> One possible way to backdoor it might be mandate that companies keep copies of the encrypted messages, tagged with a device ID. Then to decrypt you need to get the person's phone, which is a clearer analogy to getting a warrant to search someone's house to look for things they have stashed.

That doesn't work if the message has been encrypted on transit with an one-time key, which is discarded after the message has been received. The Axolotl system (used by TextSecure/Signal and AFAIK the new encryption on WhatsApp) does that. Once the message has been received (and thus the corresponding key discarded), any copy of the encrypted message becomes useless.

The current tendency in protocol design is to use ephemeral keys (usually through some Diffie-Hellman variant) whenever possible, since it's more secure.

> With SSL they can just get a warrant (or you know, don't get a warrant) and look at the server, where everything is in plain text.

That doesn't work if the server is outside their jurisdiction.