1. IPSec IPSec is communication security provided at the network layer.
Communication protected with a session key is called a security association, a triple <SPI, dest address, AH|ESP>.
Provides security for security-ignorant network applications.
Somewhat transparent to the application.
Can be implemented as bump-in-the-wire.
Flexible enough to allow
Secure branch office connectivity over the Internet.
Secure remote access over the Internet.
We will restrict ourselves to IPv4 with IPSec in tunnel mode.
Transport the original datagram in its entirety inside another one.
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CS3235, Nov 2002

2. A Simple IPSec Scenario
Public or Private
Network
IP Header
IPSec Hdr
Secure IP payload
User
System
With
IPSec
IP Hdr IPSec Hdr Secure IP payload
Server PC
Networking Device
With IPSec
IP Hdr IP Pload
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CS3235, Nov 2002

3. Some IPSec Terminology
Security Association
One-way relationship between sender and receiver.
An SA can be used with AH or ESP but not both.
It is a triple consisting of
SPI (32 bits)
IP destination address (unicast or multicast)
Security protocol identifier
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CS3235, Nov 2002

4. Some IPSec terminology
Security Policy Database (SPD)
Contains entries, each if which defines a subset of IP traffic and points to an SA for that traffic.
Each entry is defined by a set of IP and upper-layer protocol field values, called selectors.
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CS3235, Nov 2002

5. Authentication Header
Provides support for data integrity and authentication.
Prevents address spoofing attacks.
Guards against replay attacks.
The communicating parties must share a key.
Next hdr
Payload len
Reserved
SPI
Sequence number
Authentication data (variable)
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CS3235, Nov 2002

6. Authentication Header
Authenticates its payload + immutable parts of the outer IP header.
Must support HMAC-MD5-96 & HMAC-SHA-1-96.
Mutable fields set to 0 when computing ICV.
Authentication Data field set to 0 when calculating ICV.
Problem for NAT.
Payload len = The size of the AH header in 32-bit chunks – 2.
Sequence number is used to recognize replayed packets.
A new SA initializes it to 0.
Anti-replay does not permit recycling past 2^32-1.
Receiver should implement a window to check for replay.
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CS3235, Nov 2002

7. Encapsulating Security Payload
Provides confidentiality of message contents and limited traffic flow confidentiality.
Can also provide the same authentication services as AH.
SPI
Sequence number
Payload data (variable)
Padding (0 – 255 bytes)
Pad length
Next header
Authentication data (variable)
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CS3235, Nov 2002

8. Some thoughts The ESP authenticates encrypted payload.
Is there need for separate AH when ESP can provide the same functionality?
Should AH authenticate IP header fields?
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CS3235, Nov 2002