Oblivious transfer with trusted third party Trent
Alice computes $C_0$ and $C_1$ as follows:
If $b=0$ (this means $t=m$), Alice computes $$C_i = M_i \oplus R_i.$$
Otherwise (i.e. when $t=1-m$), Alice computes $$C_i = M_i \oplus R_{1-i},$$
where $i \in \{0,1\}$.

Bob computes $M_m$ as
$C_m\oplus R_t$.