69 lines
3 KiB
Text
69 lines
3 KiB
Text
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pragma cashscript ^0.13.0;
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// USD-pegged sale of a BCNR name certificate (CashTokens NFT).
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//
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// The certificate sits in this covenant while it is for sale. Anyone may
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// buy it by paying the seller the BCH equivalent of `targetCents` at the
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// oracle's current price; the seller may reclaim it at any time.
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//
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// Oracle message (40 bytes): timestamp as uint32 LE, price in USD cents per
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// BCH as uint32 LE, then the 32-byte txid of the listing's own outpoint,
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// signed by `oraclePk` (Schnorr data signature over the sha256 of the
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// message, as checkDataSig expects).
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//
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// What the chain can and cannot enforce: script can require a transaction
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// to be mined no EARLIER than a time (nLocktime), never no later, so a quote
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// cannot expire on-chain. The [floorSats, ceilSats] band the seller chooses
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// is therefore the real guarantee: whatever quote a buyer presents, the
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// payout stays inside it. Binding the quote to the listing's outpoint means
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// cancelling and relisting voids every quote ever issued. The locktime rule
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// below still stops a quote from being used before the oracle issued it.
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contract UsdListing(
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pubkey oraclePk,
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bytes20 sellerPkh,
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int targetCents,
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int floorSats,
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int ceilSats,
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int maxAgeSecs
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) {
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// Buyer path. Output 0 pays the seller; output 1 carries the certificate
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// to wherever the buyer wants it. Remaining outputs are the buyer's.
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function buy(bytes oracleMsg, datasig oracleSig) {
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require(oracleMsg.length == 40);
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require(checkDataSig(oracleSig, oracleMsg, oraclePk));
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bytes tsBytes, bytes rest = oracleMsg.split(4);
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bytes priceBytes, bytes quotedTxid = rest.split(4);
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int ts = int(tsBytes);
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int priceCents = int(priceBytes);
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require(priceCents > 0);
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require(quotedTxid == tx.inputs[this.activeInputIndex].outpointTransactionHash);
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// Not before the quote: nLocktime is checked against median-time-past
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// (about an hour behind the clock), so the buyer sets it a couple of
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// hours behind the quote and the contract allows that slack.
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require(tx.locktime + 10800 >= ts);
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require(tx.locktime <= ts + maxAgeSecs);
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int need = targetCents * 100000000 / priceCents;
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if (need < floorSats) { need = floorSats; }
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if (need > ceilSats) { need = ceilSats; }
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// Seller is paid first, in plain BCH.
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require(tx.outputs[0].lockingBytecode == new LockingBytecodeP2PKH(sellerPkh));
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require(tx.outputs[0].value >= need);
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require(tx.outputs[0].tokenCategory == 0x);
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// The certificate leaves intact: same category, same commitment.
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require(tx.outputs[1].tokenCategory == tx.inputs[this.activeInputIndex].tokenCategory);
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require(tx.outputs[1].nftCommitment == tx.inputs[this.activeInputIndex].nftCommitment);
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}
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// Seller path: reclaim the certificate (cancel the listing, or re-list
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// at a new target).
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function cancel(pubkey pk, sig s) {
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require(hash160(pk) == sellerPkh);
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require(checkSig(s, pk));
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}
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}
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