I have to disagree recursion is unecessary and clunky. What should be done is repeating a block of code n times, this is achieved by a loop of any kind not recursion. If you unroll above code you have alot of redundancy (null check and creating StringBuilder) and absolutely no benefit.

Using substring that often is very expensive.

It is a very sad thing that using an API that is deprecated since 1997(!) is considered best practice.

This coder understood the mechanism behind the encryption / decryption; recursion makes it very elegant.

O(N^2) Best practice... wow. )))