How is RSA used for Encryption in Practice?

RSA is combined with a secret-key cryptosystem, such as DES (see Question 64), to encrypt a message by means of an RSA digital envelope.

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 to message itself. This combines the high speed of DES with the key-management convenience of RSA.