Arnaud Bannier is a lecturer and researcher at (C+V)O research lab at ESIEA, France. He holds a master’s degree in cryptology from the Rennes 1 University. He is now concluding its PhD thesis in computer science which deals with block ciphers.
Block encryption algorithms are now the most widely used cipher systems in the world to protect our communications and our data. Despite the fact that their design is open and public, there is absolutely no guarantee that there do not exist hidden features, at the mathematical design level, that could enable an attacker to break those systems in an operational way. Such features are called backdoors or trapdoors. The present book intends to address the feasibility of a particular class of such backdoors based on partitionning the plaintext and ciphertext message spaces. Going from the theory to the practical aspects, it is shown that mathematical backdoors in encryption systems are possible. This book, thus, intends to initiate a new field of research.
Go to the bookTrapdoors are a two-face key concept in modern cryptography. They are primarily related to the concept of trapdoor function used in asymmetric cryptography. A trapdoor function is a one-to-one mapping that is easy to compute, but for which its inverse function is difficult to compute without special information, called the trapdoor. It is a necessary condition to get reversibility between the sender and the receiver for encryption or between the signer and the verifier for digital signature. The trapdoor mechanism is always fully public and detailed. The second concept of trapdoor relates to the more subtle and perverse concept of mathematical backdoor, which is a key issue in symmetric cryptography. In this case, the aim is to insert hidden mathematical weaknesses, which enable one who knows them to break the cipher. Therefore, the existence of a backdoor is a strongly undesirable property. This book deals with this second concept and is focused on block ciphers or, more specifically, on substitution-permutation networks (SPN). Inserting a backdoor in an encryption algorithm gives an effective cryptanalysis of the cipher to the designer.
Part of the book: Partition-Based Trapdoor Ciphers