Constant-Round Privacy Preserving Multiset Union

Abstract. Privacy preserving multiset union (PPMU) protocol allowsa set of parties, each with a multiset, to collaboratively compute a mul-tiset union secretly, meaning that any information other than union isnot revealed. We propose ecient PPMU protocols, using multiplicativehomomorphic cryptosystem. The novelty of our protocol is to directlyencrypt a polynomial by representing it by an element of an extension eld. The resulting protocols consist of constant rounds and improvecommunication cost. We also prove the security of our protocol againstmalicious adversaries, in the random oracle model. 1. IntroductionPrivacy Preserving Multiset Union (PPMU) is a set operation that allowsa set of parties, each with a multiset, to collaboratively compute a multisetunion but no party learns more information about other parties’ private inputsthan what can be deduced from the result of union.PPMU is useful in various applications such as data collection for statis-tics, where con dentiality of data is important. Typical application examplesinclude:Intrusion Detection System: Web sites need to detect malicious hostsor IP addresses. They can improve their detection accuracy by col-laboration because a given malicious host is likely to a ect more than