13-02-2013, 04:46 PM
CRYPTOGRAPHY and SECURITY
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ABSTRACT
Electronic computers have evolved from exiguous experimental enterprises in
the 1940s to prolific practical data processing systems in the 1980s. As we have come to rely on
these systems to process and store data, we have also come to wonder about their ability to
protect valuable data.
Data security is the science and study of methods of protecting data in
computer and communication systems from unauthorized disclosure and modification. The goal
of this seminar is to introduce the mathematical principles of data security and to show how these
principles apply to ATM,Smart cards,e-commerce and other purposes.
Data security has evolved rapidly since 1975. Exciting developments in
cryptography: public-key encryption, digital signatures, the Data Encryption Standard (DES),
key safeguarding schemes, and key distribution protocols. We have developed techniques for
verifying that programs do not leak confidential data, or transmit classified data to users with
lower security clearances. We have come to a better understanding of the theoretical and
practical limitations to security.
INTRODUCTION
Cryptography, art and science of preparing coded or protected communications
intended to be intelligible only to the person possessing a key. Cryptography (Greek kryptos,
“secret”; graphos, “writing”) refers both to the process or skill of communicating in or
deciphering secret writings (codes, or ciphers) and to the use of codes to convert computerized
data so that only a specific recipient will be able to read it using a key (see Encryption).
Cryptographers call an original communication the cleartext or plaintext. Once the original
communication has been scrambled or enciphered, the result is known as the ciphertext or
cryptogram. The enciphering process usually involves an algorithm and a key. An encryption
algorithm is a particular method of scrambling—a computer program or a written set of
instructions. The key specifies the actual scrambling process. The original communication may
be a written or broadcast message or a set of digital data.
In its broadest sense, cryptography includes the use of concealed messages, ciphers,
and codes. Concealed messages, such as those hidden in otherwise innocent text and those
written in invisible ink, depend for their success on being unsuspected. Once they are discovered,
they frequently are easy to decipher. Codes, in which predetermined words, numbers, or symbols
represent words and phrases, are usually impossible to read without the key codebook.
Cryptography also includes the use of computerized encryption to protect transmissions of data
and messages.
Today most communication leaves some kind of recorded trail. For example,
communications over telephone lines, including faxes and e-mail messages, produce a record of
the telephone number called and the time it was called. Financial transactions, medical histories,
choices of rental movies, and even food choices may be tracked by credit card receipts or
insurance records. Every time a person uses the telephone or a credit card, the telephone
company or financial institution keeps a record of the number called or the transaction amount,
location, and date. In the future, as telephone networks become digital, even the actual
conversations may be recorded and stored. All of this amounts to a great privacy. The ability to
encrypt data, communications, and other information gives individuals the power to restore
personal privacy.
Cryptography is important for more than just privacy, however. Cryptography
protects the world’s banking systems as well. Many banks and other financial institutions
conduct their business over open networks, such as the Internet. Without the ability to protect
bank transactions and communications, criminals could interfere with the transactions and steal
money without a trace.
Cryptography and Security
Division of Computer Science & Engineering,SOE,CUSAT 2
TYPES OF CRYPTOGRAPHY
There are many types of cryptography, including codes, steganography (hidden or
secret writing), and ciphers. Codes rely on codebooks. Steganography relies on different ways to
hide or disguise writing. Ciphers include both computer-generated ciphers and those created by
encryption methods. The different types of ciphers depend on alphabetical, numerical, computerbased,
or other scrambling methods.
Codes and Codebooks
A well-constructed code can represent phrases and entire sentences with symbols,
such as five-letter groups, and is often used more for economy than for secrecy. A properly
constructed code can give a high degree of security, but the difficulty of printing and distributing
codebooks—books of known codes—under conditions of absolute secrecy limits their use to
places in which the books can be effectively guarded. In addition, the more a codebook is used,
the less secure it becomes.
Imagine a codebook with two columns. In the first column is a list of all the words
that a military commander could possibly need to use to communicate. For example, it contains
all the possible geographic areas in a region, all possible times, and all military terms. In the
other column is a list of plain words. To create a coded message, the encoder writes down the
actual message. He then substitutes words in the codebook by finding matches in the second
column for the words in the message and using the new words instead. For example, suppose the
message is Attack the hill at dawn and the codebook contains the following word pairs: attack =
bear, the = juice, hill = orange, at = calendar, and dawn = open. The encoded message would
read Bear juice orange calendar open.
If the coded message fell into enemy hands, the enemy would know it was in code,
but without the codebook the enemy would have no way to decrypt the message. Codebooks lose
some of their value over time, however. For example, if the coded message fell into enemy hands
and the next day the hill was attacked at dawn, the enemy could link the event to the coded
message. If another message containing the word orange were captured, and the following day,
something else happened on the hill, the enemy could assume that orange = hill is in the
codebook. Over time, the enemy could put together more and more code word pairs, and
eventually crack the code. For this reason, it is common to change codes often.
Steganography:
Steganography is a method of hiding the existence of a message using tools such as
invisible ink, microscopic writing, or hiding code words within sentences of a message (such as
making every fifth word in a text part of the message). Cryptographers may apply steganography
to electronic communications. This application is called transmission security.