Buypass offer 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.
The Bx-protocol also supports features other than signing like Authentication and custom payment schemes. The protocol is highly adaptable to suite specific customer needs.