Here's a comparison that might illustrate things...
Analog is like TV off a rabbit ears antenna. Any TV will pick up analog if you just find a power socket and plug it in. Likewise, any scanner will handle analog.
Digital (P25) is like a TV with a cable box. If you have the right equipment (a cable box hooked up to your TV, or a P25-capable scanner), you can receive the intended signal.
Encryption is like a pay channel you don't subscribe to. You can tune in that channel number, but unless you get the cable company to set up your account with the info that says you paid for that channel, you won't be able to 'decode' the signal and see the channel. In the same vein, you can listen to the drunken R2D2 noises of an encrypted channel as long as you want, but unless you have been given the encryption key (password, essentially), you will not hear (or be able to transmit on) that channel.