Theodosis has a BA/MA (Honors) in Mathematics: University of Cambridge, UK, a Master of Advanced Study in Mathematics (Number Theory Group): University of Cambridge, UK, a Master of Research (MRes) in Security Science from University College London (UCL) and a PhD in Information Security and Cryptography from University College London (UCL). His PhD was fully funded by the UCL SECReT program and EPSRC.

His research interests lie in the area of cryptanalysis of both symmetric and asymmetric cryptosystems, information security, optimization techniques in cryptanalysis and applications of data analytics techniques in fraud prevention.

Theodosis has experience in both academia and industry. He has taught courses at postgraduate level at University College London (UCL) related to security science and he had worked as a Cryptologist in a project funded by Technology Strategy Board (TSB) and as a Security SME at Digital Security and Fraud at Lloyds Banking Group in London, UK. In addition, has worked as a data analyst and consultant for various London-based start-ups, consulting on data analytics and information security aspects. Recipient of the UK University Cipher Champion 2013 award.