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I have a few question about group key in WPA family:

1. How many bits GTK have? How much characters this key have?

2. From which characters GTK can be generated? From 0-9, a-z etc.?

3. Is GTK the final key to encrypt packets (GTK encrypts packets directly?)?

4. If not, what key encrypts multicast traffic? Which formula describes this key derivation from GTK?

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1. GTK has 128 bits for CCMP and 256 bits for TKIP

2. Multicast traffic is protected with another key: GTK (Group Transient key), generated from a master key called GMK (Group Master Key) which is a fixed string based on access point MAC address and a random number GNonce.

3 and 4. GTK is divided into temporary keys:

  • GEK (Group Encryption Key): Key used for data encryption (used by CCMP for auth and by TKIP for encryption).

  • GIK (Group Integrity Key): Key used for data auth (used only by Michael for TKIP).

gtp

Amazing pdf manual about this: Wi-Fi security – WEP, WPA and WPA2

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