Saturday, February 2, 2013

Encryption Techniques


Encryption Techniques:
There are two methods of all types of encryption techniques-
(1) Substitution
(2) Transposition

(1)                        Substitution:
A substitution technique is a method in which the letters of plain text are replaced by other letter or numbers or any symbol.
            “If the plain text is viewed as a sequence of bits then substitution involves replacing plain text bit patterns into a new bit pattern.”
                            i.)            Caeser cipher:
The Ceaser cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet.
For ex:
If a plain text is abcdef then cipher will be defghi.

Then algorithm can be expressed as
            C = (P+3) Mod 26
            Where C is cipher text
            And P is plain text
In generalised manner it can be represent as
            C = (P+3) Mod 26
            Where K is an arbitrary value. The resultant cipher text is known as Caeser cipher.

(2)                        Transposition:
It is an encryption in which the letters of the message are rearranged.

Transposition methods –
                                                        i.            Row wise Transposition
                                                      ii.            Column wise Transposition
                       
                       


          Major Procedure of transposition
It is the rearrangement of the plain text.
Here is an example which shows five column transposition and the rearranged form:-






Methods to create Block cipher –



1.     Electronic Code Book (ECB) Method:
It is a simplest block cipher mode in which pain text is handled 64-bit at a time and each block of plain text is encrypted using the same key.
  

The term codebook is used because, for the same given key it generates a unique cipher text for every 64 bit block of plain text. Therefore we can assume a large code book which has an entry for every possible 64 bit plain text pattern with corresponding cipher text.
            The ECB method is particularly used for short amount of data, which means we can use ECB mode to transmit a DES key. For a lengthy message it may be possible that the cipher text can be generated but it would not be secure.

2.     Cipher Block Chaining (CBC) Mode:
This mode of cipher text is particularly used in DES and AES (Authentication Key Exchange Standards). For low threat application it is simplest method for implementation. In block encryption scheme chaining means linking each block to the precious block’s value.

Process of CBC


                                                                                                                                                                             

It takes a 64-bit block of input for encryption. It generates its own 64-bit random number and uses the cipher Ci as the next random number for
ri+1
That means it takes the precious block of cipher text and uses it as the random number which is X-ORed into plain text and transmits it with along with the data. It has a initial random number IV(Initial Vector). The IV exactly works as follows –
A random chosen IV guarantees that even if the same message is sent repeatedly the cipher text will be completely different for each time. So it avoids “Chosen Plain Text Attack”.

3.     Counter (CTR) Mode:
In this mode a onetime pad is generated & X-ORed with the data (Onetime pad is a long random stream to encrypt the message). In this method the IV(Initial Vector) increments its value and encrypt the result to get successive blocks of plain text and the padding bits. The main advantage of CTR mode is that the cryptology can be pre computed and encryption is simply an X-OR and user can encrypt the message starting at any point rather than being forced to start from the beginning.
The application of CTR mode is to encrypt a randomly access file and the key will be......






4.     Cipher Feedback Mode (CFB):
It is a block cipher technique which uses 64-bit block; it is possible to convert DES into a stream cipher using either CFB mode or OFB mode so when a character stream is being transmitted each character can be encrypted and transmitted immediately using a character oriented stream cipher.
For the encryption the CFB, the input to an encryption function is 64-bit shift register which is initially set to an initialization vector. The MSB is initially set to the s bits of the output of the encryption function, are X-ORed with first pair of plain text i.e. p1 which produce the first unit of cipher text c1. Then c1 is transmitted and the contents of shift register are shifted left by its LSB.

5.     Output Feedback Mode(OFB):
It is similar to CFB. In this mode the output of the encryption function that is feed backed to the shift register in OFB.
One advantage of using OFB mode is that bit errors in transmission do not propagate.
For Ex: If a single bit error occurs in c1 only the recovered value of p1 is affected that means the recovered value of p1 will be transmitted for the next cipher text c2.
The Disadvantage of OFB is that it is more vulnerable to a message stream modification attack compared to CFB and it has the error detecting code inside which are padded with the plain text.

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