Ref T13222. Users configure "Factor Configs", which say "I have an entry on my phone for TOTP secret key XYZ".
Currently, these point at raw implementations -- always "TOTP" in practice.
To support configuring available MFA types (like "no MFA") and adding MFA types that need some options set (like "Duo", which needs API keys), bind "Factor Configs" to a "Factor Provider" instead.
In the future, several "Factors" will be available (TOTP, SMS, Duo, Postal Mail, ...). Administrators configure zero or more "MFA Providers" they want to use (e.g., "Duo" + here's my API key). Then users can add configs for these providers (e.g., "here's my Duo account").
- Factor: a PHP subclass, implements the technical details of a type of MFA factor (TOTP, SMS, Duo, etc).
- FactorProvider: a storage object, owned by administrators, configuration of a Factor that says "this should be available on this install", plus provides API keys, a human-readable name, etc.
- FactorConfig: a storage object, owned by a user, says "I have a factor for provider X on my phone/whatever with secret key Q / my duo account is X / my address is Y".
Couple of things not covered here:
- Statuses for providers ("Disabled", "Deprecated") don't do anything yet, but you can't edit them anyway.
- Some bin/auth tools need to be updated.
- When no providers are configured, the MFA panel should probably vanish.