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Subject anonymization

Ory OAuth2 and OpenID Connect offer 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.

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 sub value to each client, ensuring that the clients cannot correlate the user's activities without permission. To use the pairwise algorithm, you must enable it and set /oidc/subject_identifiers/pairwise/salt. This salt is used to obfuscate the sub value.

To enable both algorithms, use the following configuration:

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

Do not change the salt value once it is set in production. Changing it will cause all client applications to receive new user IDs from Ory, which can lead to serious complications with authentication.

Each OAuth 2.0 client has a configuration field called subject_type. The value of that subject_type is either public or pairwise. If the identifier algorithm is enabled, Ory will automatically choose 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. Check if both subject and login_hint are not empty and not the same user.

  3. If they are different, redirect the browser to request_url and append ?prompt=login.

  4. Ory will now restart the flow and ignore the existing authentication, allowing your login provider to log in a different subject.

In summary, Ory OAuth2 and OpenID Connect offer two subject identifier algorithms to provide unique user IDs to clients while preserving user privacy. You can enable either one or both algorithms and override the obfuscated sub value with your own value. If you need to log in a different subject using ogin_hint, follow the above steps to avoid an error.