I\'ve been working on the design of a longitudinal study and one requirement is:

Question

I've been working on the design of a longitudinal study and one requirement is:

- all participants will have a unique identifier that
- is not reversible from the data storage/analyst side of the study
- is defined by something easily remembered by a participant that is relatively static over several years, ex. a participant's proper name and birth date in a given format.
- Creation of the unique identifier will occur on the participant's computer and no portion of the defining string will be sent with other collected data

How do I go about meeting this goal?

Initial thoughts are to use bcrypt or something similar but that runs into the problem that if there is a list of possible participant names and birthdays it becomes trivial to determine who participated and their answers. This hypothetical situation is not very likely but concerning.

I've looked into ID based cryptography as a possible answer but the increase in complexity and high likelihood of user error are prohibitive.

Am I missing a simple answer?

Explanation / Answer

There's a simple way to accomplish this with a 2 step hash.

Take a personal identifier for someone SHA256( Firstname Middname Lastname + Birthday) and calculate this on the client side.

Send this hash to a server. Hash this with a single pre-chosen secret of high entropy (128 bits) only known to the programmer and kept secret from any and all researchers. So SHA256(secret+HashOutPutStep1). Store the output in your database as a key for that participant. The secret must of course be the same for any single study. If you like, use a integer unique identifier that maps to the SHA256 generated hash. That would give you an easy reference number for a human to utilize.

This makes it impossible to reverse the hash without knowing the secret, and the results are always the same with the same personal identifier. I believe this solution meets your requirement since the analysts can't reverse this string. The secret must be kept away from analysts, but this is a trivial matter.

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