We are on the Same Side. Alternative Sieving Strategies for the Number Field Sieve

The Number Field Sieve (NFS) is the state-of-the art algorithm for integer factoring, and sieving is a crucial step in the NFS. It is a very time-consuming operation, whose goal is to collect many relations. The ultimate goal is to generate random smooth integers mod N with their prime decomposition, where smooth is defined on the rational and algebraic sides according to two prime factor bases. In modern factorization tool, such as Cado-NFS, sieving is split into different stages depending on the size of the primes, but defining good parameters for all stages is based on heuristic and practical arguments. At the beginning, candidates are sieved by small primes on both sides, and if they pass the test, they continue to the next stages with bigger primes, up to the final one where we factor the remaining part using the ECM algorithm. On the one hand, first stages are fast but many false relations pass them, and we spend a lot of time with useless relations. On the other hand final stages are more time demanding but outputs less relations. It is not easy to evaluate the performance of the best strategy on the overall sieving step since it depends on the distribution of numbers that results at each stage. In this article, we try to examine different sieving strategies to speed up this step since many improvements have been done on all other steps of the NFS. Based on the relations collected during the RSA-250 factorization and all parameters, we try to study different strategies to better understand this step. Many strategies have been defined since the discovery of NFS, and we provide here an experimental evaluation.