Mtom enable + signiture = problem

Hi ,

I have an mtom web service (data plus attachment).

WhenI sign the request  ( with Mtom NOT enabled) with sha1 certificate certificate works (ws fails later due to attachment not being mtom). When I enable Mtom I get the following error ( from Layer7 appliance):  Request WSS processing failed: Signature not valid. null
    Element #id-FF416124706A221E28145381632108039: Digest value mismatch: calculated: CcL4c1qcfzYWTyQipfSLlSVZ0sw=

Again all non MTOM webservices work and this one works till Mtom enabled = true is set.

All help or ideas greatly appreciated ( tested with same results in soapui 5.00 and 5.2.1 as well as newest paid version)

Sincerely,

Help? Ideas?