How long did this take you? Curious as I did an undergrad research project on multiplying large integers that basically took two semesters. I implemented Karatsuba, Toom-Cook, the complex FFT, a few NTTs, and Schonhage-Strassen. Primes are basically math magic. A Friendly Introduction to Number Theory by Silverman is an amazing math book for those interested.

FYI the link on your page reads 4025051 instead of 40250519.