Security is the quality or state of being secure: - a relief from exposure to danger, and making safe against adverse contingencies. But encryption alone is not sufficient. Proper key selection, key management, physical security, people security and procedures to ensure that the plaintext does not leak out of the system via loopholes are all essential for a secure computer data system. The strength of a good cryptosystem does not depend on keeping its algorithm secret; the security of the cipher text relies solely on the secrecy of the key.


 Cryptography is the art or science of secret writing, or more exactly, of storing information (for a shorter or longer period of time) in a form which allows it to be revealed to those you wish to see it yet hides it from all others. A cryptosystem is a method to accomplish this. Cryptanalysis is the practice of defeating such attempts to hide information. Cryptology includes both cryptography and cryptanalysis.
Stewart, Ed Tiltel and Mike (2000) defined Cryptography as added security to data during processing, storage and communications. They further described the various types of cryptography such as Symmetric Key Cryptography and Asymmetric Key Cryptography or Public Key.

 Kessler (1998), described cryptography as the science of writing in secret code. He further describes it as one essential aspect for secure communication.

In data communication and telecommunication, cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the internet. Cryptography, then, not only protects data from theft or alteration, but can also be used for user authentication.

Burnett and Panini(2004) outline the security requirement which include the following:    

·         Authentication

·         Privacy/confidentiality

·         Integrity

·         Non-repudiation

 Liddell and scolt(1996) stated in their book that  in the famous Greek drama the 'Iliad', cryptography was used when Bellerophon was sent to the king with a secret tablet which told the king to have him put to death. The king tried to kill him by having him fight several mythical creatures, but he won every battle.

The Spartans used a system which consisted of a thin sheet of papyrus wrapped around a staff (now called a "staff cipher"). Messages were written down the length of the staff, and the papyrus was unwrapped. In order to read the message, the papyrus had to be wrapped around a staff of equal diameter. Called the 'skytale' cipher, this was used in the 5th century B.C. to send secret messages between greek warriors. Without the right staff, it would be difficult to decode the message using the techniques available at that time. The following version of the alphabet demonstrates the technique. First we see the wrapped version of the alphabet, then the unwrapped version.





Skytale cipher

Another Greek method was developed by Polybius (now called the "Polybius Square"). The letters of the alphabet would be laid out in a five by five square (similar to the later Playfair method) with i and j occupying the same square. Rows and columns are numbered 1 to 5 so that each letter has a corresponding (row,column) pair. These pairs could easily be signaled by torches or hand signals. Decryption consists of mapping the digit pairs back into their corresponding characters. This system was the first to reduce the size of the symbol set, and in a loose sense it might be considered the forerunner of modern binary representations of characters. Try decoding the message on the right.

        \ 1 2 3 4 5


        1|A B C D E        T=54

2|F G H I J        H=32    5344 44 4435

        3|K L M N O        I=42    4224 24 3211

4|P Q R S T        S=44

        5|U V W X Y/Z

The Polybius Square

Julius Ceasar used a system of cryptography (i.e. the 'Caesar Cipher') which shifted each letter 2 places further through the alphabet (e.g. Y shifts to A, R shifts to T, etc.). This is probably the first cipher used by most school children. In figure 2.4, the first row is plaintext, while the second row is the equivalent ciphertext. The distance of the displacement is not important to the scheme, and in fact, neither is the lexical ordering chosen. The general case of this sort of cipher is the "monoalphabetic substitution cipher" wherein each letter is mapped into another letter in a one to one fashion. Try decoding VJKU.



The Caesar Cipher

Cryptanalysis is the practice of changing ciphertext into plaintext without complete knowledge of the cipher. The Arabs were the first to make significant advances in cryptanalysis. An Arabic author, Qalqashandi, wrote down a technique for solving ciphers which is still used today. The technique is to write down all the ciphertext letters and count the frequency of each symbol. Using the average frequency of each letter of the language, the plaintext can be written out. This technique is powerful enough to cryptanalyze ANY monoalphabetic substitution cipher if enough cyphertext is provided.

El Gamely(1989) in his book stated how cryptography started to progress. All of the Western European governments used cryptography in one form or another, and codes started to become more popular. Ciphers were commonly used to keep in touch with ambassadors. The first major advances in cryptography were made in Italy. Venice created an elaborate organization in 1452 with the sole purpose of dealing with cryptography. They had three cipher secretaries who solved and created ciphers that were used by the government.

Shamir (1977) in his book declared Leon Battista Albert as "The Father of Western Cryptology" in part because of his development of polyalphabetic substitution. Polyalphabetic substitution is any technique which allows different ciphertext symbols to represent the same plaintext symbol. This makes it more difficult to interpret ciphertext using frequency analysis. In order to develop this technique, Alberti analyzed the methods for breaking ciphers, and devised a cipher which would try to render these techniques invalid. He designed two copper disks that fit into each other, each with the alphabet inscribed upon it. To start enciphering, a predetermined letter on the inner disk is lined up with any letter on the outer disk, which is written as the first character of the ciphertext. The disks are kept stationary, with each plaintext letter on the inner disk aligned with a ciphertext letter on the outer disk. After a few words of ciphertext, the disks are rotated so that the index letter on the inner disk is aligned with a new letter on the outer disk, and in this manner, the message is enciphered. By rotating the disk every few words, the cipher changed enough to limit the effectiveness of frequency analysis. Even though this technique in its stated form is very weak, the idea of rotating the disks and therefore changing the cipher many times within a message was a major breakthrough in cryptography.

Rivest(1990) explained how the next major step was taken in 1518, by Trithemius, a German monk who had a deep interest in the occult. He wrote a series of six books called 'Polygraph', and in the fifth book, devised a table that repeated the alphabet with each row a duplicate of the one above it, shifted over one letter. To encode a message, the first letter of the plaintext is enciphered with the first row of the table, the second letter with the second row, and so on. This produces a message where all available ciphers are used before being repeated. Figure 2.5 shows a changing key cipher of this sort. Notice that the assignment of code symbols to plaintext symbols changes at each time step (T1,T2,...). In this system, the key repeats every 26 letters of cipher text.








A Changing Key Cipher

Behrouz  andforouzan(2004) in their book stated that in 1553, Giovanni Batista Belasco extended this technique by choosing a keyword that is written above the plaintext, in a letter to letter correspondence. The keyword is restarted at the beginning of each new plaintext word. The letter of the keyword above the letter of the plaintext is the first letter of the cipher line to be used. In other words, if the plaintext letter is 'b', and it's keyword letter is 'r', then the line of the Trithemius cipher beginning with 'r' is used to encipher the letter 'b'.

The most famous cryptographer of the 16th century was Blaise de Vigenere (1523-1596). In 1585, he wrote 'Tracte des Chiffres' in which he used a Trithemius table, but changed the way the key system worked. One of his techniques used the plaintext as it's own key. Another used the ciphertext. The manner in which these keys are used is known as key scheduling, and is an integral part of the "Data Encryption Standard" (DES) [DESDOC77] which we will discuss later.

Robshaw (1994) stated in his book that in the year 1628, a Frenchman named Antoine Rossignol helped his army defeat the Huguenots by decoding a captured message. After this victory, he was called upon many times to solve ciphers for the French government. He used two lists to solve his ciphers: "one in which the plain elements were in alphabetical order and the code elements randomized, and one to facilitate decoding in which the code elements stood in alphabetical or numerical order while their plain equivalents were disarranged." When Rossignol died in 1682, his son, and later his grandson, continued his work. By this time, there were many cryptographers employed by the French government. Together, they formed the "Cabinet Noir" (the "Black Chamber").

Shamir and Adleman(1978) stated in their book that the father of American cryptology is James Lovell. He was loyal to the colonies, and solved many British ciphers, some which led to Revolutionary victories. In fact, one of the messages that he deciphered set the stage for the final victory of the war.

According to Panini(2004) he stated that the 'wheel cipher' was invented by Thomas Jefferson around 1795, and although he never did very much with it, a very similar system was still in use by the US navy only a few years ago. The wheel cipher consisted of a set of wheels, each with random orderings of the letters of the alphabet. The key to the system is the ordering in which the wheels are placed on an axle. The message is encoded by aligning the letters along the rotational axis of the axle such that the desired message is formed. Any other row of aligned letters can then be used as the ciphertext for transmission. The decryption requires the recipient to align the letters of the ciphertext along the rotational axis and find a set of aligned letters that makes linguistic sense as plaintext. This will be the message. There is a very small probability that there will be two sensible messages from the decryption process, but this can be checked simply by the originator. Without knowing the orderings of symbols on the wheels and the ordering of wheels on the axle, any plaintext of the appropriate length is possible, and thus the system is quite secure for one time use. Statistical attacks are feasible if the same wheels are used in the same order many times.






  A Wheel Cipher

Schneider (1996) stated that In 1844, the development of cryptography was dramatically altered by the invention of the telegraph. Communication with the telegraph was by no means secure, so ciphers were needed to transmit secret information. The public's interest in cryptography blossomed, and many individuals attempted to formulate their own cipher systems. The advent of the telegraph provided the first instance where a base commander could be in instant communication with his field commanders during battle. Thus, a field cipher was needed. At first, the military used a Vigenere cipher with a short repeating keyword, but in 1863, a solution was discovered by Friedrich W. Kasiski for all periodic polyalphabetic ciphers which up until this time were considered unbreakable, so the military had to search for a new cipher to replace the Vigenere.

El Genial (1985) stated that the 'Playfair' system was invented by Charles Wheatstone and Lyon Playfair in 1854, and was the first system that used pairs of symbols for encryption. The alphabet is laid out in a random 5 x 5 square, and the text is divided into adjacent pairs. The two letters of the pair are located, and a rectangle is formed with the two letters at opposite corners. The letters at the other two corners are the two letters of ciphertext. This is very simple to use, but is not extremely difficult to break. The real breakthrough in this system was the use of two letters at a time. The effect is to make the statistics of the language less pronounced, and therefore to increase the amount of work and the amount of ciphertext required to determine a solution. This system was still in limited use in world war II, and was very effective against the Japanese.




    CWEDX       =              =




A Playfair System

Vigenere (1972) stated that in 1859, Pliny Earle Chase, developed what is known as the fractionating or topographic cipher. A two digit number was assigned to each character of plaintext by means of a table. These numbers were written so that the first numbers formed a row on top of the second numbers. The bottom row was multiplied by nine, and the corresponding pairs are put back in the table to form the cipher text.

Hellman’(1976) started in his book that Kasiski developed a cryptanalysis method in 1863 which broke almost every existing cipher of that time. The method was to find repetitions of strings of characters in the ciphertext. The distance between these repetitions is then used to find the length of the key. Since repetitions of identically ciphered identical plaintext occur at distances which are a multiple of the key length, finding greatest common divisors of repetition distances will lead to the key length. Once the key length (N) is known, we use statistics on every Nth character and the frequency of use implies which character it represents in that set of ciphertext symbols. These repetitions sometimes occur by pure chance, and it sometimes takes several tries to find the true length of the key using this method, but it is considerably more effective than previous techniques. This technique makes cryptanalysis of polyalphabetic substitution ciphers quite straight forward.

The story of cryptography would be at an end if it weren't for the practical problem that in order to send a secret message, an equal amount of secret key must first be sent.


In 1972 the US NATIONAL BUREAU OF STANDARDS began the search for an encryption algorithm that could be tested and certified. After several false starts in 1974 IBM offered the US government an algorithm which was based on the early 1970’s LUFICER algorithm. The offer was accepted and the algorithm was tested and ‘adjusted’ by the NSA and eventually released as a federal standard in 1976.

DES is a SYMMETRIC BLOCK cypher based on a 64 bit block. The user feeds in a 64 block of plain text and is returned 64 bits of cyphertext. The same algorithm and key are used for the encryption and decryption operations.

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