Automated Cryptanalysis of Bloom Filter Encryptions of Health Records

Privacy-preserving record linkage with Bloom filters has become increasingly popular in medical applications, since Bloom filters allow for probabilistic linkage of sensitive personal data. However, since evidence indicates that Bloom filters lack sufficiently high security where strong security guarantees are required, several suggestions for their improvement have been made in literature. One of those improvements proposes the storage of several identifiers in one single Bloom filter. In this paper we present an automated cryptanalysis of this Bloom filter variant. The three steps of this procedure constitute our main contributions: (1) a new method for the detection of Bloom filter encrytions of bigrams (so-called atoms), (2) the use of an optimization algorithm for the assignment of atoms to bigrams, (3) the reconstruction of the original attribute values by linkage against bigram sets obtained from lists of frequent attribute values in the underlying population. To sum up, our attack provides the first convincing attack on Bloom filter encryptions of records built from more than one identifier.