In this case, I think, the author rolled his own RSA implementation. In general, I do not know why the developer of end software should even touch the public ex

In this case, I think, the author rolled his own RSA implementation. In general, I do not know why the developer of end software should even touch the public exponent. It can be reused, and is not a secret; the library should handle it. In fact, the library should have a function along the lines of (publicKey, secretKey)=RSA.keyGen(), and provide the simple documentation that you want to keep the secret key secret. If they have crypto option which they want to expose (such as key-length), then they can take an optional parameter that either takes a hard-coded constant (IE RSA.512), or (as you suggest) validates the sanity of the input.