Private Files: Privacy-Preserving File Hosting Service with Search Functionality

Master Thesis


Currently, file hosting services such as Dropbox and Google Drive are widely adopted for file storing and sharing. In addition to these basic functionalities, some services provide additional functionalities such as keyword search and statistical analysis of text documents, requiring more complex server-side processing on the data. While the server-side data processing enables easier scalability independent of the client, it also requires the data to be unencrypted while being processed. However, this data could be highly sensitive and often contains personal information, which should not be revealed to the service provider.

Searchable encryption (see, e.g., the survey [1]) allows selective search over the encrypted data, but might leak certain details in real-world [2]. Another approach is to secret share the data and distribute it over multiple file hosting services, where the data can now be processed using secure multi-party computation (MPC) techniques. Although the total overhead for using MPC might be greater compared to the solutions based on searchable encryption, leakage of data can be prevented in MPC. Also, we can hide the search query completely from the service providers and even extend the functionalities easily.


The first goal of this thesis is to survey the well-known file hosting services and their search capabilities. The next goal is to provide a proof-of-concept (PoC) implementation, which allows a client to secret share and retrieve its data to multiple existing file hosting services, such as Dropbox and Google Drive. Finally, the thesis aims to show how special file hosting services could provide efficient search functionality to the clients using the state-of-the-art MPC framework MOTION [3].


  • Good programming skills in C/C++
  • At least basic knowledge of cryptography
  • Basic knowledge of secure multi-party computation is beneficial
  • High motivation + ability to work independently
  • Knowledge of the English language, Git, LaTeX, etc. goes without saying