Support 3DS2 in UCP Specification #420

This issue tracks the enhancement of the UCP specification to support 3DS2 in a vendor-agnostic manner.

Proposal

In a native UCP flow, the merchant delegates the UI to the host. This makes it impossible for the merchant to perform 3DS2 challenge flows since it requires loading iframes/popups. We propose extending the UCP spec so the host can perform these actions while not having to worry about the complexities of the 3DS2 flow.

Key Concepts

• Merchant as Orchestrator: The merchant manages the flow state.
• Host as Execution Environment: The host executes standardized 3DS2 actions (rendering iframes/popups).
• complete_in_progress Status: Signals that checkout is in an intermediate state awaiting 3DS2 action.

Flow Phases

1. Initial Request: Host initiates completion.
2. Device Data Collection (DDC) Loop: Merchant requests DDC, Host submits result.
3. Challenge Loop: Merchant requests Challenge, Host submits result.
4. Final Completion.

See the detailed guide and schema changes in Pull Request #421.