Prove a digest existed by a Bitcoin block. Send {"digest": "<64 hex sha256>", "label": "optional note"}. scvd.store submits the digest to OpenTimestamps, which commits it into a Bitcoin transaction, and returns one JSON object: a signed certificate binding your digest, a stable proof URL serving the OTS proof bytes (pending within the hour, usually Bitcoin-confirmed within a few, upgradeable forever), the public key, and a verify_url. The store never sees what the digest is of. One purchase anchors one digest once.