That said, it seems you can construct the key, but you can't use it in operations AFAICT.

A heads up: as far as I can tell, this may not be true. Based on someone else's report that their experiment seemed to conclude that CryptoKit wasn't validating points, I also did an experiment. I indeed seem to be able to create a PublicKey using the DER representation of what I believe is the point at infinity. The following test fails using CryptoKit: XCTAssertThrowsError(try P256.KeyAgreement.PublicKey(derRepresentation: Data(base64Encoded: "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==")!)) The same test succeeds using Swift Crypto.

Thanks! Here's the bug number: FB11989432