IND-CPA not semantic security

[davidwagner]

Rather than introducing the new notion of semantic security just for public-key encryption, we should continue using IND-CPA security. The very same game and definition of IND-CPA security also works for public-key encryption. We could add a footnote about it being called semantic security in the literature, and a little discussion (no need for an encryption oracle, adversary receives the public key).