Cryptologist, code breaker & Senior Lecturer at University College London.
Dr Nicolas T. Courtois is a cryptologist at University College London, UK. He has authored a number of research papers about bitcoin and digital currencies. His main interest is to study the security of bitcoin against a variety of miner and network attacks. A number of events predicted by Dr. Courtois have happened later on, for example regarding the Block Withholding Attacks, see Section XI.A in arxiv.org/abs/1402.1718.
More importantly Dr Courtois is the author of the so called "Theory of Programmed Self Destruction" of crypto currencies (cf. arxiv.org/abs/1405.0534). It is about how a combination of factors such as the Longest Chain rule of Satoshi Nakamoto, the built-in monetary policies, deficiencies in timestamping, and the possibility to shift hash power rapidly from one crypto currency to another, may rapidly erode the protection against double spending and 51% attacks. This can lead to rapid decline in their hash power and their security. For example Unobtanium and Dogecoin have quite serious built-in problems unfolding in the recent months, and these currencies can hardly survive in the current form without some changes in the code, see Sections 10 and 11 in arxiv.org/abs/1405.0534.
These security questions have important practical implications for a majority of ordinary users: today people wait for tens of minutes to accept a payment transaction precisely because they fear there might be a victim of an attack. It is clear that improved security for bitcoin transactions also implies improved speed, making digital currencies more attractive for ordinary commercial transactions. In a new paper presented at SECRYPT 2014 in Vienna, Courtois, Emirdag and Nagy discuss whether there is a solution to this problem and whether bitcoin transactions could be like 100x faster, this WITHOUT changing the speed at which the blocks are generated (which is what makes it a difficult and challenging question).
Nicolas Courtois blog
Here you can find the slides of his presentation: "Bitcoin: Extending Freedom - Nicolas Courtois" (.PDF 2,2 MB)