[stunnel-users] verify option question

hello,

i am using stunnel4.x under linux in my server i set verify option level 3 and in my clinet i added stunnel.pem so verify can happen now can my isp knows my key of encrytion and decrypt ? so it is safe to set verify level 3 ? or it shouldnot ? can my isp decrpt my traffic or knows what sites i visted? thanks