Chapter 6 CDMA/802.11i IC322 Fall 2014 Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 Some material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved Introduction 1-1
Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 802.11 wireless LANs ( Wi-Fi ) 6.4 Cellular Internet Access architecture standards (e.g., GSM) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.9 Summary Wireless, Mobile Networks 6-2
Code Division Multiple Access (CDMA) unique code assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own chipping sequence (i.e., code) to encode data allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence Wireless, Mobile Networks 6-3
CDMA encode/decode sender data bits code d 0 = 1 d 1 = -1 1 1 1 1 1 1 1 1-1 - 1-1 - 1-1 - 1-1 - 1 slot 1 slot 0 Z i,m = d i. cm channel output Z i,m - 1-1 - 1 1-1 1 1 1 slot 1 channel output 1 1 1 1-1 - 1-1 - 1 slot 0 channel output received input - 1-1 - 1 1-1 1 1 1 1 1 1 1-1 - 1-1 - 1 D i = Σ Z i,m. cm m=1 M M d 1 = -1 d 0 = 1 receiver code 1 1 1 1-1 - 1-1 - 1 1 1 1 1-1 - 1-1 - 1 slot 1 slot 0 slot 1 channel output slot 0 channel output Wireless, Mobile Networks 6-4
CDMA: two-sender interference Sender 1 channel sums together transmissions by sender 1 and 2 Sender 2 using same code as sender 1, receiver recovers sender 1 s original data from summed channel data! Wireless, Mobile Networks 6-5
Chapter 8 roadmap 8.1 What is network security? 8.2 Principles of cryptography 8.3 Message integrity 8.4 Securing e-mail 8.5 Securing TCP connections: SSL 8.6 Network layer security: IPsec 8.7 Securing wireless LANs 8.8 Operational security: firewalls and IDS Network Security
WEP design goals symmetric key crypto confidentiality end host authorization data integrity self-synchronizing: each packet separately encrypted given encrypted packet and key, can decrypt; can continue to decrypt packets when preceding packet was lost (unlike Cipher Block Chaining (CBC) in block ciphers) Efficient implementable in hardware or software Network Security
Review: symmetric stream ciphers key keystream generator keystream combine each byte of keystream with byte of plaintext to get ciphertext: m(i) = ith unit of message ks(i) = ith unit of keystream c(i) = ith unit of ciphertext c(i) = ks(i) m(i) ( = exclusive or) m(i) = ks(i) c(i) WEP uses RC4 Network Security
Stream cipher and packet independence recall design goal: each packet separately encrypted if for frame n+1, use keystream from where we left off for frame n, then each frame is not separately encrypted need to know where we left off for packet n WEP approach: initialize keystream with key + new IV for each packet: Key+IV packet keystream generator keystream packet Network Security
WEP encryption (1) sender calculates Integrity Check Value (ICV) over data four-byte hash/crc for data integrity each side has 104-bit shared key sender creates 24-bit initialization vector (IV), appends to key: gives 128- bit key sender also appends keyid (in 8-bit field) 128-bit key inputted into pseudo random number generator to get keystream data in frame + ICV is encrypted with RC4: B\bytes of keystream are XORed with bytes of data & ICV IV & keyid are appended to encrypted data to create payload payload inserted into 802.11 frame encrypted IV Key ID data ICV MAC payload Network Security
WEP encryption (2) IV (per frame) K S : 104-bit secret symmetric plaintext frame data plus CRC k 1 IV key sequence generator ( for given K S, IV) k 2 IV k 3 IV k N IV k N+1 IV k N+1 IV d 1 d 2 d 3 d N CRC 1 CRC 4 802.11 header IV WEP-encrypted data plus ICV c 1 c 2 c 3 c N c N+1 c N+4 Figure new 7.8-new1: IV for each 802.11 WEP frame protocol Network Security
WEP decryption overview encrypted IV Key ID data ICV MAC payload receiver extracts IV inputs IV, shared secret key into pseudo random generator, gets keystream XORs keystream with encrypted data to decrypt data + ICV verifies integrity of data with ICV note: message integrity approach used here is different from MAC (message authentication code) and signatures (using PKI). Network Security
End-point authentication w/ nonce Nonce: number (R) used only once in-a-lifetime How to prove Alice live : Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key I am Alice R K A-B (R) Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice! Network Security
WEP authentication authentication request nonce (128 bytes) nonce encrypted shared key Notes: success if decrypted value equals nonce not all APs do it, even if WEP is being used AP indicates if authentication is necessary in beacon frame done before association Network Security
Breaking 802.11 WEP encryption security hole: 24-bit IV, one IV per frame, -> IV s eventually reused IV transmitted in plaintext -> IV reuse detected attack: Trudy causes Alice to encrypt known plaintext d 1 d 2 d 3 d 4 Trudy sees: c i = d i XOR k i IV Trudy knows c i d i, so can compute k i IV Trudy knows encrypting key sequence k 1 IV k 2 IV k 3 IV Next time IV is used, Trudy can decrypt! Network Security
Solutions to WEP: 802.11i WPA Partial implementation of 802.11i A gap-stop for older hardware (that isn t able to fully support the WPA2 protocol). Uses RC4 stream cipher (like WEP) WPA2 The complete implementation of 802.11i Uses AES block cipher Wireless, Mobile Networks
802.11i: WPA2 Fully implemented in WPA2 Adds two features to 802.11 4-Way Handshake Group Key Handshake Enterprise version uses authentication server Personal version uses AES pre-shared key Network Security
Robust Security Network via 802.1X IEEE 802.1X is the standard defined by IEEE for port based network access control. Basically a protocol to make sure only legitimate clients can use a network secured by WPA2
Robust Security Network via 802.1X STA: client station AP: access point wired network AS: Authentication server Three players are needed to run the 802.1X protocol which uses EAP or Extensive Authentication Protocol A client (STA/Supplicant) A wireless access point (AP STA/Authenticator) An authentication server (AS)
802.11i: four phases of operation STA: client station AP: access point wired network AS: Authentication server 1 Discovery of security capabilities. Agreement on security policies. 2 STA and AS mutually authenticate, together generate Master Key (MK). AP serves as pass through 3 STA derives Pairwise Master Key (PMK) 3 AS derives same PMK, sends to AP 4 STA, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity Network Security
Robust Security Network via 802.1X PMK Pairwise Master Key Sent from the AS to the Authenticator Both the Supplicant and Authenticator now have the same PMK PMK is permanent for the entire session Must generate a Pairwise Transient Key for encryption of data. Done using 4-way handshake
Robust Security Network via 802.1X Source: http://www.tldp.org/howto/8021x-howto/intro.html
Robust Security Network via 802.1X 4-Way Handshake Confirm that the client holds the PMK. Confirm that the PMK is correct and up-to-date. Create pairwise transient key (PTK) from the PMK. Install the pairwise encryption and integrity keys (MIC) into IEEE 802.11. Transport the group temporal key (GTK) and GTK sequence number from Authenticator to Supplicant and install the GTK and GTK sequence number in the STA and, if not already installed, in the AP. Confirm the cipher suite selection.
Generating the PTK 4-Way Handshake Source: http://en.wikipedia.org/wiki/ieee_802.11i-2004
Key Hierarchy Source: http://www.tldp.org/howto/8021x-howto/intro.html
WPA2-PSK Pre-Shared Key Mode Network traffic encrypted using a 256 bit PMK User enters key (Pairwise Master Key) 64 hex digits 8-63 Printable ASCII characters Takes the passphrase, salts it with SSID of AP, then runs it through 4096 iterations of HMAC-SHA-1
WPA2-PSK Authentication, Connection, Establishment of PTK and GTK. Similar process as when an AS is present except the PSK is used as the PMK. Creation of PTK and GTK is the same as in Enterprise mode.
Further Information Go to this document provided by the Linux Documentation Project as a good study reference for wireless security: http://www.tldp.org/howto/8021x-howto/intro.html Wireless, Mobile Networks