This example shows the need for Alice and Bob to have some way to ensure that they are truly each using each other's public keys, rather than the public key of an attacker.
这个例子显示了爱丽丝和鲍伯需要某种方法来确定他们是真正拿到了属于对方的公钥，而不是拿到来自攻击者的公钥。