Correctness defined as "this produces the right answer" vs correctness defined as the above with the addition of "this does not leak information / is not highly vulnerable to side-channel attacks" are very different things, and only one of those definitions is actually useful for cryptography.
Writing it yourself for the first time, or just copying some code off the internet, is going to have a vanishingly small chance of being actually secure.
Writing it yourself for the first time, or just copying some code off the internet, is going to have a vanishingly small chance of being actually secure.