# The computer security

### INTRODUCTION:

This paper dealt with basic principles and theory of cryptanalysis. Cryptology is the branch of knowledge which combines communication secret in all aspects. There are two major aspects of cryptology are cryptanalysis and cryptography.

### CRYPTOGRAPHY:

The branch of cryptology is the cryptography which has been protected for communication rather than reading by wrong people. The cryptographic system used to protect codes and cipher. The encryption key is used to make the message as unreadable by the application of codes and ciphers. Thus it is called cryptograms.

### CRYPTANALYSIS:

The branch of cryptology is the cryptanalysis and it is concern by others to solve the cryptographic system. The main objective of the cryptanalysis is to recover the cryptographic system by reading encrypted text and messages. To increase the value of potential intelligence recovery the text has made (Terminology and System Types, nd).

### THEORY OF CRYPTANALYSIS:

Opponent (Oscar) is the general assumption in cryptanalysis and he is using cryptosystem. It is referred as Kerckhoff's principle. ofcourse this task is more difficult to Oscar because if the he does not come to know whether cryptosystem is being used. There is no necessity to base the security of cryptosystem and gives an idea to Oscar about the system is being employed. It holds kerckhoff's principle by designing a goal in cryptosystem and obtaining a security.

In attack model cryptosystem there is a differentiation in different attack system. When an enemy attacks, the information available is specified from the attack model. There are numerous types of attack model.

- CIPHER TEXT ONLY ATTACK
- KNOWN PLAINTEXT ATTACK
- CHOSEN PLAINTEXT ATTACK
- CHOSEN CIPHERTEXT ATTACK
- Each Z, Q, X, J, K, V is having the probability less than 0.1.
- Each L and D is having the probability around 0.4.
- Each M, U, C, I, W, Y, G, F, B, P is having the probability between 0.015 and 0.028.
- Each A, T, O, I, S, N, H, R is having the probability between 0.06 and 0.09.
- E is having the probability around 0.120.

A string of ciphertext, y, has been obtained by the opponent.

A string of plaintext, x, is obtained by the opponent and corresponding ciphertext, y.

There is an access for the encryption machine temporarily by the opponent. The plaintext string, x, is chosen and the corresponding ciphertext string, y, is constructed.

There is an access for the decryption machine temporarily by the opponent. The ciphertext string, y, is chosen and corresponding plaintext string, x, is constructed.

The main objective of the enemy is to decide the key that was used. The ciphertext strings are decrypted by the opponent in a specified 'target' and in future if any extra ciphertext string has to decrypt the same encrypted keys are used.

Ciphertext only attack is the weaker type of attack. They assumed that there is no spaces and punctuation in English text.

In English language there are many techniques of cryptanalysis has used. Beker and piper estimate the table statistics. They have portioned the English alphabet letters 26 into five different parts.

It is consider having two or three consecutive letters n sequence. This is called diagram or trigrams and the most important 30 common diagrams are

AT, TO, NT, HA, TI, IS, IT, ET, TE, AR, HI, SE, OF, IN, TH, HE, AN, ER, RE, EP, ES, ON, ST, EN, EA, OU, AS, NG, OR.

The twelve trigrams are

DTH, FOR, WAS, ETH, NTH, THA, HER, ERE, ENT, THE, ING, AND (Stinson, 2006, pp.26).

### IMPLEMENTATION OF CRYPTANALYSIS SOFTWARE IN THE CURRENNT STATE:

In last five years symmetric key encryption is very active and it is growing in the research industry and academic area. The efforts of standardization for NESSIE, AES and CRYPTREC are to control the government in export of cryptography. In 2001 the US NIST has designed a new 128-bit block 128/192/256 bit key cipher AES from the old one 64-bit block 56-bit key cipher DES. For implementing compactness, speed and security the AES has resulted a open competition process for 15 submission which include Europe, North America and Asia. Belgian cipher known as Rijndael won the competition for various attempts in analysis.

The new techniques in cryptanalysis were developed in 1999-2000. They are impossible differential attacks, slide attacks and boomerang attack. In some cases linear techniques and classic differential techniques are less effective. An algebraic attack came to effort in the year 2002. Research has been made in this attack and hence proved it is suitable for certain class in stream ciphers (Biryukov, nd).

There is time consumption in side channel by implementing the algorithm in software or hardware. The cipher key detailed implementation in side-channel is combined with mathematical analysis techniques to find out the secret key. When comparing with pure mathematical techniques the side channel analysis is more powerful. To break a common cipher a small amount of side-channel information is enough. To recover DES (DATA ENCYPTION STANDARD) symmetric block cipher, a cipher text block between 50 and 200 is required in side channels attack from DFA (DIFFERENTIAL FAULT ANALYSIS) (Karri et al., 2002).

### PRACTICAL DIFFICULTIES:

Exponential operations are used by RSA cryptosystem to perform encryption and decryption. Exponential algorithm is used to scan the keys from left to right. Key bits are involved in the operation of squaring and multiplying. The aim of this algorithm is to multiply or square the operation against timing attack. Attacking the multiply operation is less efficient than attacking the square operation. There are four sets in the message sample secret keys.

To perform squaring and multiplying operation, a significant bit 1 is taken. If the next significant bit is needed, it is required to explain about the addition reduction is required or not, depends upon the value being 0 or 1. There is a requirement in addition reduction if the value is taken as 1 and not squaring. There are two subsets available for all messages. There is a reduction in the messages M1 and no reduction in the messages M2. It is divided into two subsets if they consider the bit as 0.

To make the decision the key bit should be either 0 or 1 in the mean time. The mean time 0 M1 and M2 sets are subtracted each other 0 to 1 from the mean time of M3 and M4 to set a different value. It should indicate the bit value is correct (Ali and Al- Salami, 2004).

### ANALYSIS:

If wrong keys bits are used there will be a decrease in the value of the given figure. The key correctness will be evaluated in this work for future. There should be no reduction in the messages so that subsets be sequential.

### SECURITY RISK:

There are three valuable components software, hardware and data. The systems are getting affected by variable assets. There is a loss in the information because of the system. We can easily identify the data and it should be protected. First we make sure that the data are not disclosed to unauthorized person. The weakness is the security in vulnerability. The computer initiated and human initiated ones are the computer system threats. The results are failure in software, design flaws hardware and human errors. These are the major risk in computing (Pfleeger and Pfleeger, 2003).