I'm on a Mac Pro (Late 2013). I hve noticed this problem since 10.13.5 although I couldn;t find anything on the forums. it also seems to happen on Mojave. Also there is no encryption.
1) Software updates fail at one of two places: 10.3 MB or if your download is less than 10.3 MB at approximately 1MB less than the download size. A 7.3 MB file would fail at about 6.3 MB
2) You are told to use the "Purchase" page to download.
3) It makes no difference if you do you'll end up at step 1 then 2 and rinse and repeat.
When the failure happens a sheet will drop down and the indicated download will be 0 bytes.
What worked for me:
1) Start the download from Updates or Purchases.
2) At about 1MB short of the 10.3 MB point (somewhere in the 9MB range) pause the download.
or at about 1MB short of the file size if less than 10.3 MB pause the download.
3) Wait for about 10-secionds. Long enough for the hand-shakes to go back and forth.
4) Resume the download. If it gets past the 10.3 MB boundary it will complete its download.
THis was happening to me on build 17G54a as well. Thank you for the workaround it worked!
You should file a bug report, as the problem can be reproduced.