java - How to use symmetric key to encrypt and sign SOAP using WSS4J -

i need build web-service client application using axis 1.4 access third party system (web service producer).

in order send valid request third party system need encrypt , sign soap request before sending them.

i make use of wss4j 1.5 task follow steps in link: calling owsm protected service axis 1.4 , wss4j

what understand that:

1- encrypt soap need use third party public key , decrypt using own private key.

2- sign soap need use own private key , should verify signature using public key need share them.

i did receiving error

  faultstring: did not understand "mustunderstand" header(s) 

from third party end. after checking third party team said:

"we use “username authentication symmetric key”, not maintain client certificates @ our trust store. there single shared key used both signing , encryption. hence need signing , encryption using our certificate."

this response not make sense me. understand need use third party certificate (i.e. public key) encrypt soap request.

but how can use same public key - which certificate shared them - sign soap request ?

i believe need private key in order that.

do need private key that? clarification helpful.