To put it another way: You (slowly and painstakingly) defeat the authentication without having to brute force, e.g. 2^128, possible keys. However, the key is s

To put it another way: You (slowly and painstakingly) defeat the authentication without having to brute force, e.g. 2^128, possible keys. However, the key is still unknown to you. It just doesn't matter, because you can forge messages without it.