How to validate X-Slack-Signature from the received shortcut from slack?

The fact is that "String request Body = req.request Body?.toString();", okay, and all the data from the shortcut is in "Map<String, String> params = req.params;" and when validating and checking:   
finalHash != slack Signature

What should I get and where? 
And how do I check if this is the correct signature or not?