Understanding Wireless LANs: Link Encryption and Security Protocols

1. CISC 210 - Class Today • Wireless LANs - recap • Link Encryption – book style • Link encryption – LAN style • WEP • WPA R. Smith - University of St Thomas - Minnesota

2. 802 Protocol in general • Traditionally evolved from Ethernet • Unreliable (unACKed, unchecksummed) • Broadcast between nearby stations • As fast and cheap as possible • Ethernet: CSMA/CD • Detect ‘free’ channel; detect collisions • Exponential backoff • Wireless (802.11): CSMA/CA • All stations can’t always hear each other; CD isn’t practical • Wireless is noisier than Ethernet; more dropped packets • Impractical to completely ignore reliability R. Smith - University of St Thomas - Minnesota

3. 802.11 Protocol • Virtual Carrier Sense • Send “RTS” to ask for permission to send • Gives source, destination, and duration of “real” transmission • If no other traffic, recipient sends back CTS • Then sender sends the actual data • Recipient sends an ACK • Collisions most likely during RTS • They’re very short messages, reduce collision risk • Other stations see the RTS/CTS, wait to transmit till done • Packets are smaller on 802.11 than Ethernet • Big packets are more likely to be corrupted by noise R. Smith - University of St Thomas - Minnesota

4. Hooking Up • Base Stations • May serve as ‘Access Point’ (AP) - Provide a link to a ‘backbone’ – i.e. Internet access • “Service Sets” • “Basic Service Set” (BSS) – Environment where everyone is within range of a single base station • “Extended Service Set” (ESS) – where two or more base stations are connected via a common backbone to provide more coverage (I do this at home) • Service Set ID (“SSID”) • That magic text string that pops up from a base station and identifies the service set you’re in (default ‘linksys’ on many) R. Smith - University of St Thomas - Minnesota

5. Link Encryption Objectives • Confidentiality on isolated set of computers • Computers only talk to one another • They do not talk to other computers • No communication with outsiders • Avoid both intentional and accidental data disclosure • Hide traffic as much as possible • Don’t disclose traffic patterns; don’t disclose data • Safety and familiarity paramount • Shouldn’t interfere with computer or network operation • Should always work with minimum of fuss • Extra cost is acceptable R. Smith - University of St Thomas - Minnesota

6. Link Level encryption: properties/features • Red/Black separation • Everything that goes out is encrypted • Everything inside is cleartext • Good algorithm; good keys • A problem with older wireless (we’ll see later) • Good keys = over 100 bits • Good algorithm = AES, maybe triple DES (slower) • Protect against replay & rewrite attacks • Duplicates must be detectable: packet serial numbers, etc. • Cryptographic checksum that outsiders can’t forge • Good stream cipher or block mode • Random data to confound “known plaintext” attacks R. Smith - University of St Thomas - Minnesota

7. Routing and LAN Encryption • Point to Point Encryption • Kind of a dead horse today • Everyone uses multipoint LANs, like Ethernet • Is everything encrypted that goes out? • What about MAC addresses? • If we need an address it must be in plaintext • A wireless router • All data on the wireless is encrypted • Including IP addresses • We strip off the wireless encryption when it leaves the wireless LAN R. Smith - University of St Thomas - Minnesota

8. Wireless Crypto • WEP, WPA – what do they encrypt? • What objectives do we achieve? • Link Encryption Objectives • Confidentiality on isolated set of computers • No communication with outsiders • Hide traffic as much as possible • Safety and familiarity paramount R. Smith - University of St Thomas - Minnesota

9. WEP versions • “Wired Equivalent Privacy” • Describes the hope, not the achievement • Shared key encryption protocol • 64-bit keys (original WEP) • 128-bit keys (WEP 2) • Uses RC-4 stream cipher (hard to use safely) • Poorly constructed encryption • 64-bit keys broken in 40-bit time • 128-bit keys broken in 64-bit time R. Smith - University of St Thomas - Minnesota

10. WEP Crypto • WEP Encryption (diagram) • Pick an IV (Initialization Vector, Nonce), 24 bits • Concatenate to ‘root key’ -> k | IV • This is the packet key (up to 128 bits) • Calculate CRC32 over the data (the “ICV”) • Encrypt data and ICV using the packet key • Transmit IV and encrypted data R. Smith - University of St Thomas - Minnesota

11. WEP Weaknesses • ICV only protects against random errors • Possible to modify a packet's contents and CRC without knowing the crypto key (think of the bit flip example) • “Related Key” attacks • Attacker knows part but not all of the key • Algorithm is vulnerable if • Knowing info about crypto with one key… • Yields info about encryption with a “similar” key • RC-4 is vulnerable. • Lots of ‘crib’ available (ARP) • “Chop chop” attack • Intercept and retransmit a packet • Change the last byte of data through trial and error R. Smith - University of St Thomas - Minnesota

12. WPA • WPA – a stopgap to replace WEP ASAP • 128-bit keys using RC-4 • Used existing hardware • Better integrity protection – MIC using ‘Michael’ • Still uses ICV function as well • Larger effective keys • TKIP • Similar to WEP, but ‘mixes’ the IV and key R. Smith - University of St Thomas - Minnesota

13. WPA2 – • Implements 802.11i enhancements • Use AES instead of RC-4 • Permanent keys to authenticate; temporary for data • Can use RADIUS authentication server • Counter Mode with CBC MAC • Integrates encryption, integrity checking, and key variation • Key update protocol • Each packet has a unique key • Derived from packet serial #, shared secret, MAC addr R. Smith - University of St Thomas - Minnesota

14. Projects • Find a project and get started! • The ‘final date due’ for your proposal is LATE

15. Obsolete but interesting technology Security implications? Political implications? All packets include LEAF Encrypted with a special shared secret key. Contains Device ID Session key used to encrypt this message Checksum on the LEAF There’s a special decryption system Has a database of DeviceID/Decryption key Escrowed Encryption R. Smith - University of St Thomas - Minnesota

16. Protocols and Layers • We use layering for several things • Organize the software • Format the packets • What it really does: Establish a relationship between software components on different computers • Layers communicate with each other at same layer • IP – IP or TCP – TCP or HTTP – HTTP • They ‘use’ the lower layers to carry their messages R. Smith - University of St Thomas - Minnesota

17. Protocol Layering Examples • Network class – bear with me • Pizza delivery example • How do we order pizza at a party? R. Smith - University of St Thomas - Minnesota

18. Network Protocol Layering Usually a ‘funnel’ shape • Top level = Applications • Lots of choices: e-mail, web, file exchange, • Uses ‘socket interface’ to talk to networks • Mid levels = “The Protocol Stack” • Transport layer: UDP/TCP • Internet layer: IP • Link layer: LAN protocols • Bottom level = device driver connections • Hardware-specific software, configuration • Uses device driver interface to link to the protocol stack • Uses a cable or antenna to link to the network R. Smith - University of St Thomas - Minnesota

19. Packets follow the layers • Upper layer data = innermoust • Lower layer data = outermost • Innermost data usually travels the network unchanged • Outermost data gets swapped with each hop through a router R. Smith - University of St Thomas - Minnesota

20. Diagramming the Crypto • Elements • Protocol stack elements • Where the crypto goes • What is encrypted • What is plaintext R. Smith - University of St Thomas - Minnesota

21. That’s it • Questions? Creative Commons License This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA. R. Smith - University of St Thomas - Minnesota