Modern cryptography......

 Symmetric-key Cryptography....

                                               

•  Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key (or, less commonly, in which their keys are different, but related in an easily computable way). This was the only kind of encryption publicly known until June 1976.

•  Symmetric key ciphers are implemented as either block ciphers or stream ciphers. A block cipher enciphers input in blocks of plaintext as opposed to individual characters, the input form used by a stream cipher.

Public key Cryptography.....

                                       

                                                    

•  Symmetric-key cryptosystems use the same key for encryption and decryption of a message, though a message or group of messages may have a different key than others.

•  A significant disadvantage of symmetric ciphers is the key management necessary to use them securely.

•   Each distinct pair of communicating parties must, ideally, share a different key, and perhaps each ciphertext exchanged as well.

•   The number of keys required increases as the square of the number of network members, which very quickly requires complex key management schemes to keep them all consistent and secret. The difficulty of securely establishing a secret key between two communicating parties, when a secure channel does not already exist between them.

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