Ditto is on Nostr but federates to the Fediverse via
nostr:npub1d03clrrrma7mlpxm0mz2dehmhkx3nh9rhxqwltgctpwyduztymustepzfs, and also implements Mastodon API for app compatibility.
,
Under the hood, it's the same as Nostr with some Mastodon-flavored bells and whistles. You own your keys, etc.