Dr. L. Christofi 1

Local & Metropolitan Area Networks

ACOE322

Lecture 8Network Security

Dr. L. Christofi 2

0. Overview• As the knowledge of computer networking and protocols has

become more widespread, so the threat of intercepting and decoding message data during its transfer across a network has increased.

• An intruder in a network can identify and remove the protocol control information at the head of each message, leaving the message contents. The message contents, including passwords and other sensitive information, can then be interpreted. This is know as listening or eavesdropping.

• In addition, an intruder can use a recorded message sequence to generate a new sequence. This is known as masquerading.

• Therefore, encryption should be applied to all data transfers that involve a network.

• In the context of the ISO reference model, the most appropriate layer to perform such operations is the presentation layer.

Dr. L. Christofi 3

Terminology • Data encryption involves the sending party in

processing all data prior to transmission so that in case it is intercepted during transmission to be incomprehensible to the intercepting party.

• Data should only be interpreted (decrypted) only by the intended recipient.

• Most encryption methods involve the use of an encryption key, which is only known by the two correspondents.

• Before encryption, message data is called plaintext and after encryption, ciphertext.

• The aim is to chose an encryption method such as an intruder cannot decipher the recorded ciphertext in a realistic time period.

Dr. L. Christofi 4

Security Requirements• Privacy (Confidentiality)

—Data only be accessible by authorized parties

• Authenticity —A host or service be able to verity the identity of a

user

• Integrity—Data can be modified only by authorized parties

• Availability—Data are available to authorized parties

• Non-repudiation—Receiver must be able to prove that a received

message came from a specific sender

Dr. L. Christofi 5

Cryptography• Original message

— Plaintext

• Encryption —Transforms the plaintext to cipher text

• Decryption—Transforms the ciphertext back to plain text

• Ciphers—Different categories of algorithms in cryptography

• Key—Number (value) that the ciphers operate on

Dr. L. Christofi 6

Cryptography Components

Dr. L. Christofi 7

Encryption and Decryption• In cryptography, the encryption/decryption

algorithms are public; the keys are secret• Two groups of algorithms

—Symmetric-key algorithms—Public-key algorithms

Dr. L. Christofi 8

Symmetric-key cryptography• Same key is used by the sender (for encryption) and

receiver (for decryption). The key is shared• In symmetric-key cryptography, the same key is

used in both directions• Often used for long messages• Algorithm is public, key is secret

Dr. L. Christofi 9

Traditional Ciphers• Substitution Ciphers• Transposition Ciphers

Dr. L. Christofi 10

Substitution Ciphers• One symbol is substitutes by another

— e.g. Caesar Cipher (a->D, b->E, c->F,… z-C).

• Monoalphabetic substitution— The relationship between a character in the plaintext to the

character in the ciphertext is always one-to-one

• Polyalphabetic substitution— The relationship between a character in the plaintext and a

character in the ciphertext is one-to-many

Dr. L. Christofi 11

Transposition Ciphers

• The characters retain their plaintext form but change their positions

• Text is organized into a two-dimensional table and the columns are interchanged according to a key

Dr. L. Christofi 12

Block Cipher• Plain text and ciphertext are block of bits• Each block is a unit of encryption/decryption

Dr. L. Christofi 13

Block Cipher (Cont.)• P-box• S-box• Product block• Data Encryption Standard (DES)• Triple DES

Dr. L. Christofi 14

Public-key cryptography• Two keys are used

—Private key• Kept by receiver• Used for decryption

—Public key• Announced to the public• Used for encryption

• Disadvantages—Complexity of the algorithm

• Key is too large, calculation time is long—Need to verify the association between an entity and its

public key• Most common algorithm – RSA (Rivest, Shamir, and

Adleman)• More efficient for short messages

Dr. L. Christofi 15

Public-key cryptography (Cont.)

Dr. L. Christofi 16

Digital signature

• Digital signature is used to provide—Authentication—Integrity—Nonrepudiation

Dr. L. Christofi 17

Signing the whole document• The sender uses private key to encrypt (sign) the

message• The receiver uses the public key of the sender to

decrypt the message.• Note: Digital signature does not provide privacy. If

there is a need for privacy, another layer of encryption/decryption must be applied.

Dr. L. Christofi 18

Signing the document• The sender creates a miniature version or digest of the document

and signs it—Using hash function is used to creates a fixed-size digest from a

variable-length message—Two common hash

• MD5: produce a 120-bit digest• SHA-1: produce a 160-bit digest

• The receiver checks signature on miniature

Dr. L. Christofi 19

Sender and receiver site

Sender Site

Receiver site

Dr. L. Christofi 20

User authentication• Message authentication

—Identity of sender is verified for each single message

• User authentication• User identity is verified once for the entire

duration of system access• With symmetric key• With public key

—Sender use private key to encrypt the message—Receiver uses sender’s public key to decrypt the

message

Dr. L. Christofi 21

Using symmetric key only• Simple approach

— Sender sends his/her identity and password in an encrypted message, using symmetric key (Kab)

— Can not prevent replay attack.— Replay attack: malicious intruders can resend the same

message

Dr. L. Christofi 22

Using symmetric key only (Cont.)• Using a nonce

— Step 1: Sender sends his/her identity and password in an encrypted message, using symmetric key (Kab)

—Step 2: Receiver challenges sender by sending a nonce, which is a large random number that is used only once (one-time number), to the receiver

—Step 3: Receiver responds to the message by sending back the encrypted nonce using the symmetric key

—Advantage: can prevent replay attack

Dr. L. Christofi 23

Public key authentication• Sender encrypts the message with his/her

private key• Receiver uses sender’s public key to decrypt

the message and authenticate the sender• Problem

—Cannot prevent man-in-the-middle attack

• Man-in-the-middle-attack:—Intruder announce his/her public key to the

receiver in place of a sender

Dr. L. Christofi 24

Firewalls • A device (usually a router or a computer) installed between the

internal network of an organization and rest of the internet• Help to protect an organization’s computers and networks from

unwanted Internet traffic• Designed to keep problems in the internet from spreading to an

organizations’ computers• Classes of firewalls

— Packet-filter firewalls— Proxy-based firewalls

Dr. L. Christofi 25

Packet-filter firewall

• A filter that uses a filtering table to decide which packet must be discarded (not forwarded).

• Filters on network layer and transport layer

• Block packets based on—Source and destination IP addresses—Source and destination ports—Type of protocol (TCP or UDP)

Dr. L. Christofi 26

Packet-filter firewall

Dr. L. Christofi 27

Proxy firewall• Focuses on application layer

• Filters messages based on information of the message at application layerStep 1: Runs as a proxy for destination process to

receive the requestStep 2: Opens the packet at the application level

and finds out if the request is legitimateStep 3: If message is legitimate, act as sender

process and sends the message to the real receiver. Otherwise drop the message and send an error message to the external sender

Dr. L. Christofi 28

Proxy firewall

Dr. L. Christofi 29

References

• F. Halsall, Data Communications, Computer Networks and Open Systems, 4th edition, Addison Wesley, 1995

• W. Stallings, Data and Computer Communications, 7th edition, Prentice Hall, 2004