Resize Canvas to 800x800 and Transform Rectangular Image to fit in those bounds

A simple action that you can batch will do it.  The tricky part is recording the Canvas size step so that 800 width and height are both recorded. Record

Step 1 Flatten (so canvas added will be filled with the background color)

Step 2 File>Automate>Fit Image... width and height set 800px. (Images aspect will be maintained so there no distortion The resulting image width or height may be less than 800 px.

Step 3 Canvas size 800px wide 800 px hight, (You must force this step to recorde correctly.) Stop recording before recording the Canvas Size step. Crop the fitted  work image to a smaller size so both the width and height are less the 800px.  Then click the record icon and record the Canvas size step 800px by 800ps leave the anchor point in the center and select the background color you want.

Stop recording the action is complete.

Menu > File > Automate > Fit Image...

// 2017, use it at your own risk;

#target photoshop

if (app.documents.length > 0) {

var myDocument = app.activeDocument;

var originalRulerUnits = preferences.rulerUnits;

preferences.rulerUnits = Units.PIXELS;

var theTarget = 800;

// get longer side;

var theFactor = theTarget / getTheLonger(Number(myDocument.width), Number(myDocument.height));

// resize;

myDocument.resizeImage (myDocument.width * theFactor, myDocument.height * theFactor, myDocument.resolution, ResampleMethod.BICUBICAUTOMATIC);

myDocument.resizeCanvas (theTarget, theTarget);

// reset;

preferences.rulerUnits = originalRulerUnits;

}

else {};

function getTheLonger(a,b) {

  if (a >= b) {

  var x = a}

  else {

  var x = b};

  return x;

  };