There are actually two rather different categories of encryption algorithms. In a previous panel, you saw that it is possible to index encryption and decryption functions with a key. In such a case, we get the equality M = D{k1}(E{k1}(M)). That is, both the encryption and decryption functions use "k1." If this equality holds, the algorithm is a "symmetric."

In 1975, Whitfield Diffie and Martin Hellman proposed a different sort of relationship between encryption and decryption keys. What if we performed encryption and decryption using two different, but related, keys? The consequences turn out to be quite radical. What we get is what is known as "public key" or "asymmetric" algorithms. For reasons discussed in the next panels, we refer to the encryption key as the "public key" and the decryption key as the "private key" in these related key pairs.