Cryptography-An Introduction...

The art of scrambling messages has actually been in use for thousands of years. However, beginning with the development of the telegraph, cryptography has been growing outside of military and highly diplomatic circles, and into the public domain. E-commerce, personal and corporate privacy have continued the pervasive need for secure communications. Additionally, cryptographic techniques are not limited to the purpose of hiding messages. For instance, archeologists can use cryptographic techniques to interpret ancient or lost languages.

Cryptography, which derives from the Greek word kryptos meaning hidden, is both an art and a science. It is the science of using mathematics to encrypt and decrypt data. The purpose of cryptography is to transmit information in such a way that access to it is restricted entirely to the intended recipient. Originally the security of a cryptotext depended on the secrecy of the entire encrypting and decrypting procedures; however, today we use ciphers for which the algorithm for encrypting and decrypting could be revealed to anybody without compromising the security of a particular cryptogram. In such ciphers a set of specific parameters, called a key is supplied together with the plain text as an input to the encrypting algorithm, and together with the cryptogram as an input to decrypting algorithm. Thus there exist three fundamental parts to the encryption/decryption process namely the message, a key and an algorithm.

The oldest encryption method is known as Secret Key (symmetric) encryption. Here the same key is used both to encrypt the plain text and then to decrypt the cipher text.

Symmetric systems are generally fast and can now be made effectively impossible to break. Like anything of this sort, there is no absolute guarantee, but it's quite reasonable to say that with the knowledge available at present and with access to current levels of computing power, a properly constructed system can be made entirely secure. For better security the key should be as large as possible. The greater the length of a key, all other things being equal, the progressively more difficult it becomes to break the encryption.

Another well-known encryption method is a system of Public (asymmetric) key cryptography. Public key systems use two keys constructed from prime numbers and which complement each other such that one key is used for encrypting plain text which can then be decrypted by, and only by, the other. One of the key pairs is kept private and is known as the private key while the other, the public key, is widely publicized. The unique nature of the process is that anyone who knows the public key can encrypt a message that can then only be decrypted by he sender using the secure private key. Conversely, a message can be encrypted with the private key and anyone decrypting it, using the freely available public key, can be confident that it came from the sender. Asymmetric keys are very long, typically ranging from perhaps 768 bits versus 56 or 128 bits.

However, inspite of the various security measures, there is a possibility of the message to get altered. For this purpose Hashing Technique is used as message authentication. It is becoming as important as information hiding. Hashing is the technique that involves a string of characters of variable length being converted to a fixed-length result, usually 128 bits. In practice, any change to the message, even one as trivial as adding or removing a space, will result in a different hash value being created when the process is run.

Cryptographic protocols and algorithms are difficult to get right. Creating cryptography algorithms requires years in professional review of the algorithm. Hence trying to create one is a sheer wastage of time and energy. Instead one should always opt for algorithms that are widely used, heavily analyzed, and accepted as secure. Algorithms for Symmetric Key Encryption, Public Key as well as for Cryptographic Hash are available worldwide.

Secret Key encryption and Public key cryptography techniques have been used for over thousands of years. However, if and when mathematicians or computer scientists come up with fast and clever procedures for factoring large integers the whole privacy and discretion of public-key cryptosystems would vanish overnight. Such an upcoming method is Quantum Cryptography. Recent work in Quantum computation shows that quantum computers can factorize much faster than classical computers. Quantum cryptography is a way to combine the relative ease and convenience of key exchange in public key cryptography with the ultimate security of a onetime pad. Here, any attempt by an enemy to obtain the bits in a key not only fails, but gets detected as well. This is due to a certain phenomena that occur at the subatomic level. The sender is required to prepare a chain of polarized photons. This chain is then sent through an optical fiber to the receiver. To obtain the original message the receiver makes a series of measurements.

The main objective of all these methods remains the same – Security. Security cannot be eliminated but can be improved with upcoming technologies. Cryptography has become an essential part of this strategy. It is regarded as backbone in various developments. The space of an introductory article is not enough to describe it. Efforts have been made to provide unprecedented levels of security. Using quantum computing, cryptographers will be able to build stronger crypto-systems, thus providing better security. However, the question of implementation of such systems and understanding of implications at machine level still prevails.

Prepared by,

Purnima Shewale,

Usha Mittal Institute of Technology,

S.N.D.T. University,

Mumbai.