Diffie-Hellman 理解

 In [DH76] Diffie and Hellman describe a means for two parties to
   agree upon a shared secret in such a way that the secret will be
   unavailable to eavesdroppers. This secret may then be converted into
   cryptographic keying material for other (symmetric) algorithms.  A
   large number of minor variants of this process exist. This document
   describes one such variant, based on the ANSI X9.42 specification.
from rfc2631

Diffie-Hellman是一种秘钥交换算法，它是一种创建秘钥的方法，而不是加密方法，所以秘钥必须和其他一种加密算法结合使用。这种秘钥交换技术的目的在于使两个用户安全的交换一个秘钥一遍后面的报文加密。