Resize Canvas to Longest Size

[Furn]

I have a batch of rectangular photos that I'd like to convert into square ones through adding white space/borders to the sides with the longer edges such that all sides are of the same length. I don't have preset dimensions (e.g. 600x600), but the square should have the same dimensions as the longest length of the existing photo.

I'm thinking of doing this through resizing the canvas to the longest size of the photo (with white background colour), and then ensuring the photo is centred in the new canvas.

Is this possible? The only options to resize canvas are absolute numbers (pixels, inches, cm or mm) or a relative scale...but not to either the shortest or longest lengths of the existing photo (which is for example an option under image resize).

Thanks!

[xnview]

it will be added