code: void verifySignature(JsonNode messageJson) {
          if (!signatureChecker.verifySignature(toMap(messageJson), fetchPublicKey(messageJson))) {
              throw new SdkClientException("Signature in SNS message was invalid");
          }
      }
docs: Verifies the signature of the SNS message by downloading the public certificate from SNS and computing
  the signature from the request properties. If the signature does not match, or if the cert is inaccessible or invalid
  and exception will be thrown.
  
  @param messageJson JSON message object.