Secure Physical Access with NFC-enabled Smartphones

Abstract

This paper presents a smartphone-based physical access control system in which the access points are not directly connected to a central authorization server, but rather use the connectivity of the mobile phone to authorize a user access request online by a central access server. The authentication of the smartphone is based on public-key cryptography. This requires that the private key is stored in a secure element or in a trusted execution environment to prevent identity theft. One of the basic requirement of our solution is the independence from third parties like mobile network operators, trusted service managers and handset manufacturers. Therefore, a SIM-based secure element as well as an embedded secure element (i.e. a separate hardware chip on the handset) were not an option and we had to concentrate on the remaining secure element architectures: Host Card Emulation (HCE) and a microSD-based secure element. We show that the HCE approach cannot solve the relay attack under conservative security assumptions and we present and discuss an implementation based on a microSD secure element that still allows the access points to connect to the authorization server upon every access albeit the access points are not connected with it.