Need best authentication model to access content

Attached is the sample data that we need to expose to internet. We need to include all our assets in the json file. (.json is file not attachable in this. so I put it in .pdf)

Initially we had a scheduler in author which will scan all the assets and put it in /content/dam/assets.json and publish the asset.json.

Later the thridparty system can access by https://www.mydomain.com/content/dam/assets.json.

Now we have a requirement that we need to include the private assets(we maintain a field in asset metadata) also in the asset.json.

In this case we cannot show the private asset path in the assets.json.

We are looking for some authentication model so that the thirdparty system will be authenticated and access the /content/dam/assets.json.

Note:

1) we can change the path of  /content/dam/assets.json 

2) we can do the the authentication either in author or publish.

we need a solution for this.