Nicolas Courtois is a Security Architect and Crypto Research Engineer at Qualcomm, Sophia Antipolis, France. He is a cryptologist and code-breaker.
Previously he was a lecturer at University College London, UK. He has done his PhD thesis in cryptology at Paris 6 University.  His H-index is 41 according to Google Scholar which lists more than 200 publications. Here is Nicolas Courtois on ResearchGate.

Industry, Smart Cards, Payment:

For 7 years he worked as a cryptologist in a biggest secure hardware manufacturer in the world Gemalto. He has filed 10+ patents on industrial applications of cryptology, embedded security, smart cards and side-channel attacks. He is the (only) author of the so called Courtois Dark Side attack [a rare sort of simultaneous differential card-only attack] which allows to extract keys from some 2 billion MiFare Classic cards sold, one of the highest impact attack in smart card history.

Blockchain and Bitcoin Security and Privacy:

He has authored several papers on bitcoin. Here are his slides on bitcoin.
He is the author of the so called “Theory of Self-Destruction of Crypto Currencies”. He has invented developed countless ways to compute private keys in bitcoin due to bad randoms, key management, brain wallets etc, see for these slides and this paper or this.  His recent works are about hiding criminal or just ordinary anonymous transactions inside the ordinary bitcoin blockchain some of which will be just totally invisible,  see ICISSP 2017 / slides.

Cryptanalysis:

He is a highly influential code-breaker and author with more than 150 publications and 9300 citations. His research focuses on the security analysis of cryptographic schemes with particular focus on realistic attack scenarios where the amount of data available to the attacker is very low.

He is responsible for the cryptanalysis of many real-life ciphers used by hundreds of hundreds of millions of people every day, such as the Bluetooth cipher E0, the automobile cipher KeeLoq and the MiFare Classic Crypto-1 system which is very widely used in public transportation (e.g. Oyster Card) and building access control cards.

A team directed by Dr. Courtois has won the UK University Cipher Champion 2013 award (older link) on 12 March 2013. In 2018 Dr. Courtois has invented a general method to backdoor a block cipher though non-linear algebraic polynomial invariants. An improved construction is given here.

Post-quantum Cryptography:

He has authored or co-authored 20 papers in Post-Quantum Cryptography. 19 are listed here: http://pqcrypto.org/mq.html (key words: HFE, Sflash, Quartz), and one more is listed here http://pqcrypto.org/code.html.

Here is his UCL home page and his UCL research portrait. and a video interview from 2015.