1 What is network security

2 Principles of Cryptography

2.1 Symmetric Key

substitution cipher: encryption on a side, decryption on another side.

DES: Data Encryption Standard

56-bit symmetric key 64-bit plaintext input

AES: Advanced Encryption Standard

3 Message Integrity

3.1 Message Authentication Code

3.2 Digital Signatures

A valid digital signature gives a recipient reason to believe that the message was created by a known sender, such that the sender cannot deny having sent the message (authentication and non-repudiation) and that the message was not altered in transit (integrity). 

-- From Wiki

Hash H(m) first, then sign it Ks(H(m))

Send both clear message m and Ks(H(m)) to receiver. Receiver Hash m and decrypt H(m), to compare them whether they are same.

Certification Authorities

The third company to verify the public key of Bob‘s.

4 End Point Authentication

4.1 Authentication

Other method can be attacked by other host playback process.

5 Securing e-mail

5.1 Secrecy

sender: 

1. random symmetric private key, Ksy

2. message m, Ks(m)

3. use receiver‘s public key to encrypt the Ks, Kry(Ks)

4. send both Ksy(m) and Kr(Ks) to receiver

receiver:

1. Use own public key to decrypt the Ksy

2. Use Ksy to decrypt message m

5.2 Sender Authentication

sender:

1. sign the message m

2. send both clear message m and signed message Ksen(H(m))

receiver:

decrypt the signed message.

5.3 Combination

Three Keys: sender sign key, receiver public key, symmetric key

6 Securing TCP: SSL

Two parts: Client/Server authentication, data encryption

6.2 Process

1. Handshake

Establish TCP connection

Authentication

2. Key Derivation

4 keys

MAC key and encrption key

3. Data Transfer

compute MAC

encrypt data and MAC

form SSL format record

6.3 Mechanism

Apply for key for encryption and keep secret between server and client.

7 Network Layer: IPsec

8 Wireless LANs

WEP

9 Firewall and IDS