Is it possible to find m' such that md5(m'+m1)=md5(m1+m2) if i know m2 and md5(m1+m2) ? is this a collision or pre-image attack.<br> Also in this case is it possible to find m' and H such that md5(m1+m')=H if i know md5(m1+m2) and m2 ? Basically a message is like : (M,md5(M|m1)) sent from server to client ( only client and server know m1 ).So can I being the middle man attack in this scenario ?