MIC culculate and Key Mixing problems
Last Post: April 30, 2007:
In Jon Edny,William a.arbaugh's book,"Real 802.11 Security" .It mentioned that MIC is computed at the MSDU level.
But when I watched CWSP study guide 2ed.In page 424~427, It sees to be culculated in MPDU level.
what is correct ?
2. Dose Temproal Key used in Phase II Key Mixing Phrase ?
Study guide 2ed metioned Temporal Key is one of the input element . but I saw other internet metirials and "Real 802.11 Security" Temporal Key is not an input element .
what's correct ?
TKIP computes and appends a MIC to an MSDU. The MSDU is then fragmented into one or more MPDUs only the last of which carries the MIC. Then each MPDU is encrypted and transmitted. You probably read about this in the first book.
CCMP fragments the MSDU into one or more MPDUs, then computes and appends a MIC to each MPDU, then encrypts, then transmits. You read about this in the pages you mention in the second book.
IEEE 802.11 18.104.22.168.1 and CWSPv2 page 392 figure 14.5 show temporal key (TK) as an input to both key mixing phases. (Actually figure 14.5 has a typo showing temporal key correctly as TK and incorrectly as TA.) The "Real" book page 240 figure 11.3 would be improved by renaming "session key" to "temporal key" and adding a second line from it to the Phase 2 box.
I hope this helps. Thanks. /criss
Thanks a lot .
I review the "Real 802.11 security " again and find out :
In chap12, page 270-271 , CCMP culculate and append MIC value in each MPDU .
In chap 11,page 246-247 , TKIP Culculate in MSDU.
That's quite different.
You really clear of my mind and remind me that the IEEE802.11-2004 standard is the best reference .
I see from the above that with TKIP, the MIC is calculated at the MSDU level, whereas in CCMP, the MIC is calculated at the MPDU level.
My question is: Where does the MIK temporal key come from for CCMP. IEEE-802.11 states clearly that for TKIP, the MIC key is a 64-bit fragment pulled from the temporal key (See section 8.61.), however there is zero information about where the key for the CCMP MIC routine comes from.
What is the source for the MIC key in CCMP?