Let me explain the problem.
I have two tables with email addresses in each of them. Table A comes from Company A and Table B comes from Company B. The end goal is to find out number of common email addresses without either company being able to identify if a particular email address exists in the other company's table.
I thought of hashing the email addresses on either side of the company using a common key and then run a compare algorithm on both lists. But the problem is that if Company A is running this, they would be able to identify using reverse lookups in their own table of hashes.
How do I prevent this and still be able to find the common email addresses?