Universally Composable Server-Supported Signatures for Smartphones

Smart-ID is a signing and authentication service available to residents of Belgium, Estonia, Iceland, Latvia and Lithuania. Such smartphone-based signing service delivers the usability of software keys with security guarantees of ID-cards by relying on a remote server to compensate for weak device protection. Security of current Smart-ID relies on multi-prime server-supported RSA, password-authenticated key shares and clone detection mechanism. Unfortunately, the security properties of the underlying protocol (Buldas et al. ESORICS 2017) have been specified only in “game-based” manner. In other words, there are no guarantees for the protocol being secure in concurrent setting. We remedy this shortcoming by presenting two ideal functionalities ℱ gSpl and ℱ Sig Spl for server-supported signing in Universal Composability model. We show that improved RSA server-supported protocol realizes ℱ gSpl and give practical example of server-supported ECDSA protocol realizing ℱ ECDSA Spl .