# Cryptography In 21st Century Computer Science Essay

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The word CRYPTOGRAPHY in Greek means "Secret Writing", which is the ancient methodology of encoding the messages so that only the dispatcher and receiver can understand them (Hingham 1999). Communication and sharing of information has become prominent these days. With the advances in technology many threats has also been increased leading to misuse of the information. Thus Cryptography which started as a modern art now has been extended its roots to every major discipline of mathematics and science. It ensures the secure transmission of data providing various additional advantageous features such as authenticity (Stallings 2006). Thus the origin and prominence of cryptography has been discussed. Various threats involved by attackers and different algorithms of cryptography used were discussed.

## Introduction:

Historically, Cryptography dates back to 1900 BC when an Egyptian tribe used an inscription involving non-standard hieroglyphs, arrived to enable people to maintain privacy for their information while each send to one another, even in the presence of threat of unauthorised access to the information. Some people argue that cryptography came into existence in-between some time immediately after writing came into existence .Later on it not only provided security but also provided many features such as integrity, authenticity and has been rooted deep and played vital role in communication systems. Earlier it has been just specified for military but now Cryptography is in widespread use, and you we might have used it without knowing it (Mactaggarat, 2002). We might have bought books online in different sites where we need to give our bank details. We might have been used online banking system where we need to give our details on web p ages. If proper security techniques were not used such as cryptography, our private data may be accessed by third party and may be misused. There is always a great threat of hackers online so we are getting privacy by means of these techniques that is with knowing we are using these techniques in our daily lives.

Since writing was invented Cryptography has been used in various ways.

It remained an art a game of hiding the data string against different beating out techniques for the greater part of history. From the past 25 years there has been great revolution in communication systems which has brought about a great change in every domain where science is applied to every part of life (Stallings, 2006). Thus the Cryptography art has also been supplemented and flavoured by science. Thus Modern cryptography became a remarkable discipline which is a part of science. Now it plays a vital role in communications system and computer domain in real time world where sharing of data has became prominent

. It is not only restricted to science but its roots also flourished to other disciplines such as mathematics and brings together different fields like number theory, computational-complexity theory involving computer algorithms and probability and stochastic theory. Now even in data and telecommunications it became very necessary while data is being transmitted over untrusted channel including any network

## What is Cryptography?

Cryptology which regards the analysis of cryptosystems can be divided into two disciplines. Cryptography is confined to of cryptosystems, and cryptanalysis analyses the breakage of cryptosystems. These two plays a vital role where setting up a cryptosystem the setup of its security plays an important role. Why the system was introduced, why some body would be using cryptographic techniques.

There are several reasons which were discussed below:

Confidentiality: Now a day the web world is having a threat of unauthorised access called hackers. So the data sent should be set up confidentially which means that no one should access the information either it is transferring or stored data. Data should be protected without being hacked while transferring from network lines.

Authentication: The sender in general wants to know that his information is sent safely to its destined destination and no one interrupts and shares this. In the same way the receiver wants that the received message is from authenticated sender.

Integrity: This is the concept of ensurance that if a person receives some data he has proof that no third person has changed the data.

Non-repudiation: This is a technique which proves that the sender itself sent the message not any one else..

Fig: Cryptography

In the earlier days the military and diplomatic services used cryptosystems in order to send secret messages and save their information during wars. As nowadays communication systems controlled by computer has been flourished in every industry and even civil and military forces, special techniques of protecting the data has been are much more important thus making cryptographic techniques play a vital role in protecting data. The protection is not only against transmitted data but also for the stored data.

## Objectives of Cryptography:

Though a wide range of problems are addressed by Modern cryptography the common problem remains as usual the classical one which should ensure security of communicated data within an untrusted channel. Let's describe this case, by introducing two members let us say sender S, and receiver R.

The first and the most basic goal is privacy which is by all means the main aim of cryptographer. Hiding the content of shared data from the third party is referred to as providing privacy

The second goal is authenticity or integrity which are discussed above

The third goal is setting up Protocols. The sender and receiver both are supplied by a specially designed protocol to achieve privacy or authenticity which are major security goals

Collection or group of programs are regarded as protocols. It is just a algorithm or software equivalent provided one for each party involved. In general case, the sender would be equipped with a program to run, and another for the receiver to run which are different and does opposite work to each other. Packaging and encapsulating the data being sent for transmission are involved in the sender's program . The receiver's program involves how to decapsulate the encapsulated data and how to recover the data from the received package

Possibly together with associated information giving the user the information regarding authenticity

The fourth goal is to design Trust models. From the above discussion we can find that the sender and receiver should be equipped with the models which are just known to those and the third party has no knowledge of it. Thus trust models play a vital role in maintaining the security of information. In general there are two central trust models:

The symmetric (or shared-key) trust model

The asymmetric (or public-key) trust model

## Various threats to Crypto graphical techniques:

Let us discuss some of the techniques which act as threat to encryption techniques.

Brute force method

Weakness in algorithm

Weakness in surroundings.

Weakness of the algorithm refers to weakness in changing plain text to cipher text which the attackers are interested in. Thus once the weakness is found and attacked he may get the knowledge the key.

## Brute force method:

This is a method which involves using up a huge amount of different keys that are destined to convert cipher text to plain text. In general the strength of the scheme depends on the number of trials made to break the key using fake keys. The number of keys to be used depends on length of the key to be broken up. Longer the key length more is the length of trials. It is a cost effective method.

## Weakness in algorithm:

It refers to weakness in changing plain text to cipher text which the attackers are interested in. Thus once the weakness is found and attacked he may get the knowledge the key.

## Weakness in Surroundings:

If attacker finds using brute force and weakness in algorithm a bit difficult in finding the key then he uses this method. For this the surroundings are observed and attacked. Let us consider an example where the person keeps the encrypted data in a file or folder which is protected by a password. At this time first the attacker attacks the surroundings that is firstly he tries to break the password of file to get encrypted data and then moves on decrypting the data/

## TYPES OF CRYPTOGRAPHIC ALGORITHMS

Cryptographic algorithms can be classified in several ways. Let us categorize based on the number of keys that are used for encrypting the data and decrypting the data. The three types of algorithms are discussed below

Secret/private Key Cryptography (SKC): For both encryption and decryption a single key is used.

Public Key Cryptography (PKC): It uses different keys one for encryption of data and other for decryption of data.

Hash Functions: These are introduced newly touching the domain of mathematics. These use mathematical equations to convert encrypted data into original form.

Fig: Encryption and Decryption

## Public-Key Cryptography

Public-key cryptography is the prominent new development in cryptography in the past few decades. Stanford University professor Mr.Martin Hellman and graduate student Mr. Whitfield Diffie first described PKC in a modernised form publicly in 1976. Their work includes two stations involving sharing of data in an untrusted environment without sharing a key.

In general PKC uses two keys which are mathematically related. Even if someone has information of one key it not help someone to determine the other. In order to encrypt the plaintext one key is used and the different key is used to decrypt the cipher text inorder to get plain text. The most fascinating factor is that it is not important which key is applied first but one thing is must that both keys are to be used. This procedure is also regarded as asymmetric cryptography as pair of keys is used up.

Public-key cryptography algorithms that are in use today for key exchange or digital signatures include:

RSA: This is the first of PKC implementation and is still mostly used keeping up its prominence. This is named after the three MIT mathematicians Ronald Rivest, Adi Shamir, and Leonard Adleman who introduced it. RSA now- a-days has its many of applications in thousands of software products and can be utilized for exchange of key, digital signatures, or in encrypting data in small blocks. The different concept set up in RSA is that key having variable size even the block size being encrypted is being varied. The pair of the key is determined from n which is a very large number. Two prime numbers are chosen by applying special rules and their product determines the value of n. The prime numbers considered may be 100 or more numbers in length each, giving out an n with nearly twice as many numbers as the prime factors. The information of n is included in public key and even the derivative of any one of the factors of n. The key concept is that an attacker cannot trace out the prime factors of n which figures out his inability to trace out the private key from this single piece of information and thus we can ensure that by using RSA algorithm the data is secured.

Draw back with RSA: But now a days as the advances in science have improved and it is not much difficult to find out prime factors of numbers with more than 300 digits. Of course one can increase the size of the key so that the range of numbers increases beyond computers computational ability but as the new updates come in it is found not an effective algorithm. Thus algorithms such as DES became prominent.

Later on various algorithms such as D-H algorithm (Diffie, Hellman) used just for secret key exchange ,Digital Signature Algorithm(DSA) used just for Digital Signature capability for authentication of messages , Elgamal algorithm used for key exchange and Elliptic curve Cryptography were introduced.

## Secret/Private Key Cryptography

A single key is used for both encrypting the data and decrypting the data while using secret key cryptography. The sender uses the key also regarded as some set of rules to encrypt the data which is given in the plain text format and sends the cipher text (it is regarded as plain text encapsulated by means of some algorithm) to the receiver. The receiver applies the same key or set of rules to decrypt the covered plain text regarded as cipher text and recover the plaintext. It is also regarded as symmetric encryption as a single key is used for both functions.

We can see that if we are using this kind of cryptography they must be sure that the key must be known to both parties the sender and the receiver and should be kept away from third party. Thus the distribution of the key is the biggest difficulty with this approach.

Secret key cryptography schemes are further classified into two types.

They are stream ciphers or block ciphers which are discussed below.

Stream ciphers implements some form of feedback mechanism operating on a single bit or byte of a computer word at a single time and is implemented such that the key is changing constantly. The second process encrypts one block of data at a single time using the same key on each block at a time and thus regarded as block cipher. In general, the same plaintext 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.

As said earlier different algorithms involved for securing the data Stream ciphers come in different algorithms out of which two are worthy to know. They are

Self-synchronizing stream ciphers: As a function of the previous n bits each bit in the stream are calculated in the key stream. As the decryption procedure always stay synchronized with the encryption process nearly by estimating how far from the n-bit key stream it is present it is regarded as "self-synchronizing". Error propagation is one of the major problems in this process. It goes on in such a way that a faulted bit in transmission will give out in n faulty bits at the receiving end and in this way the error get spreads speedily.

Non-Synchronous stream ciphers: In this process key stream is generated in a fashion which does not depend on the message stream. It uses the similar key stream generation function at both ends of sender station and receiver station. These are, by their basic behaviour, periodic and will not spread out transmission errors, so that there will be eventual repetition key stream.

In the similar fashion Block ciphers can also operate in many different modes. Let us discuss the prominent classical modes which are been used.

Electronic Codebook (ECB) mode: This is the simplest of all modes. It is the most commonly used scheme. In order to encapsulate the message the plaintext is encrypted and to form a cipher text block the secret key is used. In this scheme two similar plaintext forms, will always together produce the similar cipher text block. It is found to be susceptible to a various types of brutal attacks and hence been most common and prominent mode of block ciphers which is cost effective.

Cipher Block Chaining (CBC) mode: This scheme implements a feedback system to the encryption of data process. This uses logical Boolean functions. In this process, the message in original form is applied with the exclusive-or function with the earlier encapsulated text block before it is to encrypted. In this process, two similar blocks of plaintext never encrypt to the same cipher text.

Cipher Feedback (CFB) mode: This scheme involves a block cipher implementation similar to that of self-synchronizing process. This process classifies the data into smaller parts than the block size and allows encryption. In the encrypting interactive terminal input these kind of fragmentation is used and is used in further more similar applications such as. In the CFB mode let 1-byte of data is being used, for example, each character coming as input is placed into a shift register. The shift register used has the same size as the block. Later on the data is encrypted and then the whole block is transmitted. The cipher text is decrypted into original form and the bits in the block which are extra i.e., every data which is more than one byte are discarded at the receiving side,.

Output Feedback (OFB) mode: This process is a block cipher implementation concept similar to that of a synchronous stream. This scheme avoids the same plaintext block from producing the similar cipher text block. This also involves itself an internal feedback system which lies independent of both the plain and cipher text data streams.

Secret key cryptography algorithms that are in use today include:

Data Encryption Standard (DES): It is the most commonly used SKC scheme prominently under usage these days. IBM in the 1970s designed DES and was later adopted by the National Bureau of Standards (NBS) which is now regarded as the National Institute for Standards and Technology (NIST) in 1977 for government purposes involving applications both unclassified and commercial. It involves a block-cipher operating on 64-bit blocks which employs a 58-bit key.

Fig Data Encryption Standard

The major innovation that complicated DES is:

Triple-DES (3DES): A slight change of DES that uses three 56-bit keys and makes three times encryption/decryption while passing across the blocks.

Fig: Triple DES

## Hash Functions:

Hash functions also regarded as message digests and one-way encryption. This algorithm doesn't use a key. Despite a key it uses a fixed-length hash value which is determined depending upon the plaintext. These algorithms give out a digital fingerprint of a file's or folder's contents. This also traces out whether file has been altered by an intruder or virus. These are also in general employed by many operating systems to encrypt passwords for the systems. Some of the prominently used Hash function algorithms are Message digest algorithms (MD2, MD4, MD5), Secure hash algorithms, RIPEMD, HAVAL (Hash of variable length), TIGER and so on.

## Conclusion:

Thus the prominence of Cryptography in the 21 century is discussed. Various algorithms used in the past and latest technologies such as RSA and TDES were discussed.