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I've been reading Chapter 15 of Operating System Concepts 9th Edition. I am stuck understanding this part.

A variation of the Trojan horse is a program that emulates a login program. An unsuspecting user starts to log in at a terminal and notices that he has apparently mistyped his password. He tries again and is successful. What has happened is that his authentication key and password have been stolen by the login emulator, which was left running on the terminal by the thief. The emulator stored away the password, printed out a login error message, and exited; the user was then provided with a genuine login prompt. This type of attack can be defeated by having the operating system print a usage message at the end of an interactive session or by a nontrappable key sequence such as the control-alt-delete combination used by all modern Windows operating systems.

What I don't get is the last part.

This type of attack can be defeated by having the operating system print a usage message at the end of an interactive session or by a nontrappable key sequence such as the control-alt-delete combination used by all modern Windows operating systems.

How can having the OS print a usage message or "by a non-trappable key sequence" prevent a such a trojan horse attack?

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