As always, it depends ...<p>If you just want to learn the maths relevant to your specific interests then you can simply pick a page on wikipedia, build a tree of topics you need to cover, then start to knock them off one by one, building a web of knowledge as you go. Then ask questions on <a href="http://math.stackexchange.com/" rel="nofollow">http://math.stackexchange.com/</a> making sure you take the advice about how to ask questions the smart way[0][1].<p>If, on the other hand, you want to get into studying maths generally and build your maths study skills, then I would recommend starting with a really good maths text book and work through it, doing all the exercises, reviewing earlier material, and taking it seriously. Two options are Spivak[2][3] which claims to be about calculus, but is really about analysis, or "Sets and Groups" by Green[4]. The latter is great to create the underlying basic knowledge you need for cryptography, but more, it teaches you how to do maths properly.<p>You could also just pick something you think is interesting on Khan Academy[5] and go for it.<p>But having said all that, it's tough to get back into maths, and you need to make sure you really understand your motivation. Most people don't want to write a book, they want to <i>have written</i> a book. Most people don't want to study maths, they want to <i>have studied</i> maths. If you're not serious, you won't succeed, especially with no one to track your progress, answer questions, and generally encourage, coax, support, and inspire you, it will be tough.<p>How well motivated are you?<p>[0] <a href="http://www.catb.org/esr/faqs/smart-questions.html" rel="nofollow">http://www.catb.org/esr/faqs/smart-questions.html</a><p>[1] <a href="https://www.mikeash.com/getting_answers.html" rel="nofollow">https://www.mikeash.com/getting_answers.html</a><p>[2] <a href="http://computo.fismat.umich.mx/~fhernandez/Cursos/Calculo2015/spivak.pdf" rel="nofollow">http://computo.fismat.umich.mx/~fhernandez/Cursos/Calculo201...</a><p>[3] <a href="http://www.amazon.com/Calculus-4th-Michael-Spivak/dp/0914098918" rel="nofollow">http://www.amazon.com/Calculus-4th-Michael-Spivak/dp/0914098...</a><p>[4] <a href="https://www.amazon.co.uk/Sets-Groups-Course-Algebra-Second/dp/0710212275" rel="nofollow">https://www.amazon.co.uk/Sets-Groups-Course-Algebra-Second/d...</a><p>[5] <a href="https://www.khanacademy.org/" rel="nofollow">https://www.khanacademy.org/</a>