How each site gets a private ID

When a site signs you in through the Omxus sign-in page, it receives an ID made just for that site. Another site gets a different ID for the same account. The two IDs can't be matched with each other.

Why this matters

If every site got the same ID for you, any two sites could compare notes and join up what you do on each. Researchers have shown how easily separate records can be linked once they share even a few points in common (Narayanan & Shmatikov, 2008). A different ID for each site removes one of the easiest links.

How it works

Omxus makes each site's ID by mixing your account with the site's web address and a secret only Omxus holds. The same site always gets the same ID, so it recognises you next time. A different site gets an unrelated ID. OpenID Connect, the standard Omxus follows, calls these pairwise identifiers (Sakimura et al., 2014).

A company that runs a website and its own app can register them together so both see the same ID. They're one service, so this links nothing new.

Things that can still link you

A private ID only covers the ID itself. Sites can still match you in other ways you control:

• Your email or mobile, if you let both sites have it.
• Your username, if the sites share usernames.
• What you post, and anything else you tell each site.

Sites that show the Omxus sign-in on their own page receive your Omxus account ID rather than a private ID for the site. Signing in with your name, date of birth and secret words always gives each site a different key, wherever the sign-in appears.