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 http://math.stackexchange.com/ making sure you take the advice about how to ask questions the smart way[0][1].
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.
You could also just pick something you think is interesting on Khan Academy[5] and go for it.
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 have written a book. Most people don't want to study maths, they want to have studied 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.
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 http://math.stackexchange.com/ making sure you take the advice about how to ask questions the smart way[0][1].
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.
You could also just pick something you think is interesting on Khan Academy[5] and go for it.
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 have written a book. Most people don't want to study maths, they want to have studied 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.
How well motivated are you?
[0] http://www.catb.org/esr/faqs/smart-questions.html
[1] https://www.mikeash.com/getting_answers.html
[2] http://computo.fismat.umich.mx/~fhernandez/Cursos/Calculo201...
[3] http://www.amazon.com/Calculus-4th-Michael-Spivak/dp/0914098...
[4] https://www.amazon.co.uk/Sets-Groups-Course-Algebra-Second/d...
[5] https://www.khanacademy.org/