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Chapter: Cryptography and Network Security

Classical CryptoSystems






referred  conventional / private-key / single-key


sender and recipient share a common key


all classical encryption algorithms are private-key


was only type prior to invention of public-key in 1970plaintext - the original message


Some basic terminologies used :

ciphertext - the coded message


cipher - algorithm for transforming plaintext to ciphertext


key - info used in cipher known only to sender/receiver


encipher (encrypt) - converting plaintext to ciphertext


decipher (decrypt) - recovering ciphertext from plaintext


cryptography - study of encryption principles/methods


cryptanalysis (codebreaking) - the study of principles/ methods of deciphering ciphertext without knowing key


cryptology - the field of both cryptography and cryptanalysis


Figure Conventional Encryption



Here the original message, referred to as plaintext, is converted into apparently random nonsense, referred to as cipher text. The encryption process consists of an algorithm and a key. The key is a value independent of the plaintext. Changing the key changes the output of the algorithm. Once the cipher text is produced, it may be transmitted. Upon reception, the cipher text can be transformed back to the original plaintext by using a decryption algorithm and the same key that was used for encryption.


The security depends on several factors. First, the encryption algorithm must be powerful enough that it is impractical to decrypt a message on the basis of cipher text alone. Beyond that, the security depends on the secrecy of the key, not the secrecy of the algorithm.


Two requirements for secure use of symmetric encryption:


a strong encryption algorithm


a secret key known only to sender / receiver


Y = EK(X)


= DK(Y)


assume encryption algorithm is known


implies a secure channel to distribute key


A source produces a message in plaintext, X = [X1, X2, … , XM] where M are the number of letters in the message. A key of the form K = [K1, K2, …, KJ] is generated. If the key is generated at the source, then it must be provided to the destination by means of some secure channel.


With the message X and the encryption key K as input, the encryption algorithm forms the cipher text Y = [Y1, Y2, …, YN]. This can be expressed as


Y = EK(X)


The intended receiver, in possession of the key, is able to invert the transformation: X = DK(Y)


An opponent, observing Y but not having access to K or X, may attempt to recover X or K or both. It is assumed that the opponent knows the encryption and decryption algorithms. If the opponent is interested in only this particular message, then the focus of effort is to recover X by generating a plaintext estimate. Often if the opponent is interested in being able to read future messages as well, in which case an attempt is made to recover K by generating an estimate.




Cryptographic systems are generally classified along 3 independent dimensions:


Type of operations used for transforming plain text to cipher text


All the encryption algorithms are abased on two general principles: substitution, in which each element in the plaintext is mapped into another element, and transposition, in which elements in the plaintext are rearranged.


The number of keys used


If the sender and receiver uses same key then it is said to be symmetric key (or) single key (or) conventional encryption.


If the sender and receiver use different keys then it is said to be public key encryption.


The way in which the plain text is processed


A block cipher processes the input and block of elements at a time, producing output block for each input block.


A stream cipher processes the input elements continuously, producing output element one at a time, as it goes along.




The process of attempting to discover X or K or both is known as cryptanalysis. The strategy used by the cryptanalysis depends on the nature of the encryption scheme and the information available to the cryptanalyst.


There      are  various types  of  cryptanalytic attacks          based    on        the        amount                of


information known to the cryptanalyst.


Cipher text only A copy of cipher text alone is known to the cryptanalyst.


Known plaintext The cryptanalyst has a copy of the cipher text and the corresponding plaintext.


Chosen plaintext The cryptanalysts gains temporary access to the encryption machine. They cannot open it to find the key, however; they can encrypt a large number of suitably chosen plaintexts and try to use the resulting cipher texts to deduce the key.


Chosen cipher text The cryptanalyst obtains temporary access to the decryption machine, uses it to decrypt several string of symbols, and tries to use the results to deduce the key.




A plaintext message may be hidden in any one of the two ways. The methods of steganography conceal the existence of the message, whereas the methods of cryptography render the message unintelligible to outsiders by various transformations of the text.


A simple form of steganography, but one that is time consuming to construct is one in which an arrangement of words or letters within an apparently innocuous text spells out the real message.




the sequence of first letters of each word of the overall message spells out the real (hidden) message.


Subset of the words of the overall message is used to convey the hidden message. Various other techniques have been used historically, some of them are:


Character marking selected letters of printed or typewritten text are overwritten in pencil. The marks are ordinarily not visible unless the paper is held to an angle to bright light.


Invisible ink a number of substances can be used for writing but leave no visible trace until heat or some chemical is applied to the paper.


Pin punctures small pin punctures on selected letters are ordinarily not visible unless the paper is held in front of the light.


Typewritten correction ribbon used between the lines typed with a black ribbon, the results of typing with the correction tape are visible only under a strong light.


Drawbacks of steganography


Requires a lot of overhead to hide a relatively few bits of information.


Once the system is discovered, it becomes virtually worthless.

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