class CoindealDigest implements ParamsDigest {

  private final String secret;
  private final String apiKey;

  private CoindealDigest(String secret, String apiKey) {
    this.secret = secret;
    this.apiKey = apiKey;
  }

  public static CoindealDigest createInstance(String secret, String apiKey) {<FILL_FUNCTION_BODY>}

  @Override
  public String digestParams(RestInvocation restInvocation) {
    return new BasicAuthCredentials(apiKey, secret).digestParams(restInvocation);
  }
}

    if (secret != null || apiKey != null) {
      return new CoindealDigest(secret, apiKey);
    } else return null;
  
<no_super_class>