docs: private static PrivateKey privateKeyFromBigInteger(BigInteger priv) {
          if (priv == null) {
              return null;
          } else {
              try {
                  return ECKeyFactory.getInstance(SpongyCastleProvider.getInstance())
                          .generatePrivate(new ECPrivateKeySpec(priv, CURVE_SPEC));
              } catch (InvalidKeySpecException ex) {
                  throw new AssertionError("Assumed correct key spec statically");
              }
          }
      }