A functionality in which a sender provides 2 inputs ⚠️{$m_0, m_1$} (typically strings or single bits), and a receiver provides an input ⚠️{$b \in \{0,1\}$}. The sender receives no output (hence does not learn ⚠️{$b$}), while the receiver learns ⚠️{$m_b$} (hence does not learn ⚠️{$m_{1-b}$}).