Do NOT use Multibit to encrypt your keys.
Even it can't decrypt its own encryption.
I know because I have such experience.
You, the thread starter, are better off exporting the keys and encrypting them directly on your own, like using WinRar or any encryption software that you know to be very reliable (anyone who says WinRar is unreliable and/or untrustworthy, please state the evidence proving that is so).
Different wallets may use different methods of encryption and so decrypting the keys may not be compatible between different wallets.
Multibit is no longer supported now.
You have been warned.

Edit:
I was lucky I didn't uninstall my Multibit after encrypting my keys with it, so I managed to export the keys unencrypted, and encrypt them myself.
Or else, I would be in seriously deep shit.