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# Cryptography

INTRODUCTION

Does increased security provide comfort to paranoid people? Or does security provide some very basic protections that we are naive to believe that we don't need? During this time when the Internet provides essential communication between tens of millions of people and is being increasingly used as a tool for commerce, security becomes a tremendously important issue to deal with. There are many aspects to security and many applications, ranging from secure commerce and payments to private communications and protecting passwords. One essential aspect for secure communications is through cryptography.

Cryptography comes from the Greek words for ''secret writing.'' It has a long and colorful history going back thousands of years. Professionals make a distinction between ciphers and codes. A cipher is a character-for-character or bit-for-bit transformation, without regard to the linguistic structure of the message. In contrast, a code replaces one word with another word or symbol. Codes are not used any more, although they have glorious history. The messages to be encrypted, known as the plaintext, are transformed by a function that is parameterized by a key. The output of the encryption process, known as the cipher text, is then transmitted, often by messenger or radio. We assume that the enemy, or intruder, hears and accurately copies down the complete cipher text. However, unlike the intended recipient, he does not know what the decryption key is and so cannot decrypt the cipher text easily. sometimes the intruder can not only listen to the communication channel (passive intruder) but can also record messages and play them back later, inject his own messages, or modify legitimate messages before they get to the receiver (active intruder). The art of breaking ciphers, called cryptanalysis, and the art devising them (cryptography) is collectively known as cryptology. It will often be useful to have a notation for relating plaintext, cipher text, and keys. We will use C = EK (P) to mean that the encryption of the plaintext P using key K gives the cipher text C. Similarly, P = DK(C) represents the decryption of C to get the plain text again.

There are, in general, three types of cryptographic schemes typically used to accomplish these goals:

a) Secret key (or symmetric) cryptography

b) Public-key (or asymmetric) cryptography and

c) Hash functions.

In all cases, the initial unencrypted data is referred to as plaintext. It is encrypted into cipher text, which will in turn (usually) be decrypted into usable plaintext.

SECRET KEY (SYMMETRIC ENCRYPTION)

Symmetric encryption is the oldest and best-known technique. A secret key, which can be a number, a word, or just a string of random letters, is applied to the text of a message to change the content in a particular way. This might be as simple as shifting each letter by a number of places in the alphabet. As long as both sender and recipient know the secret key, they can encrypt and decrypt all messages that use this key.

(figure : Symmetric Key Encryption)

Public-key (or asymmetric) cryptography

Asymmetric encryption, also known as public key encryption, uses a public key-private key pairing: data encrypted with the public key can only be decrypted with the private key.

Hash Functions

Hash functions are extremely useful and appear in almost all information security applications.

A hash function is a mathematical function that converts a numerical input value into another compressed numerical value. The input to the hash function is of arbitrary length but output is always of fixed length.

CONCLUSION

Cryptography is an essential information security tool. It provides the four most basic services of information security âˆ’

• Confidentiality âˆ’ Encryption technique can guard the information and communication from unauthorized revelation and access of information.

• Authentication âˆ’ The cryptographic techniques such as MAC and digital signatures can protect information against spoofing and forgeries.

• Data Integrity âˆ’ The cryptographic hash functions are playing vital role in assuring the users about the data integrity.

• Non-repudiation âˆ’ The digital signature provides the non-repudiation service to guard against the dispute that may arise due to denial of passing message by the sender.

All these fundamental services offered by cryptography has enabled the conduct of business over the networks using the computer systems in extremely efficient and effective manner.

Some of the cryptographic algorithms that are more recognizable to the general public are symmetric key algorithms. Several of these, such as DES, 3DES, and AES, are or have been in regular use by the US government and others as standard algorithms for protecting highly sensitive data.

## Cryptography â€“ Drawbacks

Apart from the four fundamental elements of information security, there are other issues that affect the effective use of information âˆ’

• A strongly encrypted, authentic, and digitally signed information can be difficult to access even for a legitimate user at a crucial time of decision-making. The network or the computer system can be attacked and rendered non-functional by an intruder.

• High availability, one of the fundamental aspects of information security, cannot be ensured through the use of cryptography. Other methods are needed to guard against the threats such as denial of service or complete breakdown of information system.

• Another fundamental need of information security of selective access control also cannot be realized through the use of cryptography. Administrative controls and procedures are required to be exercised for the same.