Encryption, the length of the key in the algorithm and the length of the passphrase - the difference?

Question

Encryption

Hello.
There was a question, Google especially does not help to find a specific answer.
There is a symmetric encryption algorithm (AES, Serpent, not specifically) whose key length is 256 bits, i.e. the number of options is 2 to the power of 256 and there is a passphrase, for example, 100 characters long (ASCII), the number of options in which is 2 to the power of 800. The algorithm (at least Serpent) somehow "conjures" the key before it is applied.
Which of these "numbers" characterizes the resistance of a particular ciphertext to brute force brute force in the described situation (i.e. stupid number of options for brute force)? 2 in 256 or 2 in 800?

Answer 1

Depends on the algorithm. You should be interested in the length of the shortest piece of data required for decryption, it is different for different algorithms.