WordPress theme functionality

I am in the design stage for a WP theme for a client, which needs a specific feature.

Basically they are a photographer and would like their clients to be able to view their photographs on their site.

They would like this secure, but without a username / password

What they want to do is to let the client add a unique reference to a “login box” which will take them to the gallery (where they can purchase the photo’s etc).

My ideas is to add a meta box to the custom post type and then create a custom form, which will check that the reference is correct.

Is this the best possible way to do this or is there another way I could do this?

I think this would be the same functionality of like a coupon code.

So it’s like you need a id to get to that page.

Yup exactly, you need the unique reference code to see the page.