Passphrase for SharePoint saves the farm from accidental addition of servers by known or unknown admins. One way its saves but if we does not document it somewhere or lost it then the only way to retreive it is to reset the passphrase
The passphrase resides some where in SharePoint Configuration database encrypted. We need to set a new passphrase using the cmdlet Set-SPPassPhrase. Before we need to pass the new passphrase we need to convert it to secure string.
Follow the steps:
Note: Run these cmdlets on a server which is still part of a server, if the server is disconnected already from the farm then it cannot connect to configuration database to update the passphrase.