Circular Convolution and FFT of power 2
Circular convolution is just linear convolution aliased by DFT length $n$. The length of linear convolution of $a$ and $b$ will be $2n-1$. So take $FFTs$ of $a$ and $b$ , padding each of them to length nearest power of 2 more than or equal to $2n-1$. Multiply the corresponding $FFTs$ point by point to get a power of 2 length sequence and take $IFFT$ of it. This sequence is actually the linear convolution of $a$ and $b$ since we had done enough padding before taking their individual $FFT$. Let this sequence be named $c$. Now, alias in time domain by shifting copies of $c$ by $n$ and adding them on top of each other.
$$
d[m] = \sum_{k=-\infty}^{k=+\infty} c[m - nk]
$$
The final output you want is $d[m]\,\text{for}\, 0\le m \le n-1$