Transcript 14-Wireless-Security - Communications Systems Center

Security Issues Raised by Wireless LANs
Dr. John A. Copeland*
Communications Systems Center
Georgia Tech Electrical & Computer Engineering
www.csc.gatech.edu
[email protected]
*Dr. Copeland is also a founder of Lancope, Inc., www.lancope.com
Network Tunnels
Modems
Internet
VPNs
Wireless Hubs
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Network Tunnels
http://www.telecommagazine.com/default.asp?journalid=3&func=articles&page=0203t11&year=2002&month=3
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Network Tunnels
Anyone can convert their cube or office Ethernet
jack into a Wireless Hub for their Laptop
(and a public entry point into the Network)
IEEE 802.11b Wireless PC
Card - $ 19.99
Linksys Etherfast Wireless AP
+ Cable/DSL Router
with 4-Port Switch - $ 44.99
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[ The Atlanta Journal-Constitution: 3/31/02 ]
Wireless systems are simple to hack
Terrorists could use techniques to attack airliners
By DON PLUMMER
Atlanta Journal-Constitution Staff Writer
(Bill Corbitt) sits in his car in the short-term
parking lot at an airport eating a sandwich, a Pringles can
balanced on the dashboard.
After Corbitt left Hartsfield Airport, he located
more than 100 wireless networks in an hour of
driving around Atlanta. He did not actually break
into any of them but later demonstrated how he
could by intercepting signals from several
wireless systems in use at The Atlanta JournalConstitution.
"Some of these people are even transmitting
their names and the exact locations of the
wireless portals,” Corbitt said, pointing out the
identifiers as they appeared on the screen of his
laptop. Of the 120 wireless systems located,
only 32 had activated the encryption protection
included with the wireless software.
Bill Corbitt, Bulwarkz Defensive Solutions
http://www.accessatlanta.com/ajc/news/0302/31wireless.html
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“Network Stumbler” - shows 802.11 Networks
WEP
ON
No
No
Screen of laptop with Wireless LAN card
“AiroPeek” maps out who’s talking to who
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Data sniffed
off the air
from nonWEP session.
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AirSnort - a tool for Cracking WEP Messages
AirSnort is a wireless LAN (WLAN) tool which recovers encryption
keys. AirSnort operates by passively monitoring transmissions,
computing the encryption key when enough packets have been
gathered.
802.11b, using the Wired Equivalent Protocol (WEP), is crippled
with numerous security flaws. Most damning of these is the
weakness described in " Weaknesses in the Key Scheduling
Algorithm of RC4 " by Scott Fluhrer, Itsik Mantin and Adi Shamir.
Adam Stubblefield was the first to implement this attack, but he
has not made his software public. AirSnort, along with WEPCrack,
which was released about the same time as AirSnort, are the first
publicly available implementaions of this attack.
AirSnort requires approximately 5-10 million encrypted packets to
be gathered. Once enough packets have been gathered, AirSnort
can guess the encryption password in under a second.
http://airsnort.sourceforge.net
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How Does “Wired Equivalent Protocol (WEP) Work”
What’s wrong with it?
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Wired Equivalent Protocol
“The Insecurity of 802.11, an analysis of the Wired Equivalent Privacy
protocol”, Black Hat Briefings, 11 July, 2001, Ian Goldberg, ZeroKnowledge Systems, [email protected] 9with Nikita Borisov
and David Wagner, UC Berkeley).
http://www.cypherpunks.ca/bh2001/index.html
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The One-Time Pad
The most secure encryption technique is a “One-Time Pad.”
WEP uses the RC4 encryption algorithm (with a 40 or 80 bit key) to
generate a stream of random looking bits, Pi. These are XORed with
the message bits, Mi, to produce the cyphertext bits, Ci
Ci = Mi (+) Pi
The receiver has the same key, and can generate a duplicate “pad”, Pi,
and recover the original plaintext message, Mi.
Ci (+) Pi = Mi (+) Pi (+) Pi = Mi
since Pi (+) Pi = 1 and Mi (+) 1 = Mi
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“Two-Time” Pad Problem
The least secure encryption technique is a “One-Time Pad” used twice (or repeatedly).
Consider two intercepted messages (C and D are encrypted M
and N) encrypted with the same pad:
C = M (+) P
and D = N (+) P
The cracker can XOR these together to get M (+) N:
C (+) D = M (+) P (+) N (+) P = M (+) N
Whenever a byte in M (+) N is zero, M & N have the same byte.
One of six bytes in English text is “space”. so one in 36 bytes in
M+N is zero indicating both text strings have spaces there.
Given several thousand characters of M+N, the messages can
be deciphered.
Then the key sequence can be found:
P = N (+) D.
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“Two-Time” Pad Problem
The pad, or keystream, for WEP is RC4(v,k) which depends only on v
and k. “k” is a shared secret that changes rarely, if ever (in most
systems every user uses the same value of k). So the keystream
depends only on the 24-bit value of v.
Since v is transmitted in the clear, the Cracker collects messages
until he has two with the same value of v, then he is “in.”
There are 2^24 = 16 million different values of v, but because of the
“Birthday Effect,” he is likely to have two that match after collecting
only about 6,000 messages.
Number of pairs = 6000 x 5999 / 2
Even worse, Goldberg reports that all the 802.11 cards observed
reset their random number generator for v each time they are
activated, meaning they reuse the same sequence of v values
(2002).
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Defense
Use the 112-bit key mode, rather than no key or 56-bit key.
Use an access list of MAC (Ethernet) addresses at the hub.
Use a new hub that has WPA, or IEEE 802.11i, and only
network cards that are fully compatible.
Use RADIUS authentication.
Use a vendor’s equipment that has a proprietary security
feature.
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Wi-Fi Protected Access
(WPA replaces WEP)
IEEE 802.1x authentication is required.
TKIP encryption is used (unique unicast key)
8-bit “Michael” Message Integrity Check (MIC)
Frame counter prevents replay attacks
AES is optional. Some card CPU’s can not manage.
Can support older WEP cards (but no dynamic keys)
802.11 Beacon Frames contain a WEP info element.
WPA is currently supported by Windows XP and Apple
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Process
Application
Defense - Higher Level
Secure Protocols
Router
Buffers Packets that
need to be forwarded
(based on IP address).
Network
Layer (IP)
IPsec
802.11
Link Layer
802.11
Phys. Layer
WEP
Application
SSL
SSL
Transport
Layer
(TCP,UDP)
Process
Transport
Layer
(TCP,UDP)
Network
Layer (IP)
Network
Layer
802.11
Link Layer
Network
Layer
Ethernet
Data Link Layer
WEP
Ethernet
802.11
Phys. Layer
Phys. Layer
IPsec
Ethernet
Data-Link Layer
Ethernet
Phys. Layer
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Defense - War Driving
Patrol the Premises Frequently
looking for Rogue Wireless Signals
Purchase only 802.11 Hubs and PC Cards that have the soon-to-be
improved Security standards, or flash memory and can be field
upgraded.
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Wireless Sub-Nets can Safely be part
of Corporate Networks - if Properly Isolated
IDS
Wireless Hub
Wireless Subnets should be treated
like attachments to the Web, isolated
by Firewalls and Intrusion Detection
Systems
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