QLunch: Quantum ciphertext authentication and key recycling with the trap code

Speaker: Florian Speelman from CWI

Title: Quantum ciphertext authentication and key recycling with the trap code

Abstract:
How can we make sure that a quantum message remains unaltered when we send it over an insecure channel? How do we protect a quantum state from being corrupted when it is stored someplace where adversarial parties can potentially access it? And how can we have an untrusted third party perform computations on such authenticated data? The main protagonist of this talk will be a specific quantum authentication code called the "trap code" (Broadbent et al., 2013), which has proven to be very useful in several quantum cryptographic tasks. It also has a weakness, however: an adversary can learn information about the ciphertext structure by altering the ciphertext in a specific way, and observing whether or not the corrupted ciphertext is still accepted by the authentication protocol. 

The trap code is vulnerable to these types of attacks, because it lacks a property called strong purity testing (Portmann, 2017). I will discuss the strengths of authentication codes that do have this property, and I will also show how the trap code can be adjusted to become a strong-purity-testing code.  The talk is based on joint work with Yfke Dulek.