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Cover image for Blocktrails: Smart-Contract Functionality - On Bitcoin, No Extra Token Needed
Melvin Carvalho
Melvin Carvalho

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Blocktrails: Smart-Contract Functionality - On Bitcoin, No Extra Token Needed

#blocktrails #bitcoin #smartcontract #nostr

Picture a relay-race baton hurtling around a track.
Before each hand-off the runner stamps the baton with today’s secret code, then sprints ahead.
Every swap is signed, time-stamped, and etched into Bitcoin forever—so anyone can replay the entire race and verify there was never a duplicate baton.

That baton is a Blocktrail. It gives you contract-grade state changes, provable settlement, and full auditability—powered solely by Bitcoin, no new chain or side token in sight.

🏷️ What’s a Blocktrail?

A Blocktrail is a tiny, self-contained ledger riding inside Bitcoin’s main ledger.

state₀ —sha256→ tweak₀ (GENESIS UTXO) spend → state₁ —sha256→ tweak₁ (UTXO₁) spend → state₂ —sha256→ tweak₂ (UTXO₂) ...
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  • One live output at a time—Bitcoin’s double-spend rule enforces the sequence.
  • SHA-256 of your app’s state (a Git commit, JSON blob, DAG root—anything hashable) becomes the Taproot tweak for the next output.
  • Whole history on-chain: fetch the tweaks, re-hash the snapshots, and you can replay every step without trusting anyone.

Result: all the “why” and “when” of a smart contract, with nothing but ordinary transactions.

🧩 Think “Git Commits on Bitcoin”

  1. Genesis: create state₀, hash it, anchor it as the GENESIS transaction.
  2. Next commit: update your data → state₁, hash again, tweak again, spend GENESIS.
  3. Repeat: the latest UTXO is your repo HEAD; the chain of tweaks is the commit log.
  4. No forks: Bitcoin consensus permits only one spend of the live UTXO—linear history guaranteed.

🔧 Blocktrail in (Pseudo) JavaScript

Functions like createGenesisUTXO, spendUTXO, and auditChain are illustrative.
A real blocktrails-js library is the very next milestone.

// keypair controls the trail (can double as Nostr pubkey) const trailKey = generateKeypair(); // { priv, pub } // helper: hash any client-side state object const hashState = state => sha256(JSON.stringify(state)); // 1. GENESIS let state = { owner: "alice", balance: 0 }; let genesis = createGenesisUTXO({ pubkey: trailKey.pub, value: 546n, // sats tweak: hashState(state) // hash of state₀ }); console.log("GENESIS:", genesis.outpoint); // 2. first update → UTXO₁ state.balance += 100; let utxo1 = spendUTXO({ prev: genesis, privkey: trailKey.priv, value: 520n, // minus fee tweak: hashState(state) // hash of state₁ }); console.log("UTXO₁:", utxo1.outpoint); // 3. another update → UTXO₂ state.owner = "bob"; let utxo2 = spendUTXO({ prev: utxo1, privkey: trailKey.priv, value: 500n, tweak: hashState(state) // hash of state₂ }); console.log("UTXO₂:", utxo2.outpoint); // 4. audit anytime const history = auditChain({ tip: utxo2.outpoint }); history.forEach(step => console.log(step.outpoint));
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🏗️ Why You Might Care

Need Blocktrail Delivers
Smart-contract enforcement Only one valid next state—no VM required.
Double-spend safety Bitcoin consensus blocks competing branches.
Tamper-proof history Each state hash is burned into the chain.
Full replay Rebuild state locally, match hashes, verify every step.
Zero extra infra Just Bitcoin transactions. No gas, no databases.

🧪 What Can You Build?

  • 🔄 Supply-chain docs — every warehouse hashes its manifest, tweaks, spends, passes on.
  • ✅ One-person-one-vote — ballot UTXO’s tweak is sha256(choice), binding voter intent to a single spend.
  • 🎮 Game quests — quest token’s tweak is sha256(save-game), so progress is provably unique.
  • 📈 Prediction markets — each market state (order book, odds, settlement) hashes into the next tweak; payouts are locked to the final on-chain hash.
  • 🧾 Regulator-grade receipts — hash your PDF invoice, tweak, create an immutable audit link.
  • 💬 Social attestations via Nostr — hash a Nostr event (or thread root), tweak it into the next Blocktrail output, and anchor every reply or edit to Bitcoin—giving posts, likes, or channel memberships an immutable, on-chain audit path while the conversation itself stays lightweight and off-chain.

🔭 Next Up: blocktrails-js

We’re drafting an open-source JavaScript SDK that will:

  • Wrap Bitcoin Core or external signers (Ledger, Trezor, WebLN).
  • Provide simple helpers: createGenesisUTXO, spendUTXO, auditChain.
  • Ship with wallet components that surface “current tip” and commit hashes at a glance.
  • Offer explorer overlays that draw Blocktrails as subway lines on mempool graphs.

The library is the next mile on this trail—stay tuned.

TL;DR

Blocktrails link Bitcoin outputs with Taproot tweaks that are SHA-256 hashes of your app’s state.
Start with a GENESIS output, hash-tweak each new state, spend once, and keep moving.
You get smart-contract guarantees—deterministic execution, audit, provable settlement—with nothing but lean, ordinary Bitcoin transactions.

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