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Using Signal
The first time you run Signal, it will ask you to create a passphrase. This passphrase will be used to encrypt all of Signal's secret information, including the keys used to encrypt your text messages. The security of your messages depends on the strength of this passphrase, so make it good. Signal can be configured to cache this passphrase in memory for as long as its running, or for a specific length of time, so you won't need to be constantly re-entering it in order to access or send messages. This passphrase cannot be recovered if it is lost.
Once Signal has been set up, you may wish to disable the default Messaging application. This will prevent problems between the two applications, as well as ensure your SMS and MMS are encrypted upon receipt. To do this, go to Settings > Apps > All > Messaging and click 'Disable'. Reboot your phone to see this take affect.
All text messages are encrypted with your passphrase before being stored. This encryption includes the bodies of the text messages themselves, but the destination information is currently not encrypted.
Signal automatically detects when a message is received from another Signal user and prompts you to initiate a secure session. If you choose to initiate the secure session, a key exchange will ensue, and a lock icon will be displayed in the title bar of the conversation view as well as on the send button itself. A lock icon will also be displayed next to each encrypted message received, in order to confirm that it was transmitted securely.
It is prudent to verify the identity key of conversation's recipient, in order to ensure that no "man in the middle" attack has occurred. From the menu in a conversation, select "Conversation Settings" --> "Verify Identity." This will present you with an option to manually verify the recipient key's fingerprint, or to verify it by QR code scanning. If you're physically located in the same space as the recipient, you can select QR code scanning to quickly verify each-other's fingerprints. If you're remotely located, you can manually read the fingerprints to each-other over the phone.
Once you verify that the recipient's identity is correct, this information is saved and used to automatically authenticate future secure sessions with that recipient.
Encrypted backup is not supported at the moment. But you can check out Backup and restore Signal messages to try another way to transfer the encrypted data.
A lost password cannot be recovered. To continue using Signal when a password has been lost the app must be uninstalled and reinstalled, and all messages will be lost.
All messages can be deleted when starting at the conversation list using four steps:
- Press and hold on any message/conversation
- In the bottom right, press the "Select All" icon
- In the bottom left, press the "Delete" icon
- Press the "Delete" button on the warning dialog box
Contacts who are able to receive encrypted push Signal messages are shown in the "Select Contact" list with a green bar to the left of their name. The green bar indicators will only apply if you are registered for push.