Cryptography is the study of message protection in secret format. To most people, cryptography is concerned with keeping their communications private and secrete. The protection of sensitive communications has been the emphasis of cryptography throughout much of its history
There are two kinds of cryptography : symmetric and asymmetric. Symmetric cryptosystems use the same key (the secret key) to encrypt and decrypt a message, and Asymmetric cryptosystems use one key (the public key) to encrypt a message and a different key (the private key) to decrypt the encrypted message. Assymetric cryptosystems are also called public key cryptosystems.
Encryption is the transformation of data or message into a form that is as close to impossible as possible to read without the appropriate knowledge (a key; see below). Its purpose is to ensure privacy by keeping information hidden from anyone or unauthorise person, even those who have access to the encrypted data. Decryption is the reverse of encryption; it is the transformation of encrypted data back into an intelligible form.
Authentication is as fundamentally a part of our lives as privacy. Authentication is very necessary in our day to day life with any kind of transactions- when we sign our name to some document for instance – and, as we move to a world where our decisions and agreements are communicated electronically, we need to have electronic techniques for providing authentication. Cryptography provides mechanisms for such procedures. A digital signature.
Cryptography is central to the techniques used in computer and network security for such things as access control and information confidentiality. Cryptography is also used in many applications encountered in everyday life; the security of ATM cards, computer passwords, and electronic commerce all depend on cryptography.