code: public char[] getPrivateKey() throws GeneralSecurityException, IOException {
          if (privateKeyAsSeed == null) {
              throw new IllegalStateException("Public-only NKey");
          }
  
          DecodedSeed decoded = decodeSeed(privateKeyAsSeed);
          return encode(Type.PRIVATE, decoded.bytes);
      }
docs: @return the encoded private key for this NKey
  
  @throws GeneralSecurityException if there is an encryption problem
  @throws IOException              if there is a problem encoding the key