What is an authenticity proof (such as a TLSNotary proof), and what problem does it solve for oracles?

An authenticity proof lets an oracle prove the data genuinely came unaltered from a specific source (e.g. a particular HTTPS website), so a contract need not blindly trust the oracle operator's honesty.

The weakness of a single software oracle is that it could simply lie about what a website returned. An authenticity proof closes that gap by attaching cryptographic evidence tying the delivered value to the real source. TLSNotary is one classic technique: it uses the TLS (HTTPS) session between the oracle and a web server, splitting cryptographic material so a third party can attest that the oracle really received this response from that server, without the oracle being able to forge it. The contract then trusts the source and the proof — not the oracle's goodwill. This shifts the trust from "the operator is honest" to "the source is honest and the proof is sound."

Go deeper:

• TLSNotary — the project itself: cryptographically proving that web data really came from a given HTTPS server.
• Blockchain oracle (Wikipedia) — where authenticity proofs fit among the ways to make oracle data credible.