Network Security (contd.) Bijendra Jain (bnj@cse.iitd.ernet.in) 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Lecture 5: IPSec 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 IPSec: IP Security An IETF standard IPSec architecture and related standards published as refer RFC 1825 thru RFC 1829 Adrresses security issues arising from authentication and confidentiality connecting a remote host to a server Interconnecting two LANs using a public network Applications: wide-area networking of branch offices using Internet Interconnecting supplier/distributor extranets to enterprise network Telecommuting E-commerce Implemented in clients, servers or in routers 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 IPSec Scenario Public Network Enterprise LAN#1 Enterprise LAN#2 PC Server Router 11/21/2018 Tutorial on Network Security: Sep 2003

Security functions covered by IPSec   Authentication header (AH) Encapsulating security payload (ESP), without AH Encapsulating security payload, with AH Access control Yes Connection-less integrity Data origin authentication Rejection of replayed packets Confidentiality (Limited) Flow Confidentiality 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Modes in IPSec Transport Mode The payload in an IP packet is secured E.g. TCP, UDP, ICMP headers, data Tunnel Mode The complete IP packet including its header is secured 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Transport Mode IPSec Public Network Enterprise LAN#1 Enterprise LAN#2 PC Server Router End-to-end authentication and/or encryption 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Tunnel Mode IPSec Public Network Enterprise LAN#1 Enterprise LAN#2 PC Server Router End-system to ROUTER authentication and/or encryption Router-to-router authentication and/or encryption 11/21/2018 Tutorial on Network Security: Sep 2003

Transport vs. Tunnel modes ? 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 IPSec Tunnel mode Advantages: Only routers need to implement IPSec functions Implement VPN (Virtual private network) Public Network Enterprise LAN Router 11/21/2018 Tutorial on Network Security: Sep 2003

IPSec: Authentication Header Original IP packet Encoded packet in “transport mode”? Encoded packet in “tunnel mode”? Original IP hdr TCP header TCP data Authen. hdr NEW IP hdr 11/21/2018 Tutorial on Network Security: Sep 2003

IPSec: packet format for AH Reserved (16 bits) Payload length Next header Identifier (32 bits) Sequence number (32 bits) AH (variable length, default 96 bits) Based on: MD5, or SHA-1 Covers TCP/UDP/ICMP header, data and portions of “non-mutable” IP headers Payload (IP or TCP packet) Original/new IP header 11/21/2018 Tutorial on Network Security: Sep 2003

IPSec: ESP (Encryption) Original IP packet Encoded packet in “transport mode”? Encoded packet in “tunnel mode”? Original IP hdr TCP header TCP data ESP hdr ESP trailer AH (optional) NEW IP hdr 11/21/2018 Tutorial on Network Security: Sep 2003

IPSec: packet format for ESP Identifier (32 bits) Sequence number (32 bits) Payload (TCP, or IP packet with padding, pad length, next header), suitably encrypted using 3DES, RC5 or … Original/new IP header Authentication Header based on MD5, etc. encrypted authenticated Pad length, … 11/21/2018 Tutorial on Network Security: Sep 2003

Combining security functions Authentication with confidentiality ESP, with AH An AH inside a ESP (both in transport mode) Server PC Public Network Router Enterprise LAN Enterprise LAN Router 11/21/2018 Tutorial on Network Security: Sep 2003

Combining security functions An AH inside a ESP (both in transport mode), and all this within a ESP tunnel across the routers PC Server Public Network Enterprise LAN Router 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Key exchange Key generation and exchange using some “physical means” Automated generation of keys Oakley key determination and exchange Based on Diffie-Hellman key generation algorithm Oakley key exchanged protocol 11/21/2018 Tutorial on Network Security: Sep 2003

Diffie-Hellman key generation A distributed key generation scheme Given q - a large prime number a – a primitive root of q (1 <= ak mod q < q, and distinct for all 1 <= k < q) A: picks XA (keeps it secret), computes and sends YA  aXA mod q to B B: picks XB (keeps it secret), computes and sends YB  aXB mod q A A and B compute the secret shared key aXA XB YBXA or YAXB 11/21/2018 Tutorial on Network Security: Sep 2003

Diffie-Hellman key generation Man-in-the-middle attack Assumes ability to intercept, and spoof XA, A2B XE, A2B A B E XE, B2A XB, B2A aXA*XE aXB*XE 11/21/2018 Tutorial on Network Security: Sep 2003

Diffie-Hellman key generation Issues with the algorithm: What is the value of q, a? Make available several sets, and let the parties negotiate Man-in-the-middle attack Use some form of authentication Denial of service attack, arises from address-spoofing Use cookies: Replay attacks Use nonces 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Cookies Cookies: A requests B’s attention B responds with a “cookie” (a random number), K A must return K in its subsequent messages Characteristics of cookies: Should depend upon data specific to B Should use some secret information Cookie generation and verification must be fast B should not have to save the cookie Example method used: Hash sender/receiver IP address TCP port nos. and a secret value 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Oakley Key exchange 11/21/2018 Tutorial on Network Security: Sep 2003

Oakley Key exchange: part 1 A to B ID of A, ID of B Initiator cookie, CK-A Encryption, hash, authentication algorithms Specific Diffie Hellman group (q, a) public key yA = aXA mod q Nonce NA SignedKR(A)[ID of A, ID of B, NA, q, a, yA] 11/21/2018 Tutorial on Network Security: Sep 2003

Oakley Key exchange: part 2 B to A ID of B, ID of A Responder cookie, CK-B, Returned initiator cookie, CK-A Encryption, hash, authentication algorithms Specific Diffie Hellman group (q, a) public key yB = aXB mod q Nonce NA, NB SignedKR(B)[ID of B, ID of A, NA, NB, q, a, yB yA] 11/21/2018 Tutorial on Network Security: Sep 2003

Oakley Key exchange: part 3 A to B ID of A, ID of B Returned cookie, CK-B, initiator cookie, CK-A Encryption, hash, authentication algorithms Specific Diffie Hellman group (q, a) public key yA = aXA mod q Nonce NA, NB SignedKR(A)[ID of A, ID of B, NA, NB, q, a, yB yA] 11/21/2018 Tutorial on Network Security: Sep 2003

Tutorial on Network Security: Sep 2003 Thanks 11/21/2018 Tutorial on Network Security: Sep 2003