I have a different take on the same topic: <a href="https://www.nayuki.io/page/java-biginteger-was-made-for-rsa-cryptography" rel="nofollow">https://www.nayuki.io/page/java-biginteger-was-made-for-rsa-...</a><p>My article isn't written as a step-by-step tutorial and doesn't come with example numbers. But mine fills in certain things that xnacly doesn't cover: random prime generation, efficiently calculating the decryption exponent d from (n, e) by using a modular inverse, using modular exponentiation instead of power-then-modulo.<p>By the way for Python, modular exponentiation is pow(x, y, m) (since 3.0), and modular inverse is pow(x, -1, m) (since 3.8, Oct 2019). <a href="https://docs.python.org/3/library/functions.html#pow" rel="nofollow">https://docs.python.org/3/library/functions.html#pow</a>