November 24, 2007

The Tivo-Airport-WEP saga

A few months ago, my dad upgraded the house wireless router to an Airport Express, and in the process set it to use WPA2 encryption. (The previous router was capable of some encryption, but we never used it.) Unfortunately, this prevented two laptops and the Tivo from using it. The laptops were a bit annoying, but the Tivo was not getting any program information, making it only marginally better than a VCR. My mom and sister just lived with it, but I talked my dad into backing the encryption off to WEP (which all the relevant devices could handle). This should have solved all our problems, but it did not. My laptop (an old Titanium powerbook running 10.3) now worked fine; his laptop and the Tivo seemed to think that the router was still demanding WPA.

I sort of spun my wheels on that for a little while, until Dad pointed out that what he had actually set it to was "WEP (Transitional Security Network)". That meant that my laptop, which didn't even know about anything more than WEP, worked fine, but the other devices, which knew about the existence of WPA, were having problems. Unfortunately, plain-old WEP was not an option; we thought it might be if we could change the router from "802.11n (b/g compatible)" to just plain old 802.11g (or b), but that wasn't an option. A tiny bit of googling turned up a page about exactly this problem that told us about the super-secret options: if you option-click that pull-down menu, other options (including 802.11b/g compatible) appear. And when you select that, the encryption option includes WEP 128 bit, i.e. "plain-old WEP". Hurray!

Except, that brought the other laptop online but not the Tivo. Despite the fact that that webpage was explicitly about Tivos and Airports Express. Hmm. The error message had at least changed; where it used to say "Wpa not supported"*, now the Tivo was letting us type in a password, and the problem was now that it was unable to find a DHCP server. Mysterious.

Some more googling turned up another page, also purportedly solving the problem of making Tivo work with an Airport Express, which says that deep in the bowels of the Tivo online documentation, they say that to work with an Apple router you have to enter the password in hex. It also gives the way to find out the hex version of your password, except that this post is three years old and its instructions do not apply to my dad's version of the Airport Admin utility.

Now, why would we need a password in hex? What does that even mean? Well, hex is just a convenient way to represent a raw number—two hex digits make up one eight-bit byte—and of course the actual encryption algorithm uses raw numbers. In particular, the 128-bit WEP algorithm uses a 104-bit key (64-bit uses a 40-bit key), which is 26 hex digits (or 10 for the shorter one). That'd be hard for people to remember, so most manufacturers let you type an arbitrary alphanumeric password in and then convert that to a 26-digit hex number. As long as the conversion is always done the same way, the right password will translate into the right hex passkey. The problem is, different manufacturers used different algorithms, and since the modem and router are talking to each other in hex, the "right password" that you set up on (say) your Airport might not be the same "right password" you'd need on your Linksys modem!

Which is why you'll sometimes see advice about picking a password that's exactly 13 characters. The obvious way to convert a password into hex is to just take the ASCII value of each character—8 bits—and write this down in hex—two digits—and so a 13-character password would give you a 26-digit hex key. Anyone that uses this obvious method for the 13-character passwords would be able to talk to each other. If there are less than 13 characters, there might be disagreement on how to pad it; if more, disagreement on how to truncate or hash it. But we had a 13-character password, and it still wasn't working.

Because something's funny about how Tivo's doing the password-key translation, I guess. Which means I need to find out how exactly Apple is converting its passwords into hex keys. Fortunately, googling for this information turned up a page at Apple that claims that all manufacturers use this "obvious" method if the password is exactly 13 characters. This is, evidently, false; but since it's Apple making the claim, I can assume that it is at least true of Apple. So I converted the damn password to hex by hand (using an ASCII table). Then I went to type it into the Tivo, was momentarily flummoxed when there didn't seem to be any way to actually enter the key in hex; it turns out there's a message in fine print at the bottom of the TV screen that says "Press INFO for hexadecimal". Do so, and it gives you a much smaller key entry table, though the digits from 0-9 you can just type into the remote's keypad directly. The software appeared to be set up to accept hex keys up to something like forty digits, which is odd, but I typed in the 26 I had, and from there on in everything worked perfectly.


But because the info about this was so hard to track down, I figured I'd write it up. I know there's still a lot of series-2 Tivos out there, and as people upgrade their wireless routers this is bound to happen to someone else. (The disclaimer here is that by bumping the router down to WEP, your security isn't going to be as good as for WPA or WPA2, if that's a concern. Bumping it down to b/g compatible is also a slight concern, as you won't be getting the full 802.11n performance BUT if your network has any devices on it that run at 802.11g or 802.11b, having them on the network will downgrade the entire network to that slower standard, so making it actually "802.11b/g" instead of "802.11n (b/g compatible)" will not in practice make much of a difference if you've got older devices hanging around.)

*On this device. The Tivo itself can handle WPA, apparently, but the particular USB wireless modem that Tivo sold me couldn't.

"Phelps... believes in a god of hellfire, but he doesn't actually build the hellfire himself—he's got enough faith to leave it to his god to do that. ...There's plenty of actual awfulness in the world; guys like Phelps are just poseurs, like trendy college Satanists, and responding to them as an actual threat just seems like it would do nothing but feed the delusion." --Jonathan Prykop

Posted by blahedo at 5:36pm on 24 Nov 2007
Thanks! I had exactly this problem and you helped me out tremendously. Posted by Jeff at 10:26pm on 18 Jan 2012
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