Fast Fourier Transform & reverse transform algorithms?

I actually saw this once. I programmed it in two dimensions to model the optical blurring of a two dimensional image coming through a small aperture. I FFT'd the pixel rows. I FFT'd the pixel columns. I did the convolution by matrix multiplying the transformed image with a two-dimensional gaussian probability distribution, and then I detransformed it to get the blurred image. And then I lost the algorithm, and now I'd like to have it again so that I can play with it. Does anybody know where it is?