为什么输入错误密码会出现延迟？

Question

当输入正确的密码时，应用程序或操作系统（没有特定的应用程序，几乎无处不在）会立即接受这一点，但是当输入错误的密码时，会有 1-2 秒的轻微延迟。这是为什么？

Answer 1

延迟是为了减缓自动化软件的破解尝试。立即破解尝试会运行得更快。

文档：参见man pam_fail_delay

基本原理

   It is often possible to attack an authentication scheme by exploiting
   the time it takes the scheme to deny access to an applicant user. In
   cases of short timeouts, it may prove possible to attempt a brute force
   dictionary attack -- with an automated process, the attacker tries all
   possible passwords to gain access to the system. In other cases, where
   individual failures can take measurable amounts of time (indicating the
   nature of the failure), an attacker can obtain useful information about
   the authentication process. These latter attacks make use of procedural
   delays that constitute a covert channel of useful information.

   To minimize the effectiveness of such attacks, it is desirable to
   introduce a random delay in a failed authentication process. Preferable
   this value should be set by the application or a special PAM module.
   Standard PAM modules should not modify the delay unconditional.