How you go about this is up to you-there are plenty of apps and methods, depending on your make of phone and the version of Android you're using-but you could, for example, use the Files app on stock Android to share the backup to Google Drive, then download it and put it back in the same folder on your new device. You then need to manually move your most recent backup over to your new phone: It'll be in the "/Internal Storage/Signal/Backups" or "/sdcard/Signal/Backups" folder. On your old device, it's enabled through the Chats and media and Chat backups options on the Signal settings screen-again, make sure you note down the passphrase that's displayed on screen, because you're going to need it to get your chats back on a new device. Full list of changes in Appendix D.On Android, you need to make use of the backup feature we've already mentioned above. Note: Fix omission in description of initial X3DH handshake, reorganize figures for improved presentation. We have found no major flaws in the design, and hope that our presentation and results can serve as a foundation for other analyses of this widely adopted protocol. We then prove the security of Signal's key exchange core in our model, demonstrating several standard security properties. We extract from the implementation a formal description of the abstract protocol, and define a security model which can capture the ratcheting key update structure as a multi-stage model where there can be a tree of stages, rather than just a sequence.
We conduct a formal security analysis of Signal's initial extended triple Diffie-Hellman (X3DH) key agreement and Double Ratchet protocols as a multi-stage authenticated key exchange protocol. Signal includes several uncommon security properties (such as "future secrecy" or "post-compromise security"), enabled by a novel technique called *ratcheting* in which session keys are updated with every message sent. The Signal protocol is a cryptographic messaging protocol that provides end-to-end encryption for instant messaging in WhatsApp, Wire, and Facebook Messenger among many others, serving well over 1 billion active users.
Katriel Cohn-Gordon, Cas Cremers, Benjamin Dowling, Luke Garratt, and Douglas Stebila Paper 2016/1013 A Formal Security Analysis of the Signal Messaging Protocol
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