Search for global SKU and individual SKU per product and customer

Hi guys

I have an index with thousands of products. Each product has of course one SKU. Now I have customers who should be able to define the individual custom SKU for each product as they see fit. That means that one product with one global SKU can have multiple custom SKUs which only should be accessable/retrievable and searchable for those related customers.

So, my question is: how can I make that work in a secure way so that each customer can search additionally for his custom SKUs (if any given per product) and only retrieve the global SKU and his own custom SKU?