How can I encrypt(using PKI credential) a package/portfolio using Assembler Service

Hi All,

I have found a way to encrypt (using password credential) a package/
portfolio using a DDX and calling invokeDDX service of Assembler
Service. see the DDX snippet below

How  can I do the PKI based encryption on the package. I couldn't find
any DDX element for this.

snippet:
<?xml version="1.0" encoding="UTF-8"?>
  <DDX xmlns="http://ns.adobe.com/DDX/1.0/">
     <PDF result="GeneratedDocument.pdf" encryption="userProtect" >
        <PDF source="cover"/>
           <PackageFiles>
           <PDF source="attachments"/>
        </PackageFiles>
       </PDF>
      <PasswordEncryptionProfile name="userProtect">
                 <OpenPassword>abcd1234</OpenPassword>
     </PasswordEncryptionProfile>
  </DDX>

Thanks
greenday