Ditto is on Nostr but federates to the Fediverse via nostr:npub1d03clrrrma7mlpxm0mz2dehmhkx3nh9rhxqwltgctpwyduztymustepzfs, and also implements Mastodon API for app compatibility.

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Under the hood, it's the same as Nostr with some Mastodon-flavored bells and whistles. You own your keys, etc.