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Encryption - an explanation

Encryption is the transformation of data into a form unreadable by anyone without a secret decryption key. Its purpose is to ensure privacy by keeping the information hidden from anyone for whom it is not intended, even those who can see the encrypted data. For example, one may wish to encrypt files on a hard disk to prevent an intruder from reading them.In a multi-user setting, encryption allows secure communication over an insecure channel, such as the Internet. Traditional, or "secret-key", cryptography is based on the sender and receiver of a message knowing and using the same secret key: the sender uses the secret key to encrypt the message, and the receiver uses the same secret key to decrypt the message. The main problem is getting the sender and receiver to agree on the secret key without anyone else finding out.

With public-key cryptography, however, each person gets a pair of keys. These are known as the public key and the private key, and each person's public key is published while the private key is kept secret. The need for sender and receiver to share secret information is eliminated, since all communications involve only public keys, and no private key is ever transmitted or shared.

The primary advantage of public-key cryptography is increased security, since the private keys do not ever need to transmitted or revealed to anyone. In a secret-key system, by contrast, there is always a chance that a hacker could discover the secret key while it is being transmitted.

In practice, however, a public-key system such as RSA is combined with a secret-key cryptosystem, such as DES, to encrypt a message by means of an RSA digital envelope.