code: public BigInteger getPrivKey() {
          if (privKey == null) {
              throw new MissingPrivateKeyException();
          } else if (privKey instanceof BCECPrivateKey) {
              return ((BCECPrivateKey) privKey).getD();
          } else {
              throw new MissingPrivateKeyException();
          }
      }
docs: Gets the private key in the form of an integer field element. The public key
  is derived by performing EC point addition this number of times (i.e. point
  multiplying).
  
  
  @return -
  
  @throws java.lang.IllegalStateException
  if the private key bytes are not available.