Among the {{math|''b''}}-bit numbers, the most difficult to factor in practice using existing algorithms are those [[semiprimes]] whose factors are of similar size. For this reason, these are the integers used in cryptographic applications.
In 2019, Fabricea Boudot, Pierrick Gaudry, Aurore Guillevic, Nadia Heninger, Emmanuel Thomé and Paul Zimmermanngroup factored a 240-digit (795-bit) number ([[RSA-240]]) utilizing approximately 900 core-years of computing power.<ref>{{cite web| url = https://lists.gforge.inria.fr/pipermail/cado-nfs-discuss/2019-December/001139.html| url-status = dead| archive-url = https://web.archive.org/web/20191202190004/https://lists.gforge.inria.fr/pipermail/cado-nfs-discuss/2019-December/001139.html| archive-date = 2019-12-02| title = [Cado-nfs-discuss] 795-bit factoring and discrete logarithms}}</ref> The researchers estimated that a 1024-bit RSA modulus would take about 500 times as long.<ref name=rsa768>{{cite conference
