**Cryptography**

**Contents**

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 1976.

· The modern study of symmetric-key ciphers relates mainly to the study of block and stream cipher and to their applications. A block cipher is, in a sense, a modern embodiment of Alberti's polyalphabetic cipher: block ciphers take as input a block of plaintext and a key, and output a block of ciphertext of the same size. Since messages are almost always longer than a single block, some method of knitting together successive blocks is required. Several have been developed, some with better security in one aspect or another than others. They are the mode of operation and must be carefully considered when using a block cipher in a cryptosystem.

· In Symmetric key cryptography the same key is used by the sender and the receiver.

· In Symmetric key cryptography the same key is used in both direction.

· Symmetric key cryptography is often used for long message.