E:failed to verify whole-file signature I have the original Samsung firmware on my Galaxy Mega 5.8

It doesn't have any of the Google Apps on it, so I decided to flash the G-apps. I got into recovery mode, and selected the appropriate G-apps ZIP, gapps-jb-20130813-signed.zip

However, I keep getting the "E:failed to verify whole-file signature" error. I've tried downloading zips from different sources, including the CyanogenMod Wiki Downloads page, the problem seems to persist.

More

Here's a list of the steps I took: (My phone is rooted)


Download the G-apps zip file.
Put it on the root folder of the SD card.
Reboot into recovery mode.
"Install update from external memory", selected the g-apps zip file.
Got the error.


I would really appreciate any help that I can get. 

I've also tried downloading the google services framework, play store and google login service APKs separately. However, the play store simply opens and closes. (Installing Google apps; &quot;Google Services Framework has stopped&quot; after installation )
This may help some folks: In my case the error was caused by a corrupt copy of the data file to the USB drive. On the offchance that I had yanked the USB out too early, I recopied, and after that the "whole-file signature" was OK and allowed the firmware update to complete.