Information sharing helps to better protect computer systems against digital threats and known attacks. However, since security information is usually considered sensitive, parties are hesitant to share all their information through public channels. Instead, they only exchange this information with parties with whom they already established trust relationships.We propose the use of two complementary techniques to allow parties to share information without the need to immediately reveal private information. We consider a cryptographic approach to hide the details of an indicator of compromise so that it can be shared with other parties. These other parties are still able to detect intrusions with these cryptographic indicators. Additionally, we apply another cryptographic construction to let parties report back their number of sightings to a central party. This central party can aggregate the messages from the various parties to learn the total number of sightings for each indicator, without learning the number of sightings from each individual party.An evaluation of our open-source proof-of-concept implementations shows that both techniques incur only little overhead, making the techniques prime candidates for practice.

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We propose a generic construction for fully secure decentralized multiauthority predicate encryption. In such multiauthority predicate encryption scheme, ciphertexts are associated with one or more predicates from various authorities and only if a user has a set of decryption keys that evaluates all predicates to true, the user is able to recover the message. In our decentralized system, anyone can create a new authority and issue decryption keys for their own predicates. We introduce the concept of a multi-authority admissible pair encoding scheme and, based on these encodings, we give a generic conversion algorithm that allows us to easily combine various predicate encryption schemes into a multi-authority predicate encryption variant. The resulting encryption schemes are proven fully secure under standard subgroup decision assumptions in the random oracle model. Finally, by instantiating several concrete multi-authority admissible pair encoding schemes and applying our conversion algorithm, we are able to create a variety of novel multi-authority predicate encryption schemes.

We propose the first multi-client predicate-only encryption scheme capable of efficiently testing the equality of two encrypted vectors. Our construction can be used for the privacypreserving monitoring of relations among multiple clients. Since both the clients' data and the predicates are encrypted, our system is suitable for situations in which this information is considered sensitive. We prove our construction plaintext and predicate private in the generic bilinear group model using random oracles, and secure under chosen-plaintext attack with unbounded corruptions under the symmetric external Diffie-Hellman assumption. Additionally, we provide a proof-of-concept implementation that is capable of evaluating one thousand predicates defined over the inputs of ten clients in less than a minute on commodity hardware.