International Association
for Cryptologic Research

Topic modelling refers to a popular set of techniques used to discover hidden topics that occur in a collection of documents. These topics can, for example, be used to categorize documents or label text for further processing. One popular topic modelling technique is Latent Dirichlet Allocation (LDA). In topic modelling scenarios, the documents are often assumed to be in one, centralized dataset. However, sometimes documents are held by different parties, and contain privacy- or commercially-sensitive information that cannot be shared. We present a novel, decentralized approach to train an LDA model securely without having to share any information about the content of the documents with the other parties. We preserve the privacy of the individual parties using a combination of privacy enhancing technologies. We show that our decentralized, privacy preserving LDA solution has a similar accuracy compared to an (insecure) centralised approach. With $1024$-bit Paillier keys, a topic model with $5$ topics and $3000$ words can be trained in around $16$ hours. Furthermore, we show that the solution scales linearly in the total number of words and the number of topics.