Why isn’t open WiFi encrypted?
As far as I understand, WiFi networks that require no password send traffic through the air unencrypted. Those that require a password encrypt each connection uniquely, even if they're all using the same password.
If this is true, I don't understand why. Requiring a password for access and encrypting connections seem like totally separate issues.
Are they really linked this way? If so, is there some reason I'm not seeing? Can some routers be configured to allow public access with no password, yet encrypt the users connections to prevent Firesheep-style attacks?
Update
Some answers have said or implied that the password is a necessary "shared secret" that enables encryption. But it's not necessary. This problem was publicly solved in 1976.
The Diffie–Hellman key exchange method allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over an insecure communications channel. (http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange)
