Technical Report · September 2024

Bitcoin Timelocks as Unforgeable Timestamps

Using Bitcoin's OP_CHECKLOCKTIMEVERIFY for cryptographic proof that a dead man's switch was armed at a specific point in time — verifiable by anyone, forgeable by no one.

1. The Timestamp Problem

A dead man's switch needs to prove when it was armed. Without this proof:

Traditional timestamps (server logs, signed documents) require trusting the timestamping authority. Bitcoin provides an alternative: a globally synchronized clock secured by proof-of-work, where timestamps are embedded in an append-only ledger that cannot be retroactively modified.

2. Bitcoin Time

2.1 Block Height vs. Unix Time

Bitcoin has two notions of time:

Block height is more reliable for timelocks. Timestamps can drift by up to 2 hours from real time; block height is monotonically increasing and deterministic.

2.2 The Timechain

Bitcoin's blockchain is often called a "timechain" because it provides a global ordering of events. If transaction A is in block 800,000 and transaction B is in block 800,001, we know A happened before B — and this ordering is secured by the cumulative proof-of-work of all subsequent blocks.

3. OP_CHECKLOCKTIMEVERIFY

3.1 Overview

OP_CHECKLOCKTIMEVERIFY (CLTV) is a Bitcoin Script opcode that makes a transaction output unspendable until a specified block height or timestamp. It was activated in BIP-65 (December 2015).

3.2 Script Structure

OP_IF
  <locktime> OP_CHECKLOCKTIMEVERIFY OP_DROP
  <user_pubkey> OP_CHECKSIG
OP_ELSE
  <user_pubkey> OP_CHECKSIG
OP_ENDIF

// Two spending paths:
// 1. After locktime: user can spend (proves they're alive)
// 2. Before locktime: user can spend (reset the timer)

3.3 Verification Logic

When a transaction attempts to spend a CLTV-locked output:

  1. The spending transaction's nLockTime must be ≥ the script's locktime
  2. The spending transaction's nSequence must be < 0xFFFFFFFF
  3. The current block height (or timestamp) must be ≥ nLockTime

If any condition fails, the transaction is invalid and won't be mined.

4. EchoLock Integration

4.1 Purpose

In EchoLock, Bitcoin timelocks are supplementary, not required. The system functions without them — Nostr heartbeats and Shamir shares provide the core mechanism. Bitcoin adds:

4.2 Flow

// 1. User creates timelock transaction
const locktime = currentBlockHeight + (72 * 6); // ~72 hours
const timelockTx = createTimelockTransaction(
  userPubkey,
  locktime,
  amount
);

// 2. Broadcast to network
const txid = await broadcastTransaction(timelockTx);

// 3. Wait for confirmations
await waitForConfirmations(txid, 6);

// 4. Store txid in Nostr heartbeat event
heartbeatEvent.tags.push(['btc_timelock', txid]);

4.3 Guardian Monitoring

Guardians can query the timelock UTXO status: 

async function checkTimelockStatus(txid: string) {
  // Query mempool.space API
  const utxoStatus = await fetch(
    `https://mempool.space/api/tx/${txid}/outspend/0`
  );
  const data = await utxoStatus.json();
  
  if (data.spent) {
    // User spent the UTXO — they're alive, timer reset
    return { status: 'reset', spentInTx: data.txid };
  }
  
  // Check if locktime has passed
  const tx = await fetch(`https://mempool.space/api/tx/${txid}`);
  const txData = await tx.json();
  const currentHeight = await getCurrentBlockHeight();
  
  if (currentHeight >= txData.locktime && !data.spent) {
    // Locktime passed and unspent — potential trigger
    return { status: 'expired', locktime: txData.locktime };
  }
  
  return { status: 'active', locktime: txData.locktime };
}

5. Security Properties

5.1 Unforgeability

Bitcoin timestamps cannot be forged or backdated:

5.2 Public Verifiability

Anyone can verify a timelock's existence and status:

No special access, no API keys, no accounts required.

5.3 Censorship Resistance

Miners can theoretically censor transactions, but:

6. Cost Analysis

Creating and resetting a timelock incurs transaction fees:

Operation Size (vbytes) Fee @ 10 sat/vB
Create timelock ~200 ~2,000 sats ($0.60)
Reset (spend + recreate) ~300 ~3,000 sats ($0.90)
Annual cost (weekly reset) — ~156,000 sats ($47)

Assumes $30,000/BTC. Actual costs vary with fee market conditions.

Cost Optimization
EchoLock makes Bitcoin timelocks optional precisely because of cost. For users who want maximum verification, the Bitcoin layer is available. For users who prefer zero cost, Nostr heartbeats alone provide the core functionality.

7. Alternative: OP_CHECKSEQUENCEVERIFY

OP_CHECKSEQUENCEVERIFY (CSV) provides relative timelocks — the output becomes spendable N blocks after it was created, rather than at absolute block height N.

EchoLock uses CLTV (absolute) rather than CSV (relative) because:

8. Mempool.space Integration

EchoLock uses mempool.space's public API for transaction verification: 

// Recommended endpoints
const MEMPOOL_API = 'https://mempool.space/api';

// Get transaction details
GET ${MEMPOOL_API}/tx/{txid}

// Check if output is spent
GET ${MEMPOOL_API}/tx/{txid}/outspend/{vout}

// Get current block height
GET ${MEMPOOL_API}/blocks/tip/height

// Broadcast new transaction
POST ${MEMPOOL_API}/tx

Users can also run their own Bitcoin node and Electrum server for maximum sovereignty.

9. Failure Modes

Scenario Impact Mitigation
User loses Bitcoin keys Cannot reset timelock Nostr heartbeats still function
High fee environment Reset becomes expensive Extend timelock duration, reduce reset frequency
Mempool.space unavailable Verification delayed Fall back to alternative APIs or full node
Bitcoin network congestion Transaction confirmation delayed Use RBF (Replace-By-Fee) to bump priority

10. Conclusion

Bitcoin's timelock opcodes provide something no other system can: unforgeable, publicly verifiable proof that an event occurred at a specific point in time. For dead man's switches, this means:

The Bitcoin layer is optional — EchoLock works without it — but for users who need maximum assurance, on-chain timelocks provide cryptographic certainty that no centralized timestamp service can match.