Crypto Systems
Introduction
Definition
Cryptography is the study of mathematical techniques related to aspects of information security such as confidentiality, data integrity, entity authentication, and data origin authentication.
Cryptography is not the only means of providing information security, but rather one set of techniques.
Cryptographic goals
Of all the information security objectives listed in Table 1.1, the following four form a framework upon which the others will be derived: (1) privacy or confidentiality (x1.5, x1.8); (2) data integrity (x1.9); (3) authentication (x1.7); and (4) nonrepudiation (x1.6).
Confidentiality is a service used to keep the content of information from all but those authorized to have it. Secrecy is a term synonymous with confidentiality and privacy. There are numerous approaches to providing confidentiality, ranging from physical protection to mathematical algorithms which render data unintelligible.
Data integrity is a service which addresses the unauthorized alteration of data. To assure data integrity, one must have the ability to detect data manipulation by unauthorized parties. Data manipulation includes such things as insertion, deletion, and substitution.
Authentication is a service related to identification. This function applies to both entities and information itself. Two parties entering into a communication should identify each other. Information delivered over a channel should be authenticated as to origin, date of origin, data content, time sent, etc. For these reasons this aspect of cryptography is usually subdivided into two major classes: entity authentication and data origin authentication. Data origin authentication implicitly provides data integrity (for if a message is modified, the source has changed).
Nonrepudiation is a service which prevents an entity from denying previous commitments or actions. When disputes arise due to an entity denying that certain actions were taken, a means to resolve the situation is necessary. For example, one entity may authorize the purchase of property by another entity and later deny such authorization was granted. A procedure involving a trusted third party is needed to resolve the dispute.
A fundamental goal of cryptography is to adequately address these four areas in both theory and practice. Cryptography is about the prevention and detection of cheating and other malicious activities.
Cryptographic tools should be evaluated with respect to various criteria such as:
Cryptographic terms:
MODULE DESCRIPTION
The Caesar Cipher
One of the simplest examples of a substitution cipher is the Caesar cipher, which is said to have been used by Julius Caesar to communicate with his army. Caesar is considered to be one of the first persons to have ever employed encryption for the sake of securing messages. Caesar decided that shifting each letter in the message would be his standard algorithm, and so he informed all of his generals of his decision, and was then able to send them secured messages. Using the Caesar Shift (3 to the right), the message,
"RETURN TO ROME"
would be encrypted as,
"UHWXUA WR URPH"
In this example, 'R' is shifted to 'U', 'E' is shifted to 'H', and so on. Now, even if the enemy did intercept the message, it would be useless, since only Caesar's generals could read it.
Thus, the Caesar cipher is a shift cipher since the ciphertext alphabet is derived from the plaintext alphabet by shifting each letter a certain number of spaces. For example, if we use a shift of 19, then we get the following pair of ciphertext and plaintext alphabets:
Plaintext: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Ciphertext: T U V W X Y Z A B C D E F G H I J K L M N O P Q R S
To encipher a message, we perform a simple substitution by looking up each of the message's letters in the top row
and writing down the corresponding letter from the bottom row. For example, the message
THE FAULT, DEAR BRUTUS, LIES NOT IN OUR STARS BUT IN OURSELVES.
would be enciphered as
MAX YTNEM, WXTK UKNMNL, EBXL GHM BG HNK LMTKL UNM BG HNKLXEOXL.
Essentially, each letter of the alphabet has been shifted nineteen places ahead in the alphabet, wrapping around the end if necessary. Notice that punctuation and blanks are not enciphered but are copied over as themselves.
Breaking a Caesar Cipher by Hand
Can a computer guess what shift was used in creating a Caesar cipher? The answer, of course, is yes. But how does it work?
The unknown shift is one of 26 possible shifts. One technique might be to try each of the 26 possible shifts and check which of these resulted in readable English text. But this approach has limitations. For one thing how would the computer recognize "readable English text?" For another, what if a muiltiple Caesar shift was used, as is the case for a Vigenere cipher , where each letter of the keyword provides the basis for a Caesar shift. That is, if the key word is bam, then every third letter of the plaintext starting at the first would be shifted by 'b' (=1) and every third letter beginning at the second would be shifted by 'a' (=0) and every third letter beginning at the third would be shifted by 'm' (=12). Obviously we can't depend on obtaining readable English text here.
A better approach makes use of statistical data about English letter frequencies. It is known that in a text of 1000 letters of various English alphabet occur with about the following relative frequencies:
This information can be useful in deciding the most likely shift used on a given enciphered message. Suppose the enciphered message is:
K DKVO DYVN LI KX SNSYD, PEVV YP CYEXN KXN PEBI, CSQXSPISXQ XYDRSXQ.
We can tally the frequencies of the letters in this enciphered message, thus
Now we can now shift the two tallies so that the large and small frequencies from each frequency distribution match up roughly. For example, if we try a shift of ten on the previous example, we get the following correspondence between English language frequencies and the letter frequencies in the message.
English Language Frequencies
Enciphered Message Frequencies
Note that in this case the large frequencies for cipher X and Y correspond to large for English N and O, the bare spots for cipher T and U correspond to bare spots for English J and K. Also, an isolated large frequency for cipher S correpsonds to a similar one for English I. In view of this evidence we needn't even worry too much about the drastic mismatch for English E, which is usually the most frequent letter in a random sample of English text.
If we now apply this substitution to the message we get:
A TALE TOLD BY AN IDIOT, FULL OF SOUND AND FURY, SIGNIFIYING NOTHING.
Simple Substitution Cipher
These hieroglyphics have evidently a meaning. If it is a purely arbitrary one, it may be impossible for us to solve it. If, on the other hand, it is systematic, I have no doubt that we shall get to the bottom of it.  Sherlock Holmes in The Adventure of the Dancing Men
A cipher is a method for encrypting a message  i.e., for transforming the message into one that can't be easily read. The original message is called the plaintext or clear and the encrypted message is called a cryptogram or ciphertext. A substitution cipheris one in which each letter of the plaintext is replaced by some other symbol. Usually the replacement symbols are themselves letters of the alphabet, but this needn't always be the case as we see in The Dancing Men. the replacement symbols were h
An alphabet is an ordered set of symbols. For example, the normal English alphabet consists of the symbols {A,B,C,...,Z}. is an ordered set of symbols. For example, the normal English alphabet consists of the symbols {A,B,C,...,Z}. A simple substitution is one in which each letter of the plaintext is always replaced by the same ciphertext symbol. In other words, there is a 11 relationship between the letters of the plaintext and the ciphertext alphabets.
For the normal English alphabet, how many different ciphertext alphabets can be get if we use the same letters? In other words, in how many different ways can we permute or rearrange the English alphabet? The answer is 26!. That's approximately equal to the number 4 followed by 26 zeros. To understand how we got that number imagine that you are given the task of making an arbitrary permutation of the English alphabet. You have to make 26 choices. On the first choice you can choose any one of the 26 letters in the alphabet. On the second choice you can choose any one of the remaining 25 letters. On the third choice you can choose any one of the remaining 24 letters. And so on. On the last choice, there is just one letter remaining. So, in all there are 26! = 26 x 25 x 24 x ... x 1 different ways to make these choices.
The Vigenere Cipher  A Polyalphabetic Cipher
One of the main problems with simple substitution ciphers is that they are so vulnerable to frequency analysis. Given a sufficiently large ciphertext, it can easily be broken by mapping the frequency of its letters to the know frequencies of, say, English text. Therefore, to make ciphers more secure, cryptographers have long been interested in developing enciphering techniques that are immune to frequency analysis. One of the most common approaches is to suppress the normal frequency data by using more than one alphabet to encrypt the message. A polyalphabetic substitution cipher involves the use of two or more cipher alphabets. Instead of there being a onetoone relationship between each letter and its substitute, there is a onetomany relationship between each letter and its substitutes.
Hardware Specifications:
Main Memory : 64MB.
MicroProcessor : Pentium –III.
Hard Disk Drive : 4.3 GB.
Cache Memory : 512KB.
Software Specifications:
Operating System : Windows 98/NT.
Front End : Java Using AWT/SWINGS

Friday, 2 February 2018
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