I am going to awnser this from a logical standpoint, without resorting to actual character or magic (Deus ex Machina) knowlege to maintain clarity. The only magic I'm refering to is the password input, which in essence, isn't really that magical. The only actual magic in operation here is the fact that you need a password to access a broadcast (the broadcast is magical, not via a frequence)
- You need to cast a spell to access the frequence (saying the password)
- The password is different every day (Let's say it changes every 24h)
- Only anti-Voldemort will know the password (unless they force it out of someone)
Does it have advantages against other alternatives?
You mentioned bruteforce. A bruteforce attack on a password is attempting to crack the code by trying out every single possible alternative. This can be even harder if you don't know how many characters (letters, digits, words, sentences, even entoations) the password has. While it may still be easier to do if you limit it to names, Potterwatch could always change the regimen of passwords (to include places, dates or even a random combination of letters and numbers). Despite that, a bruteforce is still possible and would've been helpful except for two things:
- The radio doesn't broadcast 24/7
Which means they might nail the password but at a wrong time
- The password changes daily
Unless Voldemort's people are extremely lucky, they won't guess it in time for the next one
Static not Broadcasted Password
As in always the same password. This is a bad method as one may be forced into giving the password, thus threatening every broadcaster. Bruteforcing will also eventually work.
Dynamic not Broadcasted password
Exactly the same as the above. The password changes, which prevents bruteforce attacks to be effective, but it follows a certain order (else there wouldn't be an audience). Crack the code, you get all the passwords (or force the code out of someone)
With the two methods mentioned above (Static and Dynamic not broadcasted), is also susceptible to unintentional leaking. To get a larger audience people would have to share the password everyday should you miss a day, and owls, fireplaces (if someone actually managed to do it) and other communication methods were heavily surveilled, and it would pose a great risk. You can argue that with the system used the same would still be valid, which is true, but since they kept the same line of passwords (deceased members of the Order), people who understood the origin of the passwords would be able to catch the broadcast even if they missed one day (Like Bill). It's highly improbable that Voldemort's people know who is the current Order of the Phoenix, much less the names of the ones that have died.
Why do they use this kind of protection?
It's simple - they can maintain their viwership if they tune in
It's safe - The password changes everyday, a leak would have to be very well timed to work
It's forgiving - If you missed one broadcast you could catch the next one if you knew where to search
And no, torturing to get the password wouldn't be that helpful. Most likely a torture would yield no results as even if they got the password, which is what they'd be looking for, they wouldn't know the password is dynamic, nor they would know the participants, nor they would know the time of the broadcast, nor the wizard being tortured would (probabbly) give the password until he's sure that the password is no longer valid