Mobile Security - Wireless hacking

Wireless Security
 Wireless offers additional challenges:
 Physical media can easily be sniffed.
 War Driving

Legal?
 U.S. federal computer crime statute, Title 18 U.S.C. 1030,

Crime to knowingly access a computer used in interstate or
foreign communication "without authorization" and obtain any
information from the computer.

Crime to access a computer without authorization with "intent
to defraud" to obtain "anything of value."

But not if "the object of the fraud and the thing obtained
consists only of the use of the computer and the value of
such use is not more than $ 5,000 in any 1-year period."

Wireless Security
 Wireless offers additional challenges:
 Physical media can easily be sniffed.
 Mobile computing needs to preserve
battery power.

Calculations cost more on a mobile platform.

Especially important for sensor networks

Wireless Security:
Attackers Perspective
 Knowing the Threat
 Targets of opportunity

Goal is
 Internet access.
 Easy pickings.
 Targeted attacks

Targets assets valuable enough.
 Internal attackers

Most Dangerous

Can open an unintentional security hole

COEN 351 E-Commerce
Security
 E-Commerce Security Course Homepage
 Lecture Notes

IEEE 802.11
 Wired Equivalent Privacy (WEP)
Protocol
 Based on a shared secret k.

Distributed out of band.
 Uses CRC for internal integrity protection.
 Uses RC4 to encrypt network traffic.

WEP Protocol
 Confidentiality
 Original packet is first check-summed.
 Checksum and data form the payload.
 Transmitting device creates a 24-bit
random initialization vector IV.
 IV and shared key are used to encrypt with
RC4

WEP Protocol
 RC4
 Generates a pseudo-random stream of
bytes (keystream)

Based on a secret internal state
 Permutation S of all 256 possible bytes
 Two index pointers
 Plaintext is XORed with keystream

WEP Protocol
 RC4
 Key Scheduling Algorithm (KSA)

Initializes S based on a key
for i from 0 to 255
S[i] := i
j := 0
for i from 0 to 255
j := (j + S[i] + key[i mod keylength]) mod 256
swap(S[i],S[j])

WEP Protocol
 RC4
 Pseudo-Random Generation Algorithm
(PRGA)

Generates pseudo-random byte stream
i := 0
j := 0
while GeneratingOutput:
i := (i + 1) mod 256
j := (j + S[i]) mod 256
swap(S[i],S[j])
output S[(S[i] + S[j]) mod 256]

WEP Protocol
 RC4
 Known weaknesses

Keystream slightly biased
 Fluhrer & McGrew attack can distinguish keystream
from random stream given a GB of input.
 Fluhrer, Mantin, Shamir: statistics for output of the
first few bytes of output keystream are non-random,
leaking information about key.

WEP Protocol
 Authentication
 Station associating with access point
needs to authenticate itself.
 Both exchange the type of authentication
that is accepted.

Open: Just identification between station and
AP

Shared Secret: Participants send nonces to
each other, encrypt the nonce using WEP (and
the shared secret key), and verify the other’s
response.

WEP has no key management
 Everyone allowed to have access to a
wireless network has the same key.
 Anyone with the key can read ALL
traffic.

WEP: RC4
 RC4 uses the key and the IV to produce
a stream of pseudo-random bytes.
 Calculates cipher text from plaintext by
XORing the pseudo-random stream
with the plain-text.

WEP: Attacks on RC4
 Dictionary Attack

Build database:

224
different IVs

Build a database of 224
streams of MTU bytes
(2,312 B) for each different IV.

Takes < 40 GB storage.
 XOR two entries with the same IV.

Result are the two plaintexts XORed.

Natural language text has enough redundancy
to decrypt the XOR of two text streams.

WEP: Attacks on RC4
 Dictionary Attack
 Many packages can be completely or
partially guessed.
 XORing guessed plaintext and captured
cipher gives pseudo-random byte stream
for a given IV.
 Some implementations reset IVs poorly.
 This simplifies dictionary attacks.

WEP: Attacks on RC4
 Injection Attack
 Attacker creates packets on the wireless
connection.
 Attacker XORs plaintext and cipher.

Builds Pseudo-Random Stream database
indexed by IV.

RC4
Fluhrer, Mantin, Shamir Attack
 First few bits of several thousand
messages reveals key.
 Based on an analysis of the RC4 code.

Originally kept secret, but later leaked on the
internet.

RC4
 Key Scheduling Algorithm
 Sets up RC4 state array S
 S is a permutation of 0, 1, … 255
 Output generator uses S to create a
pseudo-random sequence.
 First byte of output is given by
S[S[1]+S[S[1]]].

First byte depends on
 {S[1], S[S[1], S[S[1]+S[S[1]]}

RC4
 Key Scheduling Algorithm
 First byte of plain text package is part of the SNAP header

0xAA for IP and ARP packages

0xFF or 0xE0 for IPX

Guessing the first byte is trivial
 Some IVs are vulnerable: “resolved”

(KeyByte+3, 0xFF, *)

Plus some more
 Easy to test whether an IV is vulnerable.
 Search for vulnerable IVs.
 They leak key bytes probabilistically.
 Large number of packets does it.

RC4
 Optimization needs about 5,000,000 to
1,000,000 packages.
 Counter-measures:
 Change key frequently.
 Change IV counters to avoid bad IVs.

WEP Message Modification
 WEP uses CRC code to ascertain integrity of
messages.
 CRC code is linear:
 CRC(x ⊕ y) = CRC(x) ⊕ CRC(y).
 Attacker knows plaintext M and desired modification
∆ for target plaintext M’ = M ⊕ ∆.
 Attacker want to substitute X = P⊕(M,CRC(M)) for
P⊕(M’,CRC(M’)).
 Attacker sends
X⊕(∆,CRC(∆)) = P⊕(M,CRC(M)) ⊕(∆,CRC(∆))
= P⊕(M’,CRC(M’))

Wireless Insecurity Problems
 WiFi card software allows users to
change the MAC address.

Wireless Security
 Casual user, low yield traffic
 WEP is good enough.
 Enterprise, Commercial
 Combine WEP with higher order security

SSH

VPN

IPSec

WPA
 Created by WiFi Alliance
 Certification started April 2003
 Uses 802.1X authentication server

Distributed different keys to each user.
 Can also be used in “pre-shared key”
(PSK) mode

Every user uses the same passphrase.

Called WPA Personal

IEEE 802.1X
http://www.linux.com/howtos/8021X-HOWTO/index.shtml
 Standard for port-
based authentication.
 Uses a third-party
authentication server
such as Radius

WPA
 Protocol changes over WEP
 CRC is replaced by “Michael” MIC.

MIC now includes a frame counter, preventing replay
attacks.

Payload bit flipping is now impossible.
 Data encryption still uses RC4, but now

Prevents key recovery attacks on WEP by using
 128b Key
 48b Initialization vector
 Temporal Key Integrity Protocol (TKIP) changes key
dynamically.

TKIP
 Temporal Key Integrity Protocol
 Ensures that every data packet has its own
encryption key.

802.11i
 Uses AES instead of RC4.
 Subset published as WPA2
 Uses 802.1X authentication

Protocol Layers
 WEP
 Privacy only.
 Very elementary security.
 WPA
 Temporal Key Exchange Protocol

Fixes WEP that scrambles keys between packages and adds a secure
message check.
 AES: Advanced Encryption Standard
 802.11i
 Military grade encryption, replaces DES
 802.1X
 General purpose and extensible framework for authentication users
and generating / distributing keys.
 Simple Secure Network (SSN)
 Recipe for authentication based on 802.1X

Mobile Security - Wireless hacking

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