IMHO, it's very telling that the FBI is recommending specifically WhatsApp, Facebook Messenger and Signal.

All of which use Axolotl/Double Ratchet.

Moreover, writing as someone who personally knows Moxie and another Signal dev, as well as having previously worked with an individual who later became CSO at Facebook.

sigh Let's just say that I don't think Crypto AG or NIST's Dual_EC_DRBG examples of backdoored cryptography were unique nor the last we've seen of such things. I would be extremely leery of anything the FBI ever recommended in E2EE apps.