Cryptography Network Security Computer Science Essay

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Network security is a complicated subject, historically only tackled by well-trained and experienced experts. However, as more and more people become ``wired'', an increasing number of people need to understand the basics of security in a networked world. This document was written with the basic computer user and information systems manager in mind, explaining the concepts needed to read through the hype in the marketplace and understand risks and how to deal with them.

Some history of networking is included, as well as an introduction to TCP/IP and internetworking . We go on to consider risk management, network threats, firewalls, and more special-purpose secure networking devices.

This is not intended to be a ``frequently asked questions'' reference, nor is it a ``hands-on'' document describing how to accomplish specific functionality.

It is hoped that the reader will have a wider perspective on security in general, and better understand how to reduce and manage risk personally, at home, and in the workplace.

Cryptography and Network Security

Does security provide some very basic protections that we are naive to believe that we don't need? During this time when the Internet provides essential communication between tens of millions of people and is being increasingly used as a tool for commerce, security becomes a tremendously important issue to deal with.

There are many aspects to security and many applications,

Ranging from secure commerce and payments to private

Communications and protecting passwords. One essential aspect for

Secure communications is that of cryptography.

Cryptography is the science of writing in secret code and is an ancient art. The first documented use of cryptography in writing dates back to circa 1900 B.C. when an Egyptian scribe used non-standard hieroglyphs in an inscription.

In data and telecommunications, cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the Internet.

Within the context of any application-to-application communication, there are some specific security requirements, including:

Authentication: The process of proving one's identity. (The primary forms of host-to-host authentication on the Internet today are name-based or address-based, both of which are notoriously weak.)

Privacy/confidentiality: Ensuring that no one can read the message except the intended receiver.

Integrity: Assuring the receiver that the received message has not been altered in any way from the original.

Non-repudiation: A mechanism to prove that the sender really sent this message. Cryptography, then, not only protects data from theft or alteration, but can also be used for user authentication.

The three types of cryptographic algorithms that will be discussed are (Figure 1):

Secret Key Cryptography (SKC): Uses a single key for both encryption and decryption

Public Key Cryptography (PKC): Uses one key for encryption and another for decryption

Hash Functions: Uses a mathematical transformation to irreversibly "encrypt" information

1. Secret Key Cryptography

With secret key cryptography, a single key is used for both encryption and decryption.

As shown in Figure the sender uses the key (or some set of rules) to encrypt the plain text and sends the cipher text to the receiver. The receiver applies the same key (or rule set) to decrypt the message and recover the plain text. Because a single key is used for both functions, secret key cryptography is also called symmetric encryption.

With this form of cryptography, it is obvious that the key must be known to both the sender and the receiver; that, in fact, is the secret. The biggest difficulty with this approach, of course, is the distribution of the key.

Secret key cryptography schemes are generally categorized as being either stream ciphers or block ciphers.

Stream ciphers operate on a single bit (byte or computer word) at a time and implement some form of feedback mechanism so that the key is constantly changing. A block cipher is so- called because the scheme encrypts one block of data at a time using the same key on each block. In general, the same plain text block will always encrypt to the same cipher text when using the same key in a block cipher whereas the same plaintext will encrypt to different cipher text in a stream cipher.

2. Public key cryptography

Modern PKC was first described publicly by Stanford University professor Martin Hellman and graduate student Whitfield Diffie in 1976. Their paper described a two-key crypto system in which two parties could engage in a secure communication over a non-secure communications channel without having to share a secret key.

Generic PKC employs two keys that are mathematically

related although knowledge of one key does not allow someone to

easily determine the other key. One key is used to encrypt the

plaintext and the other key is used to decrypt the cipher text. The

important point here is that it does not matter which key is applied

first, but that both keys are required for the process to work (Figure

1B). Because a pair of keys are required, this approach is also called

asymmetric cryptography

3. Hash Functions

Hash functions, also called message digests and one-way encryption, are algorithms that, in some sense, use no key (Figure 1C). Instead, a fixed-length hash value is computed based upon the plaintext that makes it impossible for either the contents or length of the plaintext to be recovered. Hash algorithms are typically used to provide a digital fingerprint of a file's contents often used to ensure that the file has not been altered by an intruder or virus. Hash functions are also commonly employed by many operating systems to encrypt passwords. Hash functions, then, help preserve the integrity of a file.


Secure use of cryptography requires trust. While secret key cryptography can ensure message confidentiality and hash codes can ensure integrity, none of this works without trust. In SKC, PKC solved the secret distribution problem. There are a number of trust models employed by various cryptographic schemes.

The web of trust employed by Pretty Good Privacy (PGP) users, who hold their own set of trusted public keys.

Kerberos, a secret key distribution scheme using a trusted third party.

Certificates, which allow a set of trusted third parties to authenticate each other and, by implication, each other's users.

Each of these trust models differs in complexity, general applicability, scope, and scalability.

Types of authority

Establish identity: Associate, or bind, a public key to an individual, organization, corporate position, or other entity.

Assign authority: Establish what actions the holder may or may not take based upon this certificate.

Secure confidential information (e.g., encrypting the session's symmetric key for data confidentiality).


Todays latest used cryptographic techniques:

Hash algorithms that are in common use today include:

Message Digest (MD) algorithms

Secure Hash Algorithm (SHA)

Pretty Good Privacy (PGP)

Pretty Good Privacy (PGP) is one of today's most widely used public key cryptography programs. PGP can be used to sign or encrypt e-mail messages with mere click of the mouse.

Depending upon the version of PGP, the software uses SHA or MD5 for calculating the message hash; CAST, Triple-DES, or IDEA for encryption; and RSA or DSS/Diffie-Hellman for key exchange and digital signatures. And much more techniques used.

Time is the only true test of good cryptography; any cryptographic scheme that stays in use year after year is most likely a good one. The strength of cryptography lies in the choice (and management) of the keys; longer keys will resist attack better than shorter keys

Encrypt and decrypt messages using any of the classical substitution ciphers discussed, both by hand and with the assistance of programs.

understand the concepts of language redundancy and unicity distance.

Different types of threats to network:

Application backdoors - Some programs have special features that allow for remote access . Others contain bugs that provide a backdoor , or hidden access , that provides some level of control of the program.

SMTP session hijacking - SMTP is the most common method of Sending e-mail over the Internet . By gaining access to a list of e- mail Addresses , a person can send unsolicited junk e-mail ( spam ) to thousands of users . This is done quite often by redirecting the e-mail through the SMTP server of an unsuspecting host , making the actual sender of the spam difficult to trace.

Operating system bugs - Like applications , some operating systems Have backdoors . Others provide remote access with insufficient security controls or have bugs that an experienced hacker can take advantage of .

Denial of service - You have probably heard this phrase used in news reports on the attacks on major Web sites . This type of attack is nearly Impossible to counter . What happens is that the hacker sends a request to the server to connect to it . When the server responds with an acknowledgement and tries to establish a session , it cannot find the system that made the request . By inundating a server with these unanswerable session requests , a hacker causes the server to slow to a crawl or eventually crash.

E-mail bombs - An e-mail bomb is usually a personal attack . Someone sends you the same e-mail hundreds or thousands of times until your e-mail system cannot accept any more messages .

Macros - To simplify complicated procedures , many applications allow you to create a script of commands that the application can run . This script is known as a macro . Hackers have taken advantage of this to create their own macros that , depending on the application , can destroy your data or crash your computer .

Viruses - Probably the most well-known threat is computer viruses . A virus is a small program that can copy itself to other computers . This way it can spread quickly from one system to the next. Viruses range from harmless messages to erasing all of your data .

Spam - Typically harmless but always annoying , spam is the electronic equivalent of junk mail . Spam can be dangerous though . Quite often it contains links to Web sites . Be careful of clicking on these because you may accidentally accept a cookie that provides a backdoor to your computer.

Redirect bombs - Hackers can use ICMP to change ( redirect ) the Path information takes by sending it to a different router . This is one of the ways that a denial of service attack is set up.

Network security can be done by various methods.

1. Virtual Private Network:

A virtual private network ( VPN ) is a way to use a public telecommunication infrastructure , such as the Internet , to provide remote offices or individual users with secure access to their organization's network. A virtual private network can be contrasted with an expensive system of owned or leased lines that can only be used by one organization. The goal of a VPN is to provide the organization with the same capabilities , but at a much lower cost

Implementation of network security by VPN.

Step 1. - The remote user dials into their local ISP and logs into the ISP’s network as usual.

Step 2. - When connectivity to the corporate network is desired, the user initiates a tunnel request to the destination Security server on the corporate network. The security server authenticates the user and creates the other end of tunnel.

Fig : a) A leased line private network b) A virtual private network

Step 3. - The user then sends data through the tunnel which encrypted by the VPN software before being sent over the ISP connection.

Step 4. - The destination Security server receives the encrypted data and decrypts. The Security server then forwards the decrypted data packets onto the corporate network. Any information sent back to the Remote user is also encrypted before being sent over the Internet.


A firewall provides a strong barrier between your private network and the Internet . You can set firewalls to restrict the number of open ports , what type of packets are passed through and which protocols are allowed through . You should already have a good firewall in place before you implement a VPN , but a firewall can also be used to terminate the VPN sessions .

Fig2: A fire wall consisting of two packet filters and an application gateway

3.IPSec -

Internet Protocol Security Protocol (IPSec) provides

enhanced security features such as better encryption algorithms and more comprehensive authentication . IPSec has two encryption modes : tunnel and transport . Tunnel encrypts the header and the payload of each packet while transport only encrypts the payload. Only systems that are IPSec compliant can take advantage of this Protocol . Also , all devices must use a common key and the firewalls of each network must have very similar security policies set up. IPSec can encrypt data between various devices , such as :

Router to router

Firewall to router

PC to router

PC to server

A software firewall can be installed on the computer in your home that has an Internet connection . This computer is considered a gateway because it provides the only point of access between your home network and the Internet .

4. AAA Server - AAA (authentication , authorization and accounting)

servers are used for more secure access in a remote-access VPN environment . When a request to establish a session comes in from a dial up client , the Request is proxies to the AAA server . AAA then checks the following :

Who you are (authentication)

What you are allowed to do (authorization)

What you actually do (accounting)

The accounting information is especially useful for tracking client. Use for security auditing , billing or reporting purposes .



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