This document discusses cryptography, security services, and security in the internet. It describes symmetric-key cryptography which uses a shared secret key, and asymmetric-key cryptography which uses a public-private key pair. The document outlines five security services: message confidentiality, integrity, authentication, nonrepudiation, and entity authentication. It also discusses protocols for security at the network layer (IPSec), transport layer (SSL/TLS), and application layer (PGP), as well as the use of firewalls to filter network traffic.

1. Name: Khondoker Zahidul Hossain
ID: 13103061
Section: H

2. Topic Discussed:
 Cryptography
 Security Services
 Security in the Internet

3. Cryptography
Cryptography is the science and art of transforming messages to make them
secure and immune to attacks.
Components

4. Symmetric-Key Cryptography
The same key is used by both the sender and receiver. The Key is called the
secret key.

5. Traditional
ciphers
Substitution
ciphers
Transposition
ciphers
Mono-alphabetic
Poly-alphabetic

6. Asymmetric-Key Cryptography
In an asymmetric-key cipher, a pair of keys is used. The sender uses the public
key. The receiver uses the private key.

7. Security Services
A network can provide on of the five services given below:
Message Confidentiality
Message Integrity
Message Authentication
Message Nonrepudiation
Entity Authentication

8. Confidentiality: With Symmetric-Key Cryptography, With Asymmetric-Key
Cryptography
Integrity: Document & Fingerprint, Message & Message Digest.
Message Authentication: MAC, HMAC, Digital Signature.
Message Nonrepudiation: Signature Schemes
Entity Authentication: Passwords, Challenge Response.

9. Security in the Internet
 IPSec for IP Protocol.
 SSL for TCP Protocol
 PGP for SMTP Protocol.
 Firewalls

10. IP Security (IPSec)
IP Security(IPSec) is a collection of protocols designed by the IETF to provide
security for a packet at the network level.
Two Modes: Transport Mode, Tunnel Mode
Two Security Protocol: AH Protocol, ESP Protocol

11. SSL/TLS
It is a transport layer protocol that give end-to-end service for transactions on
the Internet.
SSL Protocols TLS Protocols
Handshake Handshake
Alert Alert
Record Record
Change Cipher Spec Protocol

12. PGP
Provide security at the application layer.
Services:
Plaintext
Message Authentication
Compression
Confidentiality with One Time Session Key
Code Conversion
Segmentation

13. Firewalls
A firewall is a device installed between the internal network of an organization and
the rest of the Internet.
Packet-Filter Firewall: It filters at the network or transport layer.
Proxy-Filter Firewall: It filters at the application layer.