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My friend wants to keep his home network super-secure, due to some trouble that he's had with unwelcome Wi-Fi intruders. So, naturally, he devised an ultra-long WPA2 passphrase (on the order of 100+ characters) to thwart any would-be intruders.

Now, to my knowledge, even a simple 10-character alphanumeric passphrase would be quite secure for everyday use. Nonetheless, he insists on overkill. My question is, can this gigantic passphrase be detrimental to the performance of the network, or somehow cause slowdown at all? And, why or why not?

This question arose because I was having trouble connecting an older laptop, running Xubuntu, to his WPA2 network, whereas the laptop connects and functions fine with other WPA2 networks.

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Most routers I've encountered max the PSK at 63 characters. Are you sure that password is "100+ characters"? –  Iszi Apr 29 '12 at 15:34
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@Iszi: Well, I don't have it off hand to test my theory, but it definitely looked ridiculously long. For the sake of argument, let's say that the password is indeed "just" 63 characters - I think that's still too long to be useful. –  voithos Apr 29 '12 at 17:16
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Is there such a thing as an unnecessarily long password? –  Steve Apr 29 '12 at 17:39
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@voithos For something that you have to enter one time per system it's not so bad. Mine's 63 characters, randomly generated, stored on a text file in an encrypted volume. Add on MAC address filtering for the network, and you'd better have a really good reason to want to use my Wi-Fi when you ask. ;-) –  Iszi Apr 30 '12 at 11:49
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@voithos Ah, and I forgot to throw this in earlier. Quote from a friend of mine. Not sure if it originates with him, or elsewhere. "Overkill is underrated." –  Iszi Apr 30 '12 at 16:51

2 Answers 2

up vote 10 down vote accepted

No. It doesn't matter how long your passwords are because that value is never transmitted during the WPA-PSK Key Exchange. Instead a CMAC is calculated based on the secret key, the client and server id, as well as client and server provided large random values.

Regardless of how large your password is the resulting CMAC for WAP2-PSK will always be 128 bits:

MAC_P = CMAC-AES-128(AK, ID_P||ID_S||RAND_S||RAND_P)
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Is this mechanism used for other wireless connections besides WPA2? –  Pacerier Feb 16 at 8:06
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@Pacerier, please don't use the comment thread to ask new questions. This is not a discussion forum: comments should be used only to improve the answer. This question is about WPA2. If you have other questions about other schemes, they should be posted as a separate question. –  D.W. Feb 16 at 11:27

No. A long passphrase will not cause slowdown.

The passphrase you type in is first converted to a fixed-size cryptographic key (e.g., a 128-bit key); no matter how long the passphrase, the cryptographic key will be the same size. Encryption and decryption take exactly the same amount of time, regardless of the length of your passphrase.

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xkcd.com/285 –  Pacerier Feb 16 at 8:06

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