External Temporal Anchoring for Transparency Services

This document defines a mechanism for external temporal anchoring of digital artifacts by committing cryptographic hashes to an independent, publicly verifiable ledger -- specifically, the Bitcoin blockchain via the OpenTimestamps protocol. The resulting proof is independently verifiable by any party with access to ledger state, without reliance on a trusted third party. The SCITT Architecture [RFC9943] is used as the primary integration example, but the anchoring primitive is applicable to any system requiring externally verifiable temporal proof. No changes to the SCITT architecture are required.