Indistinguishability in the presence of an eavesdropper attack scenario The property of Indistinguishability in the presence of an eavesdropper
as described in the following experiment:



(tract from: Introduction to Modern Cryptography)

It must be interpreted as a scenario of  Ciphertext-only attack or Known-plaintext attack?
Thanks
It's CTO, you are trying to figure out the plaintext given two ciphertexts. M1 and M0 are just bits of either 1 or 0.