Key Reinstallation Attacks:
Forcing Nonce Reuse in WPA2
Mathy Vanhoef — @vanhoefm

CCS 2017, 1 October 2017

Overview
Key reinstalls in4-way handshake

Misconceptions

Practical impact

Lessons learned
2Overview
Key reinstalls in
4-way handshake

Misconceptions

Practical impact

Lessons learned
3

The 4-way handshake
Used to connect to any protected Wi-Fi network

Two main purposes:› Mutual authentication› Negotiate fresh PTK: pairwise temporal key
Appeared to be secure:› No attacks in over a decade (apart from password guessing)› Proven that negotiated key (PTK) is secret1› And encryption protocol proven secure7
44-way handshake (simplified)5

4-way handshake (simplified)

PTK = Combine(shared secret,ANonce, SNonce)6

4-way handshake (simplified)

Attack isn’t about
ANonce or SNonce reuse

PTK = Combine(shared secret,ANonce, SNonce)7

4-way handshake (simplified)8

4-way handshake (simplified)9

4-way handshake (simplified)

PTK is installed104-way handshake (simplified)

11

Frame encryption (simplified)
Nonce

Plaintext data(packet number)

PTKMixPacket key(session key)Nonce
 Nonce reuse implies keystream reuse (in all WPA2 ciphers)124-way handshake (simplified)

Installing PTK initializes
nonce to zero13

Reinstallation Attack
Channel 1

Channel 6

14

Reinstallation Attack

15

Reinstallation Attack

16

Reinstallation Attack

Block Msg4

17

Reinstallation Attack

18

Reinstallation Attack

In practice Msg4
is sent encrypted19Reinstallation Attack

Key reinstallation!
nonce is reset20Reinstallation Attack

Same nonce
is used!
21

Reinstallation Attackkeystream

Decrypted!

22

Overview
Key reinstalls in
4-way handshake

Misconceptions

Practical impact

Lessons learned
23General impact
Transmit nonce reset
Decrypt frames sent by victim

Receive replay counter reset
Replay frames towards victim

24

Cipher suite specific
AES-CCMP: No practical frame forging attacks

WPA-TKIP:› Recover Message Integrity Check key from plaintext4,5› Forge/inject frames sent by the device under attack
GCMP (WiGig):› Recover GHASH authentication key from nonce reuse6› Forge/inject frames in both directions
25

Handshake specificGroup key handshake:› Client is attacked, but only AP sends real broadcast frames› Can only replay broadcast frames to client
4-way handshake:› Client is attacked  replay/decrypt/forge
FT handshake (fast roaming = 802.11r):› Access Point is attacked  replay/decrypt/forge› No MitM required, can keep causing nonce resets

26

Implementation specific
Windows and iOS: 4-way handshake not affected› Cannot decrypt unicast traffic (nor replay/decrypt)› But group key handshake is affected (replay broadcast)
wpa_supplicant 2.4+
› Client used on Linux and Android 6.0+
› On retransmitted msg3 will install all-zero key
27

Overview
Key reinstalls in4-way handshake

Misconceptions

Practical impact

Lessons learned28

Misconceptions I
Updating only the client or AP is sufficient› Both vulnerable clients & vulnerable APs must apply patches
Need to be close to network and victim› Can use special antenna from afar

No useful data is transmitted after handshake› Trigger new handshakes during TCP connection
29Misconceptions II
Obtaining channel-based MitM is hard
› Nope, can use channel switch announcements
Attack complexity is hard
› Script only needs to be written once …
› … and some are already doing this!

30

Overview
Key reinstalls in
4-way handshake

Misconceptions

Practical impact

Lessons learned31

Limitations of formal proofs› 4-way handshake proven secure› Encryption protocol proven secure

The combination was not proven secure!
32Model vs. implementation
Abstract model ≠ real code
› Must assure code matches specification
The wpa_supplicant 2.6 case

› Complex state machine & turned out to still be vulnerable› Need formal verification of implementations

33

On a related note…
Workshop on:

Security Protocol Implementations:
Development and Analysis (SPIDA)
Co-located with EuroS&P 2018
“focuses on improving development & analysisof security protocols implementations”
34Thank you!
Questions?
krackattacks.com

References
1. C. He, M. Sundararajan, A. Datta, A. Derek, and J. Mitchell. A Modular
Correctness Proof of IEEE 802.11i and TLS. In CCS, 2005.
2. S. Antakis, M. van Cuijk, and J. Stemmer. Wardriving - Building A Yagi
Pringles Antenna. 2008.
3. M. Parkinson. Designer Cantenna. 2012. Retrieved 23 October 2017 fromhttps://www.mattparkinson.eu/designer-cantenna/
4. E. and M. Beck. Practical attacks against WEP and WPA. In WiSec, 2009.
5. M. Vanhoef and F. Piessens. Practical verification of WPA-TKIP
vulnerabilities. In ASIA CCS, 2013.
6. A. Joux. Authentication failures in NIST version of GCM. 2016.
7. J. Jonsson. On the security of CTR+ CBC-MAC. In SAC, 2002.
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Countermeasures
Problem: many clients won’t get updates

Solution: AP can prevent (most) attacks on clients!
› Don’t retransmit message 3/4› Don’t retransmit group message 1/2
However:› Impact on reliability unclear› Clients still vulnerable when connected to unmodified APs37

Handshake specificGroup key handshake:› Client is attacked  replay broadcast frames to client› Because client never sends real broadcast frames!

Unicast38