Offline bruteforce attack on WiFi Protected Setup

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Offline bruteforce attack on
WiFi Protected Setup
Dominique Bongard
Founder
0xcite, Switzerland
@reversity

§  Protocol aiming at easily connecting to protected WiFi networks
§  Two main modes: Push-Button and 8 digit PIN code
§...

Stefan Viehböck

Stefan Viehböck

§  Brute force each half of the PIN
§  Maximum 10‘000 tries + 1‘000 tries
§  No limitation on number of tries in many A...

STA
Nonce
E-Hash1 E-Hash2 HMAC
AES(HMAC(PIN1),E-S1) AES(HMAC(PIN2),E-S2)

§  If we can guess E-S1 and E-S2, we can the
brute force PIN1 and PIN2 offline!
§  Pixie dust attack!

§  Usually with pseudo-random generators (PRNG)
§  Often insecure PRNG
§  No or low entropy
§  Small state (32 bits)
§...

$int rand_r( unsigned int *seed ) { unsigned int s=*seed; unsigned int uret; s = (s * 1103515245) + 12345; // permutate see...$

AP Nonce Description PK

§  Linear Congruential Generator
§  32 bits state
§  No external entropy
§  E-S1 and E-S2 generated right after the No...

§  Do the WPS protocol up to message M3
§  Get the Nonce from M1
§  Bruteforce the state of the PRNG
§  Compute E-S1 a...

§  Linear Feedback Shift Register (LFSR)
§  Broken
§  Doesn‘t matter the keys are always NULL !!

§  Some AP have the same state at each boot
§  Make a list of common states after reboot
§  Attack the AP right after b...

§  Trigger the breakers
§  DDOS the AP
§  Jam the signal until the target reboots the AP

§  Looks okay
§  Uses /dev/random
§  Found in Atheros SDK
§  But you never know
§  Several papers attack the entropy ...

§  It‘s complicated
§  Many of the implementations are the reference
code for the chipset
§  Only the GUI is reskinned
...

§  Disable WPS now !
§  Reverse engineers: Check other AP for bad PRNG
§  Cryptographers: Check if good PRNG are okay

Offline bruteforce attack on WiFi Protected Setup

Offline bruteforce attack on WiFi Protected Setup

Offline bruteforce attack on WiFi Protected Setup