CSCE 715: Network Systems Security Chin-Tser Huang huangct@cse.sc.edu University of South Carolina

Security in Network Layer Implementing security in application layer provides flexibility in security policy and key management Problem is need to implement security mechanism in every application individually To reduce the overhead, implement security in network layer to provide security for all applications between selected pair of computers 02/23/2009

IPSec Current security standard for IP layer Provide general security services for IP Authentication Confidentiality Anti-replay Key management Applicable to use over LANs, across public and private WANs, and for the Internet 02/23/2009

Scenario of IPSec Uses 02/23/2009

Benefits of IPSec Provide strong security to all traffic crossing the perimeter if installed in a firewall/router Resistant to bypass IPSec is below transport layer, hence transparent to applications Can be transparent to end users Can provide security for individual users if desired 02/23/2009

IP Security Architecture Specification is quite complex Defined in numerous RFC’s RFC 2401/2402/2406/2408 many others, grouped by category Two protocols Authentication Header (AH) Encapsulating Security Payload (ESP) Mandatory in IPv6, optional in IPv4 02/23/2009

Security Association (SA) A unidirectional relationship between sender and receiver that affords security for traffic flow Each IPSec computer maintains a database of SA’s Defined by 3 parameters Security Parameters Index (SPI) IP Destination Address Security Protocol Identifier 02/23/2009

SA Parameters Sequence Number Counter Sequence Number Overflow Anti-Replay Window AH and ESP information Lifetime IPSec Protocol Mode Path MTU 02/23/2009

Authentication Header (AH) Provide support for data integrity and authentication of IP packets end system/router can authenticate user/app prevent address spoofing attacks guard against replay attacks by tracking sequence numbers Based on use of a MAC HMAC-MD5-96 or HMAC-SHA-1-96 MAC is calculated over IP header fields that are either immutable or predictable, AH header other than authentication data, and entire upper-level protocol data Parties must share a secret key 02/23/2009

Authentication Header 02/23/2009

Transport vs Tunnel Mode AH Transport mode is used to authenticate IP payload and selected portion of IP header good for host to host traffic Tunnel mode authenticates entire IP packet and selected portion of outer IP header good for VPNs, gateway to gateway security 02/23/2009

End-to-End vs End-to-Intermediate Authentication 02/23/2009

Scope of AH Authentication 02/23/2009

Encapsulating Security Payload (ESP) Provide message content confidentiality and limited traffic flow confidentiality Can optionally provide the same authentication services as AH Support range of ciphers, modes, padding DES, Triple-DES, RC5, IDEA, CAST etc CBC most common pad to meet blocksize, for traffic flow 02/23/2009

Encapsulating Security Payload 02/23/2009

Padding Serve several purposes expand the plaintext to required length make Pad Length and Next Header fields aligned to 32-bit word boundary conceal actual length of payload 02/23/2009

Transport vs Tunnel Mode ESP Transport mode is used to encrypt and optionally authenticate IP data data protected but header left in clear can suffer from traffic analysis but is efficient good for ESP host to host traffic Tunnel mode encrypts entire IP packet add new header for next hop can counter traffic analysis good for VPNs, gateway to gateway security 02/23/2009

Transport vs Tunnel Mode ESP 02/23/2009

Scope of ESP Encryption and Authentication 02/23/2009

Combining Security Associations SAs can implement either AH or ESP, but each SA can implement only one Some traffic flows may require services of both AH and ESP, while some other flows may require both transport and tunnel modes To address these concerns, need to combine SAs to form a security association bundle 02/23/2009

Authentication plus Confidentiality Which one first? Three approaches to consider ESP with Authentication Option Transport mode or tunnel mode Authentication after encryption Transport Adjacency A bundle of two transport SAs, with the inner being an ESP SA and the outer being an AH SA Transport-Tunnel Bundle A bundle consisting of an inner AH transport SA and an outer ESP tunnel SA Authentication before encryption 02/23/2009

Combining Security Associations 02/23/2009

Key Management Handle key generation and distribution Typically need 2 pairs of keys 2 per direction for AH & ESP Manual key management sysadmin manually configures every system Automated key management automated system for on demand creation of keys for SA’s in large systems Oakley and ISAKMP 02/23/2009

OAKLEY A key exchange protocol Based on Diffie-Hellman key exchange Add features to address weaknesses of Diffie-Hellman cookies to counter clogging attacks nonces to counter replay attacks key exchange authentication to counter man-in-the-middle attacks Can use arithmetic in prime fields or elliptic curve fields 02/23/2009

Usage of Cookies Three basic requirements Must depend on specific parties Impossible for anyone other than issuing entity to generate cookies that will be accepted by issuing entity Cookie generation and verification must be fast To create a cookie, perform a fast hash over src and dst IP addresses, src and dst ports, and a locally generated secret value 02/23/2009

ISAKMP Internet Security Association and Key Management Protocol Provide framework for key management Define procedures and packet formats to establish, negotiate, modify, and delete SAs Independent of key exchange protocol, encryption algorithm, and authentication method 02/23/2009

ISAKMP Header 02/23/2009

ISAKMP Payload 02/23/2009

ISAKMP Exchange 02/23/2009

ISAKMP Exchange 02/23/2009

Next Class Denial-of-Service (DoS) attack Hop Integrity 02/23/2009