Decrypts the envelope's subject with the content key the recipient's
private key opens (the reference's decrypt_subject_to_recipient): the
first sealed message the key opens yields the content key, which must
decode as a SymmetricKey.
EnvelopeError UnknownRecipient when no sealed message opens;
Cbor (dcbor error: <message>) when the sealed plaintext is not a
symmetric key; the errors of recipients and decryptSubject
Decrypts the envelope's subject with the content key the recipient's private key opens (the reference's
decrypt_subject_to_recipient): the first sealed message the key opens yields the content key, which must decode as aSymmetricKey.