Introduction
Buypass offers (today - see above!) one protocol for doing end-user document signing:
The Buypass Bx-protocol (implemented by IPS)
The protocol does not require server side components installed at the Merchant/Relying Party.
Client libraries are available to ease the technical integration.
Bx-protocol (IPS)
The Bx-protocol is a message base protocol develop by Buypass. It has proven itself for years and is based on public key cryptography. This means that all messages are signed and encrypted making the protocol secure even over open networks (does not require HTTPS).
The Bx-protocol requires a Buypass issued "Merchant Certificate" for identification of the Merchant/Relying Party.
An overview of the basic flows and of the protocol (and IPS) is documented here: IPS grensesnittet - signering personID.
Specific documentation for doing signing with the Bx-protocol is documented here: IPS-PKI signering med IPS-Bx