Rotem Arnon — Page 2

arXiv2012

Limits of privacy amplification against non-signalling memory attacks

The task of privacy amplification, in which Alice holds some partially secret information with respect to an adversary Eve and wishes to distill it until it is completely secret, is known to be solvable almost optimally both in the classical and quantum world. Unfortunately, when considering an adversary who is only limited by non-signalling constraints such a statement cannot be made in general. We here prove that under the natural assumptions of time-ordered non-signalling system, which allow past subsystems to signal future subsystems (using the device's memory for example), super-polynomial privacy amplification by any hashing is impossible. This is in great relevance when considering practical device independent key distribution protocols which assume a super-quantum adversary.

quant-ph↗

arXiv2012

Towards the Impossibility of Non-Signalling Privacy Amplification from Time-Like Ordering Constraints

In the past few years there was a growing interest in proving the security of cryptographic protocols, such as key distribution protocols, from the sole assumption that the systems of Alice and Bob cannot signal to each other. This can be achieved by making sure that Alice and Bob perform their measurements in a space-like separated way (and therefore signalling is impossible according to the non-signalling postulate of relativity theory) or even by shielding their apparatus. Unfortunately, it was proven in [E. Haenggi, R. Renner, and S. Wolf. The impossibility of non-signaling privacy amplification] that, no matter what hash function we use, privacy amplification is impossible if we only impose non-signalling conditions between Alice and Bob and not within their systems. In this letter we reduce the gap between the assumptions of Haenggi et al. and the physical relevant assumptions, from an experimental point of view, which say that the systems can only signal forward in time within the systems of Alice and Bob. We consider a set of assumptions which is very close to the conditions above and prove that the impossibility result of Haenggi et al. still holds.

quant-ph↗