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In my previous question I learnt something about Group Temporal Key (GTK), but after searching the internet I have another questions:

1. How is Group Master Key (GMK) derived? What is GMK derived from (which formula describes the derivation of GMK)?

2. How is Group Temporal Key (GTK) derived? Is GTK derived only from GMK or do I need more factors than only GMK?

3. How many bits does GMK have?

4. How is Group Encryption Key (GEK) derived? Is GEK derived only from GTK or do I need more factors than only GTK?

5. How many bits does GEK have?

6. Which sets of characters can be used in Group Keys (GMK, GTK, GEK)? Do these keys contain lowercase, uppercase, digits and special signs? Can't these keys use any of the above specified sets?

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Bartosz Królak
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1 Answers1

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  1. The GMK is generated by the Authenticator, not derived.
  2. By definition:

group temporal Key (GTK): A random value, assigned by the group source, which is used to protect group addressed medium access control (MAC) protocol data units (MPDUs) from that source. The GTK might be derived from a group master key (GMK).

Still generated by the Authenticator and sent to the Supplican/STA and is 16 bits long, two bits for the ID and 14 Reserved

  1. GMK is 256 bits long.
  2. I'm not sure what you mean by GEK but I'll guess you're referring to both PTK and GTK since these are used for the final encryption of unicast traffic and broadcast/multicast respectively, both are derived from PMK or PSK. Or maybe the Temporal Encryption Key (TEK) which is generated in TKIP and used to encrypt the packets, this one derived from PTK or GTK.
  3. Uhm.... nope, sorry.
  4. Well.... the passphrase has a limit of 256 bits, or 64 octets when in hex.

It is difficult for a user to correctly enter 64 hex characters. Most users, however, are familiar with passwords and pass-phrases and feel more comfortable entering them than entering keys. A user is more likely to be able to enter an ASCII password or pass-phrase, even though doing so limits the set of possible keys.

[...]

  • A pass-phrase is a sequence of between 8 and 63 ASCII-encoded characters. The limit of 63 comes from the desire to distinguish between a pass-phrase and a PSK displayed as 64 hexadecimal characters.

  • Each character in the pass-phrase must have an encoding in the range of 32 to 126 (decimal), inclusive.

Not sure but I really hope this helps though.

Azteca
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