Hashed Email

A hashed email is the output of running an email through a one-way hash function to produce a stable identifier for matching users instead of storing the raw email.

The idea is that a hash is one-way: you go from email to hash but cannot rebuild the email from the hash, so privacy is seemingly preserved while two records of the same person can still be matched. But a dangerous misconception lives here: a hashed email is not 'anonymous'. Because the hash is deterministic, anyone who has the same email can produce the same hash and re-link the records to the person; this is why many privacy frameworks still treat a hashed email as personal data, not truly anonymized data. Its legitimate use is matching more than protection: linking a user across two systems without passing the raw email. But if the goal is real anonymization, hashing is not enough.