I want to post message in chatter (case) services using API. how can i create a sales force access token without username and password. 

I have get documentation to generate token. but, i am not able to find how to Upload an X509 Certificate to a Java Key Store (JKS) and where to upload.

I am try to create token using OAuth 2.0 JWT Bearer Flow for Server-to-Server Integration.

So, How to create rest API access token without username and password?