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RSA Laboratories

3.1.7 How is the RSA algorithm used for privacy in practice?

In practice, the RSA system is often used together with a secret-key cryptosystem, such as DES (see Section 3.2), to encrypt a message by means of an RSA digital envelope (see Question 2.2.4).

Suppose Alice wishes to send an encrypted message to Bob. She first encrypts the message with DES, using a randomly chosen DES key. Then she looks up Bob's public key and uses it to encrypt the DES key. The DES-encrypted message and the RSA-encrypted DES key together form the RSA digital envelope and are sent to Bob. Upon receiving the digital envelope, Bob decrypts the DES key with his private key, then uses the DES key to decrypt the message itself. This combines the high speed of DES with the key management convenience of the RSA system.

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