Device-independent quantum secret sharing using Mermin-type contextuality

We present a new quantum secret sharing protocol based on recent advances in Mermin-type contextuality scenarios, which is provably secure against postquantum nonsignaling attackers. It is a fundamental assumption of secret sharing protocols that not all players are trusted parties, and that some may collude amongst themselves and with eavesdroppers to break confidentiality. To this extent, quantum secret sharing introduces a new layer of security, enabling eavesdropping detection via entangled states and noncommuting observables. A more thorough security analysis, however, becomes crucial if the protocol relies on untrusted devices for its implementation: for example, it cannot be excluded that some players may collude with the device supplier. In this paper, we put recent developments in Mermin-type contextuality to work in a new quantum secret sharing protocol. The maximal contextuality (aka maximal non-locality, or zero local fraction) demonstrated by the measurement scenarios results in strong device-independent security against nonsignaling attackers -- be they classical, quantum or postquantum -- which can be operationally established by means of minimal statistical analysis.