Encrypt DB Fields preserving search functionality

I would actually not advise encrypting something like a user id. There should be practically no risk to anyone seeing it and you're really making a lot more work for yourself than necessary. An id identifying a user is not what I imagine a lot of people are going to call "sensitive information".

I'm actually having a hard time trying to think of a case of sensitive information that you would like to use a WHERE clause on in SELECT. I wouldn't never want to search against SSNs, for example.

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If you wanted to compare user input against something like an SSN there's generally other information such as name and DOB that's entered alongside the SSN. I would pull all records out into an array with that match that information (name, dob) and then check any resultant entries with the SSN provided and the SSN from the entry, decrypted.