COEN 350

Mobile Security

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

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

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 := 0j := 0while 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: RC4

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 Fluhrer, Mantin, Shamir Attack 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 Fluhrer, Mantin, Shamir Attack 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 Fluhrer, Mantin, Shamir Attack

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