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Potterwatch is a clandestine radio program:

Potterwatch, didn’t I tell you that’s what it was called? The program I keep trying to get on the radio, the only one that tells the truth about what’s going on! Nearly all of the programs are following You-Know-Who’s line, all except Potterwatch, I really want you to hear it, but it’s tricky tuning in…”

Its security is described like this:

  • You only need to know one password to listen to it.
  • There is no protection against brute force attacks.
  • The password changes each time, but they say the next one at the end of the program.
  • The password is usually related to the Order, e.g. ‘Albus’.
  • They use pseudonyms, but since they don't distort the voices, they can be recognized.

This doesn't seem much safe. The bad guys will eventually find a password and be able to listen to the program. To prevent that, they could use stronger security. Or they could just broadcast with no password, and have a bigger potential audience of good guys.

Therefore,

  • Why do they use this kind of protection?
  • Does it have any advantage over other alternatives?
  • Is there any evidence that this is intentional, because they are using Potterwatch to leak false information to the enemy, in a non-suspicious way?
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    I don't think wizards are especially savvy at info security. This seems a lot like Radio Free Europe and Radio Londres, resistance stations that would change frequency frequently to stymie attempts to block their signal. – Valorum Apr 4 '15 at 23:32
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    It is highly unlikely that JKR had a security professional analyze the security of this approach. She may well have modeled it upon real security devices (like RSA.s SecurID token) or may have simply made it up from whole cloth. – atk Apr 5 '15 at 2:30
  • @Oriol - Much better. My downvote is now an upvote. – Valorum Apr 5 '15 at 6:24
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I am not aware of canon information that directly addresses this problem, and I don't know how thoroughly J.K. Rowling thought it through. Perhaps when the extra DH information is put on Pottermore, she will offer a more thorough explanation. Given the similarities between the Second Wizarding War and World War II, I think it is likely that she based the idea on secret radio stations run by the Resistance. Like Potterwatch, some of the Resistance stations would include comic segments along with more serious news. They were sometimes used to organize active resistance, but more often they were focused on countering propaganda and raising morale, as Potterwatch seems to be.

Why do they use this kind of protection?

Exactly what "brute force" attacks could be conducted against magical radio is unclear--as is the nature of the magic used to develop magical radio stations. In WWII, the Axis attacks on Resistance stations mainly consisted of jamming the airwaves, because they could not decipher the messages quickly enough to prevent operations from being carried out. The best they could do was prevent the Resistance stations from being heard. If magical radio is similar, then perhaps the attacks that can be leveled against it are limited.

The best protection that the station seems to have is its irregularity, since the passwords would be easy for anyone familiar with the Order of the Phoenix to guess. Still, it did take Ron weeks to find Potterwatch, even when he knew how it worked (thanks to Bill and Fleur). The Death Eaters might not be so patient, especially since the radio station is not being used to organize outright warfare. The book does not explain whether the protection used for Potterwatch is the Order's preference, or the only option available.

That the Potterwatch commentators do not distort their voices somehow does seem risky, since the major danger to Potterwatch is apparently that Death Eaters might listen in. Most likely the commentators assumed that none of the Death Eaters knew them well enough to recognize their voices. Kingsley's fellow Aurors might recognize his voice, but--since Voldemort does not seem to have truly trusted the Ministry--Ministry workers would probably not be tasked with finding secret radio stations. And if the Death Eaters found one, they would not know whom to ask to identify the voices.

Does it have any advantage over other alternatives?

We do not have enough information about how magical radio functions to know whether there are alternative ways of protecting a magical radio station. Better passwords seem like one very obvious improvement; but that could prevent opponents of Voldemort from finding the station again if they missed an episode.

Is there any evidence that this is intentional, because they are using Potterwatch to leak false information to the enemy, in a non-suspicious way?

I am aware of no evidence that Potterwatch was being used to leak false information. DH does not even record Potterwatch coordinating resistance operations--the program Ron, Harry, and Hermione listen to seems to be primarily focused on countering false rumors and raising the resisters' morale. Perhaps Potterwatch is used to coordinate resistance operations, but, since wizards can Apparate, members of the Order might think that meeting in person is safer than sending coded messages over a radio station to which Death Eaters could conceivably tune in.

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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:

  1. The radio doesn't broadcast 24/7

Which means they might nail the password but at a wrong time

  1. 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

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You're thinking like a modern muggle. Remember Hermione's quote from the Sorcerer's Stone: "This isn’t magic – it's logic – a puzzle. A lot of the great wizards haven’t got an ounce of logic, they’d be stuck in here forever."

The security Potterwatch has is the best its creators can think of. Not very good by our modern, computer-savvy muggle standards, but for people who aren't renowned for understanding even basic muggle technology or for logic...

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