Detail best practices for when a user switches to a new device, focusing on how to migrate end-to-end encryption keys securely.

Migrating end-to-end encryption (E2EE) keys securely when switching to a new device is a critical process that must be handled carefully to avoid compromising the security of past and future communications. Improper key migration can expose users' encrypted messages, therefore following best practices is essential to ensure a smooth and secure transition. The general approach is to backup your keys and transfer them securely to the new device.

First, understand the key types you need to transfer. In most E2EE systems, you typically have several types of keys, such as the main identity key, session keys, and prekeys. The identity key is the most important long-term key used to verify a user's identity, and its private part must be transferred securely. Session keys are ephemeral keys used for short-lived communications and can be usually re-negotiated. Prekeys are used to initiate new conversations. It is important to know which keys need to be transferred and which can be regenerated on the new device. Also, ensure that you have a backup of all your keys available, as the process might go wrong.

The recommended approach is to use a built-in key migration or backup and restore feature provided by the messaging application that is being used. Most modern E2EE messaging apps offer such a mechanism and it is the most recommended method. These systems are often designed to automatically encrypt and securely transfer keys using cryptographic methods. For example, the messaging app may use a method of creating an encrypted backup using a password or a PIN that you specify. The backup is then stored securely, usually on your device or in the cloud. When the new device is initialized, the user uses the password or PIN to decrypt and import the backup, transferring their keys securely. Always ensure that the software version on the new device is at least as current as the old device as a precaution.

A common option for key migration is through a QR code. The E2EE messaging app can present a QR code on the old device containing your encrypted keys. The new device uses the camera to scan the QR code, decrypting and importing the keys, which is often done with a password that the user has specified. This approach is secure if the QR code is displayed directly on the user's old device because no intermediary can intercept it. This method can only be used if both devices are available and it is very important to protect the physical location to prevent man-in-the-middle attacks.

Another key migration method may involve the use of a cloud backup. In this approach, keys are backed up to a secure cloud storage solution provided by the app. The keys should be encrypted with a password or other strong cryptographic key, to protect keys from unauthorized access. When setting up a new device, you should be required to log in with your credentials and then use a recovery password to restore the keys to the new device. With this system the user must be aware of the dangers of cloud storage and ensure they choose a strong password. Also, the user must trust the cloud storage system because if the cloud system is compromised, so will be the user's keys.

If your messaging app does not offer a convenient migration method, it might require manual transfer of keys which is generally not recommended and should be done with extreme care. This usually involves transferring the keys through another medium, such as a file transfer via USB or using a secure messaging application. However, manually transferring keys is a difficult and error-prone process and not recommended unless all the other methods fail. If you must do this manually, it is vital to ensure the keys are encrypted and transferred securely. You could use a secure file transfer method to transfer encrypted key files from the old device to the new one. It is also very important that the key files are deleted from the old device once the transfer is complete.

Regardless of the key migration method used, certain safeguards should be implemented. It is essential that the old device is wiped or reset securely after migrating to a new one, and to do this make sure that all data, including the private keys, are removed and cannot be recovered. Doing this can help prevent any unauthorized access to the device or the user’s encryption keys. Also, before completing any key migration method, always check that all security codes are the same on both devices. The security codes, which are usually displayed as a series of numbers or QR codes, are used to visually verify that the devices have been successfully connected, which also verifies that they have exchanged their encryption keys successfully, preventing MitM attacks.

Always use a strong password or PIN to protect the transferred keys. It is recommended to use strong, unique, complex passwords that are not used anywhere else. Also, the systems should use a key derivation function that is resistant to brute force attacks. Enable two-factor authentication (2FA) on the messaging app to provide an extra layer of protection during login and key transfer, particularly on the new device.

In summary, when switching to a new device, follow built-in key migration tools in the app where possible. If not, then use the QR code method, or the cloud backup method as alternative options. Carefully evaluate which key migration method to use and always make sure the old device is wiped and that all security codes match, and that two-factor authentication is enabled. Also, if the keys must be transferred manually, it should be done with extreme care. By adhering to these best practices, users can ensure that their keys are securely migrated, protecting their E2EE communications.