1. Network Security (contd.)
Bijendra Jain
11/21/2018
Tutorial on Network Security: Sep 2003

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

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

23. 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

24. 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

25. 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

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