Complex sentence: There exist information-theoretically secure schemes that cannot be broken even with unlimited computing power--an example is the one-time pad--but these schemes are more difficult to implement than the best theoretically breakable but computationally secure mechanisms.
Split into simpler sentences: There exist information-theoretically secure schemes that cannot be broken even with unlimited computing power. An example is the one-time pad--but these schemes are more difficult to implement than the best theoretically breakable but computationally secure mechanisms.