Signal and WhatsApp, Softer Than Tofu?

As a result, Keybase uses a key hierarchy so that by transferring 32 bytes of secret data from the old device to the new, the new device can see all long-lived cryptographic data (see the FAQ below for more details). When Alice provisions a new device, her old device knows the secret half of her PUK and the new device’s public key. She encrypts the secret half of the PUK for new device’s public key, and the new device pulls this ciphertext down via the server.

Source: keybase.io