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3.2.2 Why use RSA rather than DES?

RSA is not an alternative or replacement for DES; rather it supplements DES (or any other fast bulk encryption cipher) and is used together with DES in a secure communications environment. (Note: for an explanation of DES, see Question 3.5.1)

RSA allows two important functions not provided by DES: secure key exchange without prior exchange of secrets, and digital signatures. For encrypting messages, RSA and DES are usually combined as follows: first the message is encrypted with a random DES key, and then, before being sent over an insecure communications channel, the DES key is encrypted with RSA. Together, the DES-encrypted message and the RSA-encrypted DES key are sent. This protocol is known as an RSA digital envelope.

One may wonder, why not just use RSA to encrypt the whole message and not use DES at all? Although this may be fine for small messages, DES (or another cipher) is preferable for larger messages because it is much faster than RSA (see Question 3.2.3).

In some situations, RSA is not necessary and DES alone is sufficient. This includes multi-user environments where secure DES-key agreement can take place, for example by the two parties meeting in private. Also, RSA is usually not necessary in a single-user environment; for example, if you want to keep your personal files encrypted, just do so with DES using, say, your personal password as the DES key. RSA, and public-key cryptography in general, is best suited for a multi-user environment. Also, any system in which digital signatures are desired needs RSA or some other public-key system.


next up previous
Next: 3.2.3 How fast is Up: 3.2 RSA Previous: 3.2.1 What is RSA?
Denis Arnaud
12/19/1997