I aways wondered if the communication channel between my wireless keyboard and the usb receiver-antena is secure. I never bother to reseach this. Today I figured out the practical way. I turned on my pc at work and I tried to type the first letter of my password. Nothing hapened. Then I started spamming that letter. Still nothing, until the person next to me said “my keyboard is typing all by itself”. It turns out she has a wireless mouse with a seemigly identical receiver-antena usb.
The moral of the story. If it was so easy to almost leak my password unintentionally due to this flaw of wireless keyboard communication, imagine wad a bad actor can do intentionally. Why try to brute force, social engineer e.t.c. when your password can be stollen in transit from your keyboard to your pc.
“Encryption” most importantly, preferably encryption checked out by third parties. It’s no guarantee, but firmware updates for peripherals such as keyboards and receivers are usually a good sign; most hardware can use a firmware update down the line, but only the shitty brands don’t make any updates after release. The lists I linked also show how companies responded to flaws in their wireless communications: no response is a bad sign, if there is a response that’s usually a good sign (but may come with instructions like “customer should buy a new, up to date receiver”).
Wired keyboards are also fine, of course, if you don’t want to deal with security risks.
If you do want to go wireless, I would personally look at Logitech’s offerings, in my experience their hardware is usually quite good and they do eventually patch their bugs (unlike, say Microsoft).
I’ll probably going to update to wired. It has all of the advanteges except portability. The only reason I got that wireless keyboard was that I needed something small, chaeap and portable.