Hi Işık!
I am new to cayley but from conversation in the chat (#cayley on freenode) I was told that there is no authentication and this is something you'll have to do yourself.
For example, if you want to build HTTP endpoints that can access your DB, you'll have to write a Node.js server (express or anything else you are familiar with) and that server will do the authentication/authorization and if the request is from a valid user that is allowed to access your DB, that server will make another HTTP call against the cayley API.
If you don't like that idea of making 2 HTTP calls you can embed Cayley inside your app. The only constrain with this approach is your server side must be writen in Go. This is not fully documented yet but you can take a look here until we'll have better docs:
https://github.com/google/cayley/wiki/Cayley-Go-API-(as-a-Library-in-your-own-projects).
Also, please join the chat #cayley on freenode. you can use whatever IRC client you like or even the web interface -
https://webchat.freenode.net/?channels=cayley
Let me know if it helps.