PRE2: Acceptable identifier types are configured outside the system.
Flow
User provides activation key and identifier type/name (or the actual Person)
If activation key exists and is currently valid, successful indicator is returned.
If activation key is invalid the particular failure is indicated:
Not in validity date range.
Post Conditions
Business Rules
BR1: Verifying the key enables one to "lock" exclusive use of it for a configurable period of time (configured via system, not by user). Invalidating/Deleting the key obviously frees the lock also. New keys can still be generated, which makes the existing lock useless.