sequenceDiagram
      autonumber
      accTitle: Account to Account Payment
      accDescr: Sequence diagram for A2A using SSI technology

      participant merchant as Merchant Checkout API
      participant mssi as Merchant Simulator
      actor Holder
      participant bssi as Bank Simulator
      participant bank as Bank Payment API

      rect rgb(255,211,92)
            note over merchant, bank: Initiation of the flow starts with the payment selection on the checkout page.<br/>The customer select the A2AP rail.
            Holder->>merchant: Pay with A2AP
            merchant->>+mssi: Request payment with invoice data
      end

      note over merchant, bank: this is where the A2AP simulation starts. 
      mssi->>Holder: Merchant presents QR Code of Holder.<br/>This is an offer for a VC
      Holder->>mssi: Customer accepts the VC offer
      mssi->>Holder: deliver invoice VC and<br/>QR Code to request payment promise

      rect rgb(164,196,0)
            note over merchant, bank: This part of the flow uses a different Window or an Overlay of the current window.<br/>In a mobile-only situation this could also be achieved with app switches.<br/>The Holder connects to the Bank<br/>to authenticate with Bank ID VC and<br/>present the invoice VC
            Holder->>bssi: Start invoice upload flow
            bssi->>Holder: QR Code to request Bank ID and Invoice
            Holder->>bssi: Present VP with invoice VC and Bank ID

            note over merchant, bank: The authentication was successful, <br/>the validation of the invoice and the Merchant were successful, and <br/>the Bank now processes the invoice.
            bssi->>bssi: validate Bank ID and invoice VC

            note over merchant, bank: This step is outside the PoC simulation
            bssi->>bank: request payment promise from API 
            bank->>bssi: returns payment promise details
            note over merchant, bank: The Bank now presents another QR Code to offer the payment promise to the Holder 
            bssi->>Holder: QR Code offers payment promise
            Holder->>bssi: Accept payment promise
            bssi->>Holder: Deliver payment promise
      end

      note over merchant, bank: Now we can finally read the QR Code from above and present the Payment Promise to the Merchant
      Holder->> mssi: Scan QR Code from Merchant
      mssi->>Holder: Request Payment Promise from Holder
      Holder->>mssi: Present payment promise to Merchant
      mssi->>mssi: Validate Payment promise

      note over merchant, bank: Now we can finally read the QR Code from above and present the Payment Promise to the Merchant
      mssi->>-merchant: Response on the intial request<br/>with data extracted from the payment promise

      rect rgb(255,211,92)
            note over merchant, bank: This step is out of scope of the PoC.<br/>It is done some time after the payment promise was issued.<br/>maybe a batch process during the night.  
            bssi->>mssi: payment execution over SIC
       end