code: protected void configureSigningKey(final String signingSecretKey) {
          try {
              if (ResourceUtils.doesResourceExist(signingSecretKey)) {
                  configureSigningKeyFromPrivateKeyResource(signingSecretKey);
              }
          } finally {
              if (this.signingKey == null) {
                  setSigningKey(new AesKey(signingSecretKey.getBytes(StandardCharsets.UTF_8)));
                  LOGGER.trace("Created signing key instance [{}] based on provided secret key", this.signingKey.getClass().getSimpleName());
              }
          }
      }
docs: Sets signing key. If the key provided is resolved as a private key,
  then will create use the private key as is, and will sign values
  using RSA. Otherwise, AES is used.
  
  @param signingSecretKey the signing secret key