How to Secure amazon s3 bucket web URL of a resource against unauthorised user

First, make your bucket private.

Then, generate signed URL to access your content.

The format will be https://<bucket>.s3.amazonaws.com/objectname?AWSAccessKeyId=<accesskey>&Expires=<expiretime>&Signature=<signature string>

Please see https://s3.amazonaws.com/doc/s3-developer-guide/RESTAuthentication.html

You can also try it with this tool: http://www.dancartoon.com/projects/s3-siggenerator/