docs: public String toStringWithPrivate() {
          StringBuilder b = new StringBuilder();
          b.append(toString());
          if (privKey != null && privKey instanceof BCECPrivateKey) {
              b.append(" priv:").append(Hex.toHexString(((BCECPrivateKey) privKey).getD().toByteArray()));
          }
          return b.toString();
      }