Symmetric key cryptography

Traditionally, all cryptography was symmetric key cryptography. In a symmetric key cryptosystem, the encryption key Ke and the decryption key Kd are the same, denoted simply by K. The key K must be kept secret, and it is also important that an eavesdropper who sees repeated encryptions using the same key can not learn the key. The simple substitution cipher described earlier is an example of a symmetric key cryptosystem. 

Symmetric key cryptography uses a polyalpha-betic encryption algorithm, such as a block cipher, a stream cipher, or a hash function to convert the data into binary code. A block cipher apphes the same key to each block of data. Stream ciphers encrypt the data bit by bit. They can operate in several ways, but two common methods are self-synchronizing and synchronous. The self-synchronizing cipher encrypts the data bit by bit, using an algebraic function applied to the previous bit. Synchronous ciphers apply a succession of functions independent of the data. 

Often, Ke = that is, the encryption and decryption keys are identical, and in this case we refer to it simply as the key or the secret key. This is called symmetric or secret key cryptography. In contrast, in asymmetric or public key cryptography, the encryption keys and decryption keys are different from each other, and only the decryption key needs to be kept secret. In public key cryptography, the decryption key is also sometimes called the secret key. 

Cryptography can be divided into two groups symmetric and asyrrrmetric encryption. In symmetric encryption the sender and receiver use the same key. This method is very fast but the sender needs to send not only the message but also the key, otherwise the receiving party carmot read the message. 

Finally, the defensive measures that are proposed to deal with the presented attacks and that are suitable for the unique characteristics of a WSN (either in the form of a WBAN or a WPAN) include key estabhshment and encryption. Especially, symmetric cryptography with the use of shared keys is preferred over techniques that include use of both private and public keys, because of the greater energy consumption needed by the latter. 

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