Network Security
Contents
• Security Requirements and Attacks
• Confidentiality with Conventional
Encryption
• Message Authentication and Hash
Functions
• Public-Key Encryption and Digital
Signatures
• IPv4 and IPv6 Security
Security Requirements
• Confidentiality
• Integrity
• Availability
Passive Attacks
• Release of message content
(eavesdropping)
– Prevented by encryption
• Traffic Analysis
– Fixed by traffic padding
• Passive attacks are easier to prevent than
to detect
Active Attacks
• Involve the modification of the data stream
or creation of a false data stream
• Active Attacks are easier to detect than to
prevent
Active Attacks (cont.)
•
•
•
•
Masquerade
Replay
Modification of messages
Denial of service
Conventional Encryption
Decryption
algorithm
Encryption
algorithm
Transmitted
ciphertext
Plain text
Plain text
Shared
secret key
Conventional Encryption
Requirements
• Knowing the algorithm, the plain text and
the ciphered text, it shouldn’t be feasible to
determine the key.
• The key sharing must be done in a secure
fashion.
Encryption Algorithms
• Data Encryption Standard (DES)
– Plaintext: 64-bit blocks
– Key: 56 bits
– Has been broken in 1998 (brute force)
• Triple DES
• Advanced Encryption Standard (AES)
– Plaintext: 128-bit blocks
– Key: 128, 256 or 512 bits
Location of Encryption Devices
PSN
PSN
PSN
PSN
End-to-end encryption device
PSN
Packet Switching Node
Link encryption device
Key Distribution
• Manual
– Selected by A, physically delivered to B
– Selected by C, physically delivered to A and B
• Automatic
– The new key is sent encrypted with an old key
– Sent through a 3-rd party with which A and B
have encrypted links
Message Authentication
• Authentic message means that:
– it comes from the alleged source
– it has not been modified
Message Authentication
Approaches
• Authentication with conventional
encryption
• Authentication without message
encryption:
– when confidentiality is not necessary
– when encryption is unpractical
Message Authentication Code
• Uses a secret key to generate a small
block of data
MACM = F (KAB, M)
One-way Hash Function
• Message digest – a “fingerprint” of the
message
• Like MAC, but without the use of a secret
key
• The message digest must be
authenticated
Secure Hash Requirements
•
•
•
•
H can be applied to a block of any size
H produces a fixed-length output
H(x) is easy to compute
Given h, it is infeasible to compute x s.t.
H(x) = h
• Given x, it is infeasible to find y s.t.
H(x) = H(y)
• It is infeasible to find (x,y) such that
H(x) = H(y)
Secure Hash Functions
• Message Digest v5 (MD5)
– 128-bit message digest
– has been found to have collision weakness
• Secure Hash Algorithm (SHA-1)
– 160-bit message digest
Public-Key Encryption
• Each user has a pair of keys:
– public key
– private key
• What is encrypted with one, can only be
decrypted with the other
Encryption
Bob’s
public key
Bob’s
private key
Transmitted
ciphertext
Plain text
Plain text
Alice
Bob
Authentication
Alice’s
private key
Alice’s
public key
Transmitted
ciphertext
Plain text
Plain text
Alice
Bob
Digital Signature
• Like authentication, only performed on a
message authenticator (SHA-1)
Public-Key Encryption Algorithms
• RSA (used by PGP)
• El Gamal (used by GnuPG)
Key Management
• Public-Key encryption can be used to
distribute secret keys for conventional
encryption
• Public-Key authentication:
– signing authority
– web of trust
IPv4 and IPv6 Security
• Provides encryption/authentication at the
network (IP) layer
• IPSec applications:
– Virtual Private Networking
– E-commerce
• Optional for IPv4, mandatory for IPv6
IP Header with IPSec
Information
Two Types of IPSec Security
Protocols
Advantages of IPSec
How an AH is Generated in
IPSec
AH Fields
The ESP Header Format
Encapsulated Security Payload
Tunnel Versus Transport Mode
AH Header Placement in
Transport Mode
AH Header Placement in Tunnel
Mode
ESP Header Placement in
Transport Mode
ESP Header Placement in
Tunnel Mode
Security Association
• One-way relationship between two hosts,
providing security services for the payload
• Uniquely identified by:
– Security Parameter Index (SPI)
– IP destination address
– Security Protocol Identifier (AH/ESP)
SA Security Parameters
IPSec Process Negotiation
Key Management
• Manual
– used for small networks
– easier to configure
• Automated
– more scalable
– more difficult to setup
– ISAKMP/Oakley
IKE Use in an IPSec
Environment