Security Protocol Design

A{
  var certificate_A;
  var amount;
  borrow(id_A/certificate, signed_request{pk_A & amount})
  receive()
}

B{
  var certificate_B
  var transaction;  //hash
  send_request(id_B/certificate,
          signed_transaction{ request{pk_A & amount},
          pk_B })
  receive()
}

S{
  var message;
  response{id_C/certificate, signed_response_message(
            update_balance_B{update_balance_A, pk_A}, pk_B},
            pk_C)
}

1. A signs transaction and sends own certif. and amount to B
2. B signs lending contract (message from A) with B's private key
3. B sends request to MobilePay
4. Server S responds to B with encrypted message with updated balances of A & B (B cannot decrypt the message for A, because doesnt have pk of A.)
5. B decrypts the message and updates its own balance & sends update to A
6. A makes update

Link to our presentation