Improving the Efficiency of Code-Based Cryptography

Abstract

Recent public-key cryptography is largely based on number theory problems, such as factoring or computing of discrete logarithm. These systems constitute an excellent choice in many applications, and their security is well defined and understood. One of the major drawbacks, though, is that they will be vulnerable once quantum computers of an appropriate size are available. There is then a strong need for alternative systems that would resist attackers equipped with quantum technology. One of the most well-known systems of this kind is the McEliece cryptosystem, introduced in 1978, that is based on algebraic coding theory. There are no known vulnerabilities against quantum computers, and it has a very fast and efficient encryption procedure. However, it has also one big aw, the size of the public key, that makes it impractical for many applications. The first part of this thesis is dedicated to finding a way to significantly reduce the size of the public key. Latest publications achieve very good results by using codes with particular structures, obtaining keys as small as 4,096 bits. Unfortunately, almost all of the variants presented until now have been broken or proven to be insecure against the so-called structural attacks, i.e. attacks that aim to exploit the hidden structure in order to recover the private key. My work is based on Generalized Srivastava codes and represents a generalization of the Quasi-Dyadic scheme proposed by Misoczki and Barreto, with two advantages: a better flexibility, and improved resistance to all the known attacks. An efficient implementation of the above scheme is also provided, as a result of a joint work with P.-L. Cayrel and G. Hoffmann. In the next chapters, other important aspects of code-based cryptography are investigated. These include the study of a higher security standard, called indistinguishability under a chosen ciphertext attack, in the standard model, and the design of a code-based key encapsulation mechanism (KEM), which is an essential component of the hybrid encryption protocol. The last chapter is about digital signatures, a fundamental protocol in modern cryptography; existing code-based signatures schemes are reviewed and a negative result is obtained, showing that the design of an efficient signature scheme based on coding theory is still an open problem.