Nostr: How do I keep my Nostr private key safe?

Short answer

Keep the private key out of posts, screenshots, cloud notes, and unfamiliar websites. Use a trusted signer when supported so a client can request signatures without directly handling the key. Keep an offline or otherwise well-protected backup, and review each signing request. A relay only needs your public key and signed events; it should never ask for your nsec.

Sources

NIP-19: Human-readable encodings, NIP-07: Browser signers, NIP-46: Remote signing.