Complex sentence: The key space to be searched by brute force increases by a factor of 2 for each additional bit of key length (assuming, importantly, random choice of keys) which alone increases the degree of difficulty for a brute force search very rapidly.
Split into simpler sentences: The key space to be searched by brute force attacks increases by a factor of 2 for each additional bit of key length (assuming the keys were constructed randomly). This alone increases the degree of difficulty for a brute force search very rapidly.