<Li> In a known message attack, the attacker is given valid signatures for a variety of messages known by the attacker but not chosen by the attacker . </Li> <Li> In an adaptive chosen message attack, the attacker first learns signatures on arbitrary messages of the attacker's choice . </Li> <P> They also describe a hierarchy of attack results: </P> <Ol> <Li> A total break results in the recovery of the signing key . </Li> <Li> A universal forgery attack results in the ability to forge signatures for any message . </Li> <Li> A selective forgery attack results in a signature on a message of the adversary's choice . </Li> <Li> An existential forgery merely results in some valid message / signature pair not already known to the adversary . </Li> </Ol>

Important aspect that establishes trust in digital signature