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

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.

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🏷️ 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)
          ...

• 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.

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🧩 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.

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🔧 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.

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🧪 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.

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🔭 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.

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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.