Subject anonymization

Ory OAuth2 and OpenID Connect offers two subject identifier algorithms: public and pairwise. These algorithms are used to provide a unique identifier for each user that can be used by the clients without revealing the user's identity. You can enable either one or both algorithms and override the obfuscated sub value with your own value.

Public algorithm

The public algorithm is the default algorithm in Ory OAuth2 and OpenID Connect. It provides the same subject value (sub) to all clients. You can set this algorithm using the Ory CLI:

ory patch oauth2-config {project.id} \
  --replace "/oidc/subject_identifiers/supported_types=[\"public\"]"

Pairwise algorithm

The pairwise algorithm provides a different subvalue to each client, ensuring that clients can't correlate user activity without permission. To use the pairwise algorithm, you must enable it and set /oidc/subject_identifiers/pairwise/salt. The salt is used to obfuscate the sub value.

To enable the pairwise algorithm, run this command:

ory patch oauth2-config {project.id} \
  --replace "/oidc/subject_identifiers/supported_types=[\"pairwise\"]" \
  --replace "/oidc/subject_identifiers/pairwise/salt=\"{16-character-long-salt}\""

danger

Don't change the salt value once it's set in production. When you change the salt, all client applications receive new user IDs from Ory. This can cause serious complications with authentication in your system.

Each OAuth 2.0 client has a subject_type configuration field that can take a public or pairwise value. If the identifier algorithm is enabled, Ory automatically chooses the right strategy.

While Ory handles sub obfuscation out of the box, you can also override this value with your own obfuscated sub value by setting force_subject_identifier when accepting the login request in your custom OAuth2 login app.

Using 'login_hint' with a different subject

If a user has already logged in with a subject (for example, "user-A"), and she wants to log in as another user using login_hint (for example, login_hint=user-B), directly accepting the latter login request in your login provider will cause an error:

Subject from payload doesn't match subject from previous authentication

To solve this issue, follow these steps:

1. Get the OAuth 2.0 Login Request.
2. Make sure that subject and login_hint aren't empty and don't point to the same user.
3. If they point to different users, redirect the browser to request_url and append ?prompt=login.
4. Ory restarts the flow and ignores existing authentication which allows your login provider to log in a different subject.