Internship Cryptographer


The Blockchain Group is a global umbrella organization of engineers and entrepreneurs with the end mission to create a blockchain-powered ecosystem that allows businesses to harness the power of decentralized energy through its various divisions

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The Blockchain technology builds upon many cryptographic primitives such as digital signatures, encryption and zero-knowledge proofs. In cryptocurrencies, elliptic-curve based signatures are used to authenticate transactions, and the ability to generate a signature is equivalent to the ability to spend one’s money. Real world cryptography asks to store keys in a manner that is both easy to use and resilient to theft and loss.

Threshold cryptography [1, 2] is an approach for this. It covers encryption and signature schemes. It is part of multiparty computation and is parametrized by two integers: t and n. There are n parties, they compute individual key shares with an algorithm of distributed key generation and store them on different devices. The maximal number of corrupted shares is the corruption threshold t. In this setting, t+1 shares are required to sign or decrypt messages, no unique key is employed and t corrupted shares do not permit to forge a signature or ciphertext.

Threshold signature scheme has been a research topic for over two decades [3]. It has gained increased interest recently due to the adoption of ECDSA in Bitcoin, Ethereum, and EdDSA in Tezos, for instance. In recent years, there has been an plenty of protocols for threshold ECDSA [5-9]. The protocols that stand out here in terms of overall efficiency are the ones by Gennaro and Goldfeder [5], Lindell et al. [6] and Doerner et al. [4], and the recent work of Castagnos et al. [9]. Besides ECDSA, Schnorr [12] and other Schnorr-like signature schemes such as EdDSA [13] are seeing an increase in interest from the cryptocurrency community, as their design is more adapted to threshold setting[14]. At least a dozen companies are now integrating threshold signatures into their commercial products.

Security against mobile adversary has been a question in the recent threshold ECDSA studies. In this model, an attacker may have access to the shares of all n parties. However, at any given point in time, not more than a threshold number of them are compromised. This approach was first considered by Ostrovsky and Yung [10] who devised the notion of a mobile adversary, which may change which devices are compromised during time. To thwart such an adversary, each party re-randomize regularly key shares. This is called as “proactive key-refresh”. This technique ensures that the views of different parties at different epochs in time are independent, and can not be combined to reveal any meaningful information about shared secrets by a mobile attacker. Recently, this paper [11] focuses on key-refresh technique. They propose an interoperable solution, that is, compatible with Schnorr, EdDSA and ECDSA protocols, limited to the two-party case.

This is a research direction to design an interoperable key-refresh solution secure against mobile adversary, in the multi-party setting.

What you will do (Responsibilities and Duties)

  • Study the proactive key-refresh solutions for threshold ECDSA and EdDSA protocols.
  • Develop a scalable solution for certain applications such as blockchains using non-interactive threshold ECDSA protocols.
  • Submit/present your work in conference/workshop/journal.


  • Duration of the internship: 6 months full-time
  • Type of contract: approved internship at the end of studies (pre-employment)
  • Avantages : salary + ticket restaurant card 9€ + Transport (100%)
  • Availability: 1 Mars 2022

Joining The Blockchain group means :

  • A career follow-up with your technical, HR and commercial referents
  • Unlimited access to our training platforms, certifications, in-house training around BI, Big Data, Machine Learning, Blockchain, web development and software
  • Workshops on innovative projects
  • A CSE with afterworks, escapes games, and other team activities!
  • Home office every week.

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