Some months ago I found in some paper that we cannot compare symmetric algorithm with asymmetric algorithm based on the key length and more important we cannot compare asymmetric algorithm using only the key length, while a comparison of only symmetric algorithm in this term is reasonable... I explained it basing on the fact that:
in symmetric encryption algorithm the key length identifies the number of attempts necessary to decrypt the communication in both way, while in asymmetric encryption we are not sure that for a key of n bits, we will use all the possible numbers, so using ONLY the key length as a term of comparison is not sufficient, we could have key of 300 billion of bits but available only 2...
My question so is: is it true that all the possible symmetric algorithm uses all the possible numbers of n bits as possible keys or among these there are some of that not usable?
Thanks