change SAML AuthnRequest with iRule, how to inflate?

im using the BIG-IP as SP and trying to change the AuthnRequest send to the IdP. as suggest by Kevin before i created a virtual server which has the actual IdP as poolmember and attached an iRule to that virtual server.

 

the SAML AuthnRequest is in order URL encoded, base64 encode and then deflated by the SP (this is how SAML 2.0 wants it and can't be changed from SP i believe) before it is send to the IdP. so to get the XML i need to URL decode it (URI::decode, check), base64 decode it (b64decode, check) and finally inflate it and that is where i get stuck. how to inflate the base64 decoded string, can't find a function to do this. with php i could get the XML fine, but with an iRule im stuck.

 

am i missing some obvious here or ...?