Function responder_is 

pub fn responder_is(resp: &ChallengeResponse, expected_did: &str) -> bool

The “actually Bob” check: does the response come from the DID we expected?

verify_response proves the signature is valid for the DID the responder claims; this confirms that claimed DID matches the one the initiator knows out-of-band. Both together establish “when Alice connects to Bob, it’s Bob”.