anchor.diff:  /// [HmacSecretKey.importRawKey] or generated using [HmacSecretKey.generateKey].
   ///
   /// [1]: https://doi.org/10.6028/NIST.FIPS.180-4
  +@sealed
   abstract class HmacSecretKey {
     HmacSecretKey._(); // keep the constructor private.
   
  
positive.code:   Future<Map<String, dynamic>> exportJsonWebKey();
  }
  
  abstract class AesCbcSecretKey {
    AesCbcSecretKey._(); // keep the constructor private.
  
  
negative.code: /// key-pair.
  ///
  /// [1]: https://tools.ietf.org/html/rfc3447
  @sealed
  abstract class RsassaPkcs1V15PrivateKey {
    RsassaPkcs1V15PrivateKey._(); // keep the constructor private.
  
  