Since it came up once again that you can't keep secrets on the fediverse, sure you can. I'll make this visible for those using software platforms that don't support it. On our platform, you just click the lock icon and enter the key (which presumably you and I previously agreed to "somehow") and view the message. Go ahead and try if you want, but you're probably wasting your time. This is level 3 encryption. I can't legally describe what is involved in higher numbers, but let's just say there is no upper limit.
This is not suitable for state secrets. The internet is not suitable for state secrets. Your device is not strong enough for state secrets. Your infosec discipline is unlikely to be suitable for state secrets. But this isn't too bad for your day-to-day "I want 'xyz' to read this but I don't want either of our site admins or their service providers to read this".
[ crypt alg='AES-128-CCM' hint='65255431382d7a405272793221']eyJpdiI6InoweWJoUW1CMFJVbGJwd2JIejNXZ3c9PSIsInYiOjEsIml0ZXIiOjEwMDAwLCJrcyI6MTI4LCJ0cyI6NjQsIm1vZGUiOiJjY20iLCJhZGF0YSI6IiIsImNpcGhlciI6ImFlcyIsInNhbHQiOiJqVnZSY0hPN3d3bz0iLCJjdCI6IjZoNUpyWTVUT1dvcmlKdzQ3MWkrekMwMTZzR1hlVGNwZU9lRUFmKzBoQT09In0=[/crypt ]