RAR Compression
Due to restrictions in access to external binaries, final version 1.1.0 and newer has no RAR compression support but you can (at your own risk) remove Keka's signature and run it un-sandboxed with RAR support.
Without sandbox, Keka supports RAR compression if the rar binary is found on your machine.
If you don't have the binary on your machine, you can download a trial version here or buy the full version here.
Note you'll need the command line version compiled for Mac OS X/macOS.
Where to put the rar binary
The paths that can be used are these:
/bin/rar/opt/homebrew/bin/rar(since v1.5.0)/usr/local/bin/rar~/bin/rar
Easy way to put it there
Manually
cp <DOWNLOADED_PATH/rar> /usr/local/bin/
Via Homebrew
brew cask install rar
Remember to restart Keka.
Many thanks to Alexander Roshal and the RARLAB team for it's great work.
Un-sandbox Keka
Note you'll need to do this every time you update Keka. This is an unsupported feature so run it at your own risk. If you have issues with this step, be sure to try with the web version and not the App Store version.
Simply execute this command in the Terminal:
find /Applications/Keka.app/Contents/MacOS -type f -exec codesign --force --sign - {} +
In case this command fails with "resource fork, Finder information, or similar detritus not allowed" , execute this first:
/usr/bin/xattr -cr /Applications/Keka.app
Keka will now run un-sandboxed and will be able to run your own rar binary.
Fix the security issue
On modern macOS versions the rar binary can be blocked. You will see a dialog similar to the one below:

Allow the binary:
- Open the - "System Settings..."
- Select "Privacy and Security" on the left menu
- Scroll to the bottom where you'll see something like "rar" was blocked to protect your Mac. and click on "Allow Anyway"

Repeat the compression process
Now you'll probably see a dialog similar to the one below. Click on "Open Anyway" and type your password if asked. You should be set now, repeat the compression if the process failed again.

Content source: Keka official GitHub Wiki (https://github.com/aonez/Keka/wiki)
