What You Will Be Able To Do

• Explain oracles and the oracle problem.
• Describe cross-chain messaging and its trust.
• Compare ActivityPub and Nostr.
• Sign and verify an event in code.
• List local-first principles.

Why Systems Must Talk

• Each system keeps its own state.
• Contracts need outside facts.
• Users want to move value between chains.
• People want to follow friends on other servers.

Oracles

• A contract reads only its own chain.
• An oracle brings outside data on chain.
• Examples: prices, weather, sports results.
• The contract inherits the oracle's trust.

web price  -> oracle -> chain data
chain data -> lending contract

Narrowing The Oracle Problem

• Many independent reporters, median result.
• Time-averaged prices from on-chain markets.
• Bonded disputes in optimistic oracles.
• Signed data from the original publisher.

Three Oracle Questions

• Who reports the data?
• What does it cost them to lie?
• How long do we wait before acting?

Cross-Chain Messages

• Chain B cannot see chain A directly.
• Someone must report events from A to B.
• B must decide whether to believe the report.
• A bridge carries messages and assets.

chain A: lock 10 tokens
   -> message
chain B: mint 10 wrapped tokens

Four Bridge Designs

• External verifiers: trust a signer group.
• Light clients: trust the source chain consensus.
• Optimistic: trust watchers in a challenge window.
• Zero-knowledge: trust a proof.

Bridge Habits

• Move only the value you need.
• Prefer public trust assumptions.
• Limit what one bridge can hold.
• Plan for bridge failure.

Federation

• Many servers speak one protocol.
• Email is the classic example.
• Users pick a server and its rules.
• Servers exchange messages on behalf of users.

ActivityPub And Mastodon

• W3C standard published in 2018.
• Actors have an inbox and an outbox.
• Mastodon, PeerTube, and Pixelfed use it.
• The network of servers is the fediverse.

Ana@server1 posts note
server1 -> inbox of Bo@server2
server2 shows note to Bo

Nostr

• Your identity is a key pair.
• Posts are signed JSON events.
• Relays store and forward events.
• Clients verify every signature.

{"pubkey": "...", "kind": 1,
 "content": "hello", "sig": "..."}

Federation Or Keys

• ActivityPub: identity lives on a server.
• Nostr: identity lives in a key.
• Federation gives strong local moderation.
• Keys give portability and key safety duties.

Local-First Software

• Your data lives on your devices first.
• The app works offline.
• The network syncs copies.
• Written up in 2019 by Kleppmann and colleagues.

CRDTs

• Separately edited copies merge automatically.
• Order of merging does not change the result.
• Automerge and Yjs are popular libraries.
• Encryption keeps relays from reading content.

A edits offline: add 'x'
B edits offline: add 'y'
merge(A, B) == merge(B, A)

Recap

• Oracles import facts, so budget the cost of lying.
• Bridges differ in who they trust.
• Federation ties identity to servers, Nostr to keys.
• Local-first keeps data with its owner.

Next Week: Communities And Institutions

• Network states and pop-up cities.
• Cooperatives and Ostrom's commons.
• Running a decentralized community.

Questions And Exercise

• Exercise: sign and verify a Nostr-style event.
• Test tampering and write what you see.
• Read the local-first essay.