private key in Guard, since Guard is also protected by a password. We have no means of restoring the plaintext of your mails. If we could, encryption would be pointless and would not provide you with any security benefit.

Therefore, always keep a backup of your private key in a secure place, for example in an encrypted container file. For the corresponding passwords, we recommend a backup in an encrypted password database such as KeePass or KeePassX.

Public keys of communication partners

In the key management, you can also manage the keyring with the public keys of your communication partners. You can find the list under

All settings | mailbox Guard | Guard default settings | Your PGP keys.

By clicking on Your PGP keys you can upload additional public keys (e.g. as .pgp or .asc file).

Furthermore, you can also manage the keys of your communication partners directly in the mailbox Guard. To do this, open the Guard settings and select the section PGP keys of recipients. In the displayed list, you can add new public keys (e.g. as .pgp or .asc file) by clicking on the field Upload recipient key and thus extend the keyring.

Figure 1: Contact card overview.

Uploading keys – maximum file size

Note: You can upload public keys with a maximum size of 65k. This is sufficient for most keys. If the key contains photos or a very large number of signatures and exceeds 65k, upload errors may occur.

To reduce oversized keys, you can import them into your local key management with GnuPG and then export them with the option --export-options export-minimal for upload:

gpg2 --armor --export-options export-minimal --export Your-Key-ID > key4upload.asc

If you do not have or do not want to use a local installation of GnuPG, you must ask your respective communication partner to provide such a minimal key.

Deleting an old key

Warning: If you have already encrypted mails or data with an inactive, old key, you will no longer be able to decrypt them once you delete that key. This also affects your inbox encryption, if enabled.

Inactive keys can be deleted by clicking on the trash can icon. If this involves a private key, the Guard password will be requested.

Which PGP standards does Guard support?

The mail encryption used by the mailbox Guard is 100% compatible with the (Open)PGP standard.

PGP/MIME and PGP/Inline encrypted mails can be sent, received, signed, and verified in the mailbox web client. This allows you to communicate securely with users of other PGP-compatible mail clients or plug-ins.

When you start the mailbox Guard for the first time, an RSA 2048 bit key pair is created. As of December 2020, this is considered sufficiently secure.
Those who require a particularly high level of security can also manually create an ECC Curve25519 PGP key pair.

Manual creation of a Curve25519 key with GnuPG (Linux)

If you require a particularly high level of security, you can manually create your own ECC Curve25519 PGP key pair using GnuPG.

Requirements

• Installed gpg2 (GnuPG)

Step-by-step guide

Open a terminal and enter the following command:

gpg2 --expert --full-gen-key

You will then be asked:

Please select what kind of key you want: ... ECC and ECC ...

Then choose:

Please select which elliptic curve you want: Curve 25519 ...

The generated key pair can then be imported into the mailbox Guard.

How the private key is protected

When you upload a PGP key (1) or create one via the Guard, this key is encrypted with a password known only to you. This password is never stored on our systems and must be entered by you separately when decryption (e.g. to read an encrypted mail or edit an encrypted text file) is required. As long as you do not enter the Guard password, the key files remain encrypted on the server – and thus inaccessible (even to us).

Secure use of your key

When you log in, your key (1) is decrypted for exactly one action: to be encrypted with a newly generated random key (2) for this login and temporarily stored at mailbox. The temporary password is cached in your browser, and the temporarily encrypted PGP key (2) is stored on our mailbox server. Neither your respective passwords nor your PGP key (1) are ever permanently stored unencrypted on disk or in the program memory of our servers.

Each side only has "half" the knowledge of the information required to access the PGP key (1) – and therefore your encrypted data. Even if a third party could gain access to one of the two pieces of information, they could not compromise your sensitive data.

Once you log out, the temporary, encrypted copy of the key (2) is deleted. Even if someone later reads out the temporary password from your login session, it has already become useless once you have logged out.

Attack scenarios

If an attacker intercepted the temporary key (2) during transmission to the user's browser, they would have no way of accessing the genuine PGP key (1), which continues to be stored only on the server. An attacker’s login would initiate a new, independent session in which the stolen temporary password would be worthless.

An attacker, however, who has (direct or remote) access to your device and thus already local control over your web browser, could take over the current session of your mailbox login from the browser and thus also gain possession of the temporary key (2) – although this would only be valid briefly during your current login and therefore could not be used permanently by the attacker. At this point, however, the attacker would already have local access to your device and your browser and therefore also to your mails and all data currently being processed there – regardless of whether they are encrypted or decrypted locally on your PC with a PGP installation or server-side by our Guard.

Thus, the question of the security of each user's electronic device arises. The use of the mailbox Guard does not open up any security gaps or attack vectors that are not already inherent in the hardware, the operating system, and installed third-party software. On the contrary: one could even say that when using the Guard, the attacker at least cannot immediately access the user's PGP keys, since these are stored on the mailbox servers.