Known-/chosen-key distinguishing attacks apply in the "open key model" instead.
[1] They are known to be applicable in some situations where block ciphers are converted to hash functions, leading to practical collision attacks against the hash.
[2] Known-key distinguishing attacks were first introduced in 2007 by Lars Knudsen and Vincent Rijmen[1] in a paper that proposed such an attack against 7 out of 10 rounds of the AES cipher and another attack against a generalized Feistel cipher.
Their attack finds plaintext/ciphertext pairs for a cipher with a known key, where the input and output have s least significant bits set to zero, in less than 2s time (where s is fewer than half the block size).
[3] These attacks have also been applied to reduced-round Threefish (Skein)[4][5] and Phelix.