Digital signature S/EDIFACT (ISO9735-5) or S/MIME for AS2 Messages

Dear ladies and gentlemen,

we start a project with a Portugues Company..they expect, that we have to signature our invoices with S/EDIFACT (ISO9735-5) or S/MIME.

Knows somebody, what must we do, that we get this signature for each message ?

We transfer the invoice from ERP 4.7 System to PI. In the PI system we map the IDOC to a AS2 message and send it

to a AS2 Server.

Thanks indeed.