Why is a known-plaintext attack considered a complete break? I read somewhere that if a cipher has a known-plaintext attack, then it is considered completely broken.

Say there is a cipher that someone uses where the algorithm is understood and a known-plaintext attack is trivial to perform, however a ciphertext only attack is as impractical as brute force. The XOR-Cipher with the key length equal to message length, for example.

If the user changes the key for every message sent, then what use is a known-plaintext attack?

It's always been my understanding that if one finds the key to a message they already know but to nothing else, then that is just as useless as not being able to perform a known-plaintext attack at all.

So what use is a known-plaintext attack in a world where a key is never used more than once (effectively a one-pad pad)? How is that "a complete break"?
if a cipher has a known-plaintext attack, then it is considered completely broken.


Yes, pretty much...


  [Paraphrased] But can't we come up with a case where this isn't true, such as a One Time Pad?


Yes, we can come up with cases like that; however the requirements of such a case (key as long as the plaintext, no key reuse) make such a cipher impractical.

Instead, when we talk about such a cipher:


We generally use the same key to encrypt multiple messages (for example, consider TLS, where the the symmetric key is used to encrypt all records going in the same direction)
And, even when we use a key to encrypt only one message, the attacker might be able to guess parts of it; if he can use those guessed parts as known plaintexts, and recover the parts he didn't know (or even to verify if his guess is accurate), well, the cipher didn't do its job.


"Known plaintexts implies total break" is an excellent rule-of-thumb, even if you can come up with cases where it is not true.