Cryptographic primitives, such as digital signatures or encryption schemes are widely deployed to secure all-digital communication. Most of these primitives are inspired by an analogue counterpart in the physical world: The analogue of a handwritten signature is a digital signature scheme and it is used to prove the authenticity of documents resp. digital information. Interestingly, many cryptographic primitives, such as digital signatures, offer stronger security guarantees in the digital world than their counterparts in the physical world. Consider the following examples:
Authentication. Most authentication systems deployed in practice rely on the two-factor authentication paradigm. The basic idea of this paradigm is that a user authenticates himself by a possession and a knowledge component. The most prominent example of a deployed system are ATMs, where the user inserts a credit or debit card in the ATM and proves knowledge of the corresponding secret personal identification number (PIN) in order to gain access to his