I am stuck in the proof where it says $$p_0=1/|M|$$ , I just do not understand how we can infer this equality.
Also, am I right in understanding that B is defined as choosing his two messages at random each time (i.e sampled uniformly from $M$)?
This is the link to two pictures: the first one containing the definition of Semantic Security, the second containing the definition of Message Recovery Security and the Proof I cannot follow: https://imgur.com/a/ZP6xtgq
They are from Dan Boneh and Victor Shoup's free e-book 'A Graduate Course in Applied Cryptography'"A Graduate Course in Applied Cryptography"