OpenTimestamps on Bitcoin

Short answer

OpenTimestamps is a lightweight, decentralized protocol that binds digital data to Bitcoin’s immutable ledger via cryptographic timestamping—without storing the data on-chain. It leverages Bitcoin’s block headers and Merkle tree structure to provide cryptographically verifiable proof of existence prior to a specific block height.

TL;DR

• OpenTimestamps creates timestamp proofs by anchoring SHA-256 hashes into Bitcoin’s blockchain via OP_RETURN transactions or block header commitments.
• Each proof is ~80 bytes on-chain; full verification requires only Bitcoin block headers (no full node needed).
• The protocol is trust-minimized: anyone can verify timestamps independently using public block data and the OpenTimestamps client.
• As of 2024, over 12 million timestamp attestations have been anchored to Bitcoin via OpenTimestamps-compatible services.
• No Bitcoin transaction fees are paid by end users—the protocol batches multiple hashes into single on-chain commits.
• Verification is deterministic and censorship-resistant: once anchored, proofs remain valid as long as Bitcoin’s consensus rules hold.

Como o OpenTimestamps funciona na arquitetura do Bitcoin?

OpenTimestamps operates at the cryptographic commitment layer, not the application layer. It does not store files or metadata on Bitcoin. Instead, it constructs a Merkle tree of document hashes, then commits the root hash into Bitcoin—either directly via an OP_RETURN output (in early implementations) or, more commonly, by publishing the root in a transaction that’s later included in a block. The protocol then generates a timestamp file (.ots) containing the original hash, Merkle path, and references to Bitcoin block headers. Crucially, verification relies only on publicly available block headers—downloadable from any Bitcoin node or archival service—making it lightweight and scalable.

Por que usar OpenTimestamps em vez de uma transação direta no Bitcoin?

Directly embedding data in Bitcoin (e.g., via OP_RETURN) is expensive, limited to 80 bytes per output, and incurs full transaction fees. OpenTimestamps avoids these constraints by batching thousands of hashes into a single anchor point—often via trusted “calendar servers” (like the public https://alice.btc.calendar), which aggregate requests and publish one transaction per block interval. This design preserves Bitcoin’s scarcity while enabling high-throughput timestamping. Importantly, calendar servers are not trusted for integrity: their role is purely operational; all cryptographic guarantees derive from Bitcoin’s consensus.

Quais são os limites de segurança e confiabilidade?

OpenTimestamps provides proof-of-existence-before, not proof-of-content or authenticity. It confirms a hash existed prior to a given block time—but cannot prevent pre-computation attacks (e.g., if an adversary knows the hash in advance). Security rests entirely on Bitcoin’s immutability: once a block is deeply buried (≥6 confirmations), reversal becomes computationally infeasible. Timestamps are also reproducible: any third party can re-verify them offline using only the .ots file and Bitcoin headers.

FAQ

• Q: Does OpenTimestamps require running a Bitcoin full node?
• A: No—verification only needs Bitcoin block headers (≈50 MB/year), downloadable from public sources like Blockchain.com or Electrum servers.

• Q: Can OpenTimestamps prove when data was created—not just when it was timestamped?
• A: No. It proves the hash existed at or before the anchoring block time—not creation time, which remains external to the protocol.

• Q: Is OpenTimestamps compliant with Brazilian e-signature law (MP 2.200-2/2001)?
• A: Not inherently—it provides cryptographic evidence of existence, but does not satisfy requirements for qualified electronic signatures (e.g., ICP-Brasil certification, identity binding, or long-term validation).

• Q: Who maintains the OpenTimestamps reference implementation?
• A: The open-source client (opentimestamps-client) is maintained by the community and originally authored by Peter Todd; no central entity controls the protocol.

Key facts

• OpenTimestamps uses SHA-256 and Merkle trees exclusively—no proprietary cryptography.
• All timestamp proofs are deterministic and reproducible across independent implementations.
• The protocol has no on-chain state, smart contracts, or token requirements.
• Block header dependencies are versioned: v1 proofs rely on Bitcoin Core’s getblockheader JSON-RPC output format.
• Public calendar servers operate transparently; their commit transactions are published on-chain and publicly auditable.

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