CSCE 715: Network Systems Security Chin-Tser Huang University of South Carolina

02/18/20092 Network Security Designs After discussion of cryptographic tools, we turn to look at various network security designs at different layers in protocol stack Link layer – secure address resolution Network layer – IPsec, hop integrity Transport layer – SSL/TLS Application layer – Kerberos, X.509 certificate, firewall design

02/18/20093 Ethernet Most widely used LAN technology Low cost and high flexibility Versions of different speed: 10Mbps, 100Mbps, Gigabit Use globally unique media access control (MAC) address (hardware address) for every interface card

02/18/20094 Use of Hardware Address Need an address to send a message to receiver on same Ethernet IP address is not usable because the data link does not recognize IP address Use hardware address to identify receiver’s interface Need to resolve receiver’s hardware address from receiver’s IP address

02/18/20095 Address Resolution Protocol ARP maps each IP address to corresponding hardware address in subnetwork For computer i to get hardware address of computer j, i broadcasts a rqst message with IP address of j to the subnetwork Internet i j r default router switch rqst(ipa.j)

02/18/20096 Address Resolution If j sees a rqst message from i with its IP address, j sends a rply message with its IP address and hardware address to i Internet i j r default router switch rply(ipa.j,hda.j)

02/18/20097 Functions of ARP Three functions of ARP Resolving IP addresses Supporting dynamic assignment of addresses Detecting destination failures

02/18/20098 ARP Spoofing Attack To stop traffic from i to j, an adversary sends to i a spoofed rply message with IP address of j and a non-existent hardware address Internet i j A r default router switch rply(ipa.j,hda.x)

02/18/20099 Another ARP Spoofing Attack To stop traffic from i to default router r, an adversary sends to i a spoofed rply message with IP address of r and its own hardware address Internet i j A r default router switch rply(ipa.r,hda.A)

02/18/ Countering ARP Spoofing Attacks Proposed solutions include ARPWATCH and static ARP caches ARPWATCH monitors transmission of rqst and rply messages over Ethernet and check them against a database of (IP addr, hardware addr) pairings Static ARP cache stores permanent (IP addr, hardware addr) pairings of trusted hosts to avoid sending rqst and rply messages over Ethernet

02/18/ Insufficiencies of Proposed Solutions ARPWATCH does not support dynamic assignment of IP addresses Static ARP caches does not support dynamic assignment of IP addresses and detection of destination failures

02/18/ Need for Secure Address Resolution When a computer receives a message m, it needs to determine whether m was indeed sent by claimed source, or was inserted, modified, or replayed by an adversary Use secure address resolution protocol between each computer and a secure address resolution server

02/18/ Architecture of Secure Address Resolution Protocol

02/18/ Adversary Adversary can perform three types of actions to disrupt communication between server s and any computer h[i] on the Ethernet Message loss Message modification Message replay

02/18/ Secure Address Resolution Protocol Use three mechanisms to counter adversarial actions timeouts to counter message loss shared secrets to counter message modification nonces to counter message replay

02/18/ Invite-Accept Protocol Periodically, server s sends out an invt message to every computer on Ethernet Every up computer is required to send back an acpt message including its IP address and hardware address s updates its address database according to received acpt messages

02/18/ Invite-Accept Protocol s  h[0..n-1]: invt(nc, md) where md=MD(nc;scr[0])||MD(nc;scr[1])||…||MD(nc;scr[n-1]) h[i]  s: acpt(nc, ipa[i], hda[i], d) where d=MD(nc;ipa[i];hda[i];scr[i])

02/18/ Request-Reply Protocol When a computer needs to resolve a destination’s hardware address, it sends a rqst message to server s If destination’s hardware address is still valid, s sends back a rply message with address information If destination’s hardware address is not valid anymore, s sends back a rply message with no address information

02/18/ Request-Reply Protocol h[i]  s: rqst(nc, ipa[j], d) where d=MD(nc;ipa[j];scr[i]) If found, s  h[i]: rply(nc, ipa[j], hda[j], d) where d=MD(nc;ipa[j];hda[j];scr[i]) If not found, s  h[i]: rply(nc, ipa[j], 0, d) where d=MD(nc;ipa[j];0;scr[i])

02/18/ Extensions Four extensions of secure address resolution protocol Insecure address resolution Backup server System diagnosis Address resolution across multiple Ethernets

02/18/ Next Class IPsec Authentication Header (AH) Encapsulation Security Payload (ESP) key management Read Chapter 16