Glossary
What is Hashed email?
Also called: HEM, email hash
A hashed email is an email address turned into a fixed code by a one-way hash, so it can be matched for advertising without sharing the address itself.
हिंदी में पढ़ेंA hash is a one-way function: the same input always gives the same output, and it is designed so the output cannot simply be turned back into the input. Hashing a reader’s email gives a code that two systems can compare. If both hashed the same address the same way, the codes match, though neither shared the address.
Several identity tools start from an email. UID2 takes a logged-in email or phone number, salts and hashes it with SHA-256, and turns the result into a rotating encrypted token. ID5 can take a hashed email from your page through its pd parameter. PAIR, an IAB Tech Lab standard, matches a publisher’s and an advertiser’s email lists in a clean room, with each list hashed and locked with secret keys so neither side sees the other’s addresses.
Hashed emails only exist where readers give you an email, through logins or newsletter sign-ups. Hashing protects the address, but it does not replace consent, so use hashed emails only where your consent setup allows.
Example
A recipe site has 200,000 newsletter emails and a cookware brand has 50,000 customer emails. With PAIR, both lists are hashed and encrypted, and say 8,000 match. Only the brand's DSP learns which 8,000; the site never learns who matched, and no raw address changes hands.
Why it matters to publishers
Logged-in and newsletter readers are the audience email-based IDs can reach. Counting them shows how much of your traffic these tools can actually cover.