Innovative Polygram Substitution Cipher Method Computer Science Essay

Abstract: Information security has become an important aspect of computing systems. In this internet driven world, virtually every computer is connected to every other computer. We use computers for everything from banking and investing to shopping and communicating with others through email or chat programs. The communication may not be “top secret,” but strangers should be avoided to read email, use computer to attack other systems, send forged email from computer, or examine personal information stored on computer. The world, business and organizations worldwide have to live with a constant threat from attackers and hackers who have a variety of techniques and tools in order to break computer systems, steal information, change data and cause havoc. Each of these aspects in message security can be addressed by standard methods in cryptography. This paper proposes modified polygram substitution method in a linear way, to generate ASCII values of the given text and then applying conversion, transposition with the features of Cryptography. The analysis shows the possibility of hijacking the key used.

Key Words: Cryptography, Polygram Substitution Cipher.

1. INTRODUCTION

Information hiding technique is a new kind of secret communication technology. Keeping secrets is not easy in fact; human tendency is such that when told something is secret and asked to keep it a secret, people are actually quite eager to share that secret with everyone else!

For keeping data secret, the term cryptography is used. Cryptography is the art and science of manipulating message to keep it secure. [8] The term is derived from the Greek language.

• krytos – secret

• graphos – writing

A cryptographic algorithm, or cipher, is a mathematical function used in the encryption and decryption process. [5] Cryptanalysis is one of the major challenging areas of intense research in the discipline of security. It is a process of looking for weakness in the design of ciphers. A cryptosystem takes as input a plaintext and a known key and produces an encrypted version of the plaintext known as the ciphertext. [6]

This paper introduces a new cryptographic algorithm which is a modified version of polygram substitution cipher method. Polygram substitution cipher has been employed in developing novel information security system in multimedia files.

The rest of the paper is organized as follows. In section 2, we present some of the related works .Section 3 describes how the basic Polygram substitution cipher works. In section 4, we describe the modified polygram substitution cipher .Evaluation result has been presented in section 5, application of the proposed system in section 6 and finally we conclude the paper in section 7.

2. RELATED WORK

Cryptographic algorithms to increase the security have been extensively studied in the past .Many algorithms have been proposed but only a few of them are widely adopted.

The substitution cipher is widely used for encryption and decryption of message. Recently, many efforts have been made to propose different algorithms based on substitution cipher.

3. BASIC POLYGRAM SUBSTITUTION CIPHER METHOD

In polygram alphabetic substitution ciphers the plaintext letters are enciphered differently depending upon their placement in the text. As the name polygram alphabetic suggests this is achieved by using several two, three keys and random keys combinations instead of just one, as is the case in most of the simpler crypto systems. [3] A polygram substitution cipher is a cryptosystem in blocks of characters are substituted in groups. For instance (for a particular key) AA could map to NO, AB to IR, JU to AQ, etc.

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These cryptosystems make cryptanalysis harder by destroying the single character frequencies preserved under simple substitution ciphers. In polygram substitution cipher technique, a block of alphabets is replaced with another block. For instance, BECOME could be replaced by XAYKJA, but COME could be replaced by IUSK totally different cipher .This is true despite the last four characters of the two blocks of text (COME) being the same.

This shows that in polygram substitution cipher, the replacement of plain text happens block by block rather than character by character .Polygram substitution cipher technique replaces one block of plain text with a block of cipher text -it does not work on a character by character basis.

3.1 PROBLEMS IN POLYGRAM SUBSTITUTION:

1) Different keys are required for each block of characters ,in above example first key is subtracting 3 from each character and second key is adding 5 to each character .it is easier to find as the keys are very simple .

2) The encryption and decryption technique is applied to each and every block of characters of text differently even the block is repeated in plain text .so it requires more time for encryption .In above example character C’ is replaced by first key into’ Z’ and by second key into ‘I’.

3) It occupies more storage for maintaining the encoded data.

4) The character is replaced by character itself in each block differently .it will make ease for the intruder to identify the characters, thus by trapping the message in easier way, this results in security lapses.

5) The final result is stored in numerical form which is not understood by the computer.

3.2 SOLUTION SUGGESTED

1) First reverse each block of plain text before applying the polygram substitution method.

2) Each character is substituted by another character using following key generation algorithm.

3) In this algorithm a key ‘sphinx ‘is taken and the difference of each character of the key is calculated from ‘a’ and the same difference is added to each corresponding character of the reversed plain text.

4) Each character is then converted to ASCII coded form, adding different keys for each block of character as in the same way as polygram substitution method does.

5) The ASCII value of the corresponding character of the key is added to the corresponding substituted character.

6) If the BLOCK is repeated then also apply the same procedure.

7) As the ASCII values are used for both encryption and decryption so it is not easily rectified by the intruder.

8) Adding and subtracting the consecutive numbers from each ASCII value of the character increases the security and it is not simply identified. [2]

4. MODIFIED POLY SUBSTITUTION METHOD

A. Encryption Process

1) Take the example text “BECOME”.

2) Reverse the above block as “EMOCEB”.

3) Substitute the reverse character as per key generation algorithm. The difference of key from character ‘a’ is calculated and the difference is then added to reverse character.

4) The ASCII value of the character of the key is added to the corresponding substituted character.

5) Add the ASCII value of the algorithm.

6) After adding ASCII value of all values of given text, the resultant text is an encrypted message.

7) Convert the corresponding ASCII value into its equivalent binary value.

8) Transposition takes place in each character after all the process is over that is moves or change one bit either LSB or MSB, the end result is increasing security. If MSB is taken then alternative sign is changed.

9) Finally takes the decimal values of each updated character in the given text and this process shown in Table 1.

B. Decryption Process

1) Takes the ASCII values of each updated character in the given text and converted into binary format.

2) Transposition takes place in each character after all the process is over that is moves or change one bit either LSB or MSB. Sign bit is changed is MSB alters its value.

3) Subtract ASCII value of all values of given text, the resultant text is a decrypted message.

4) Subtract corresponding ASCII value of the character of the key from the corresponding character of the reversed plain text.

5) Transposition takes place in each character after all the process is over that is moves or change one bit either LSB or MSB, the end result is some binary value.

6) Finally takes the decimal values of each updated binary value in the given text and print.

7) Then again reverse the character value generated by the ASCII values.

8) Decrypted message is given as “BECOME” and this process shown in Table 2.

5. EVALUATED RESULT

Encryption Result

Key – SPHINX, message – BECOME

ASCII value for S=83, P=80, H=72, I=73, N=78, X=88

Table 1: Encryption Process Result

CHARACTER

REVERSE

KEY

ASCII VALUE OF KEY

(1)

SUBSTITUTED CHARACTER AFTER ADDING DIFFERENCE

ASCII VALUE(2)

ADD (1)+(2)

BINARY VALUES

ALTER LSB

FINAL RESULT

B

E

S

83

X

88

171

10101011

10101010

170

E

M

P

80

C

67

147

10010011

10010010

146

C

O

H

72

V

86

158

10011110

10011111

159

O

C

I

73

L

76

149

10010101

10010100

148

M

E

N

78

S

83

161

10100001

10100000

160

E

B

X

88

Y

89

177

10110001

10110000

176

The Encrypted message is {170,146,159,148,160,176} i.e. cipher text.

Decryption Result

The Encrypted Text is applied to decrypted formula by applying the reverse process.

Table 2: Decryption Process Result the Plain text is “BECOME”

CYBER RESULT

BINARY VALUES

ALTER LSB

DECIMAL EQUIVALENT(1)

KEY

ASCII VALUE OF KEY (2)

SUBTRACTING(2) FROM (1)

EQUIVALENT CHARACTER

SUBSTITUTED CHARACTER AFTER SUBTRACTING DIFFERENCE

PLAIN TEXT

170

10101010

10101011

171

S

83

88

X

E

B

146

10010010

10010011

147

P

80

67

C

M

E

159

10011111

10011110

158

H

72

86

V

O

C

148

10010100

10010101

149

I

73

76

L

C

O

160

10100000

10100001

161

N

78

83

S

E

M

176

10110000

10110001

177

X

88

89

Y

B

E

6. APPLICATION OF THE PROPOSED SYSTEM

Information Hiding & Embedding Text in Audio Signal that embeds the text with encryption which gains the full advantages of cryptography. In audio steganography, secret message is embedded into digitized audio signal which result slight altering of binary sequence of the corresponding audio file. For more security not only altering the bits of audio files, we embed high security innovative substitution cipher algorithm in audio signal for data hiding.

7. CONCLUSION

The Proposed methodology will give the new area of research on cryptography. The present work is compared with the standard algorithm i.e. polygram substitution method used for encryption and decryption. It is observed that not only the algorithms are to be secure but the keys which are used for encryption and decryption are to be secured more. The basic polygram substitution method used for encryption and decryption is modified for more secure .The proposed algorithm is having more security as compared to previous method at low computational effort .Some modification is made in previously defined algorithms and keys which are used for encryption and decryption and designed this algorithm as refined algorithm for better security and more enhanced features in terms of memory storage an time required to decrypt the key.