Projects

• Sustainable Cryptocurrencies. Bitcoin is the first successful digital currency. Its popularity comes from the fact that it is decentralised, so no central authority controls it. To achieve security despite decentralisation a huge amount of computing power is constantly wasted towards generating Proofs of Work, this is economically and ecologically problematic. We work on more "sustainable" cryptocurrencies.
  
  Towards this goal we introduced the concept of "Proofs of Space" [DFKP'15],[ACK+'16],[Pie'19] as a replacement for Proofs of Work. An academic proposal using proofs of space is the Spacemint cryptocurrency [PPK+'15]. We are involved with the Chia network which will combine proofs of space with verifiable delay functions (VDF).

• Memory-hard Functions (MHF) are functions which require a large amount of memory to be evaluated. Despite having found many applications, until recently there were no meaningful security proofs MHFs at all. MHFs come in two flavours, data-dependent and data-independent. In [ACPRT'17] (Best Paper Award at Eurocrypt'17) we show that Scrypt is an "optimal" data depentend MHF. In [ABP'16] we show how "depth robust" graphs give rise to data independent MHFs with optimal parameters. In a series of papers [AB'16a],[AB'16b],[AGK+'16b] we break most of the proposals of data-independent MHFs.

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• Symmetric Cryptography. We develop new tools to construct and analyze symmetric (aka secret key) primitives. One focus lies is determening the exact security of popular schemes. We gave such tight bounds for HMAC [GPR'14] (and variants  [GPT'15]), PMAC [GPR'17] and CBC [GPT'15].