I was reading Serious Cryptography book. I am a newbie in cryptography field who try to learn as possible as he can. In this book, there is the following section of phrases:
For example, if a ciphertext is 128 bits long (meaning the plaintext is 128 bits as well), there are 2^128 possible ciphertexts; therefore, there should be 2^128 possible plaintexts from the attacker’s point of view. But if there are fewer than 2^128 possible keys, the attacker can rule out some plaintexts. If the key is only 64 bits, for example, the attacker can determine the 264 possible plaintexts and rule out the overwhelming majority of 128-bit strings. The attacker wouldn’t learn what the plaintext is, but they would learn what the plaintext is not, which makes the encryption’s secrecy imperfect.
I understand in OTP, plaintext and key should have a same size to generate a ciphertext with same size too. Also I understand the concept of randomness behind the key which that generate random output all the time.
But in this section I couldn't understand what he said when he mentioned if our key was less than 128 bit, the attacker can rule out some plaintext. What he mean? What is the meaning of rule out in this context? Sorry if this question is so basic.