key assignment的用法和样例：
例句
- Users can make their public keys freely available or place them at a key distribution centre for others to access.
用户可以让公开密钥自由使用或者把公开密钥放在可由别人存取的密钥分配中心。 - The private key can guarantee the security of communication, while the distribution is still a problem.
私钥系统虽然安全，但是密钥分配的安全性在经典系统中一直难以得到保证。
