# Corelab Seminar

*2020-2021*

### Marios Georgiou

* Unclonable Secret Keys
*

**Abstract.**

In this talk, I will survey an emerging area called Hybrid Quantum/Classical Cryptography, in which secret keys are quantum and unclonable whereas all other parameters (such as public keys and communication) remain classical. I will motivate this setting and present several initial results:

- I will introduce the notion of equivocal collision resistant hash functions (CRHF). These are CRHF with the additional property that a quantum algorithm can sample an image y and, later, given a bit b, can find a pre-image of y that starts with b. Although such functions are impossible classically, we will see that they exist relative to an oracle, which we can then heuristically obfuscate using existing indistinguishability obfuscation techniques.

- I will present applications of equivocal CRHF including a way to design one-shot signatures. In such signatures, the signing key is a quantum state that can sign only one message. We will see that one-shot signatures give an elegant way to build a decentralized cryptocurrency without the need for a blockchain.

- I will then move to the encryption setting and show that we can combine one-shot signatures with witness encryption to build encryption schemes in which decryption keys are impossible to copy, even in the paradoxical scenario where they are generated adversarially.

- Last, I will discuss some open problems.

Part of this talk appeared at STOC 2020 and it is a joint work with Ryan Amos, Aggelos Kiayias and Mark Zhandry.

Short Bio: Marios just obtained his PhD on Cryptography from the City University of New York under the supervision of Professor Nelly Fazio and he will soon join Galois. Before CUNY, he obtained the Parisian Master for Research in Computer Science from the University of Paris 7 Diderot. Before that, he obtained his Electrical and Computer Engineering Diploma from the National Technical University of Athens.