OTS Bitcoin Anchoring

Status

Draft. Reference implementation: crates/confium-transparency/src/ots.rs.

Motivation

A transparency log’s tree head can be anchored to a public blockchain (Bitcoin, by default) so that retroactive modification of the log becomes computationally infeasible (would require rewriting Bitcoin history). OpenTimestamps (OTS) provides the standard format.

Scope

• Periodic anchoring of tree heads to Bitcoin via OTS
• Proof format per RFC 8941 / OTS v1
• Verification of an anchor proof

Out of scope

• Anchoring to other chains (Ethereum, Solana) — same pattern, separate implementation
• Pay-to-anchor services (we use the OTS calendar public API by default)

Specification

Anchor event

On a schedule (default: every 1 hour, when ≥1 new entry appended):

. Compute the current tree head H = sha256(tree_size || root_hash). . Submit H to the OTS calendar API (https://a.pool.opentimestamps.org). . Receive a .ots proof file; store it alongside the log.

Proof verification

To verify a tree head H was anchored:

. Load the .ots proof file. . Verify against the Bitcoin header chain at the claimed block height. . Confirm the on-chain transaction commits to H.

Security considerations

• Anchoring interval is a tradeoff: shorter = more Bitcoin fees, longer = larger “vulnerable window” during which a rewrite is undetectable.
• Verification requires access to Bitcoin headers — clients either run a full node or trust a header service.

References

• OpenTimestamps: https://opentimestamps.org/
• Confium source: crates/confium-transparency/src/ots.rs