There aren't much tools and protocol-level solutions to key loss or theft atm. But that's just because nobody is trying to use them for mission critical stuff, once demand comes, solutions will come. I will keep repeating that nostr keys have lots in common to bitcoin keys, and we do hope to make bitcoin the core of our future economy, so how is nostr different in principle? More here: nostr:nevent1qqs0qkyxmykx2a5f98e88c2ayyz44z53h8ntvqp0fusge4r62m9m7mcql9f4x