The widely used algorithm $3$DES is a triple composition of DES and has a security level of $112$ bits. Why did cryptographers not try $2$DES $($a double composition$)$ with two independent $56$ bit keys, for a total key length of $112$ bits? In your answer please make clear that you know the difference between key length and security level. Be sure to explicitly define what "bits of security" means in cryptography.