why ctrl-alt-delete to logon is not backwards

[XerXis]

So in the supposedly april fools windows logon screen news I see a lot of comments from people that go like "it must be a joke because it says ctrl-alt-delete to logon". Obviously, those people don't use domains otherwise they would have known that it is a security measure that has always been and will always be a part of windows NT.

But why that key combination? Actually, that's very simple to answer, ctrl-alt-delete is the only key combination that is actually a hardware interupt, way before the win 9x line made it famous to reboot a computer it could already be used to send an interupt which the kernel could afterwards process. This means that software isn't able to send that key combination and fire the interupt. And that is why microsoft uses it before the logon screen in corporate environments as it essentially makes it impossible for trojans to try an automated login.

update: You're never too old to admit you're wrong and learn new stuff, so here it is, I was wrong! And i want to rectify this as I hate false information on the net ;). The ctr-alt-delete is not a special hardware interrupt, even though many site say it is. It is however a software interrupt, which is probably what causes the confusion. Windows handles it in his own way, and the ctr-alt-delete sequence in the login screen has to come from hardware unless group policies tell otherwise. So it's function is the same, prevent trojans from automated login attempts, however it is not because of a hardware interrupt. My apologies for my mistake, I will now kill the tutor who once told me this :p

[Nick H. Supervisor]

[Mippie]

I'm familiar with the logon-procedure, and I knew the reason for it. What really makes me wonder though, is that in Teamviewer (Remote graphical desktop access software) I can send a Ctrl+Alt+Del signal, which then brings up the Taskmanager or the 'Ctrl+Alt+Del Lockscreen' depending on the version of Windows. How is this possible if Ctrl+Alt+Del is a hardware interrupt?

[Bhav]

That's actually quite interesting!

[Berserk87]

So in the supposedly april fools windows logon screen news I see a lot of comments from people that go like "it must be a joke because it says ctrl-alt-delete to logon". Obviously, those people don't use domains otherwise they would have known that it is a security measure that has always been en will always be a part of windows NT.

But why that key combination? Actually, that's very simple to answer, ctrl-alt-delete is the only key combination that is actually a hardware interupt, way before the win 9x line made it famous to reboot a computer it could already be used to send an interupt which the kernel could afterwards process. This means that software isn't able to send that key combination and fire the interupt. And that is why microsoft uses it before the logon screen in corporate environments as it essentially makes it impossible for trojans to try an automated login.

I knew ctrl+alt+del to login was legit, but I never knew why they used that combination (Y).

[argonite]

I'm familiar with the logon-procedure, and I knew the reason for it. What really makes me wonder though, is that in Teamviewer (Remote graphical desktop access software) I can send a Ctrl+Alt+Del signal, which then brings up the Taskmanager or the 'Ctrl+Alt+Del Lockscreen' depending on the version of Windows. How is this possible if Ctrl+Alt+Del is a hardware interrupt?

It's just a normal keyboard hook. However, the only process that is allowed to respond to the keyboard hook is Winlogon (the welcome screen/security console). Any process can send the keystroke, however, they just can't handle it themselves.

[Dan~]

Yup it's like if your in the BIOS and do control alt delete, it'll restart the PC.

[XerXis]

I'm familiar with the logon-procedure, and I knew the reason for it. What really makes me wonder though, is that in Teamviewer (Remote graphical desktop access software) I can send a Ctrl+Alt+Del signal, which then brings up the Taskmanager or the 'Ctrl+Alt+Del Lockscreen' depending on the version of Windows. How is this possible if Ctrl+Alt+Del is a hardware interrupt?

http://www.simple-help.com/simplehelp/support/knowledgebase?fname=Sending_Ctrl_Alt_Del_to_remote_computer

that's a better explanation of how it works on remote login software :)

so yeah: i should have said, most software isn't able to send that key combination to login :p. Your other question, teamviewer afaik essentially just sends a software signal to the host that opens the task manager, it's not a real ctrl-alt-delete (but this is just guessing from my part I admit ;))

[Descartes]

So is that why even when I'm viewing a full screen application, which is hung, and doesn't respond to anything (incl. ctrl+shift+esc, alt+tab etc), when i do ctrl+alt+del it fires up instantly and takes over the focus? :p

[Detection]

Actually that is pretty interesting to know, and I guess its why its always the last key combination we know will work if all else fails (As long as the system has not crashed)

[what]

That's pretty cool. My university uses CTRL + ALT + Del for the login menu, I never knew the reason for it though (or even that there was a reason).

[BeerFan]

And why is this in the Microsoft Beta forum?

[what]

And why is this in the Microsoft Beta forum?

Because it explains the screenshot of the Windows 8 lock screen posted on the home page yesterday. Problem?

[MrA]

So in the supposedly april fools windows logon screen news I see a lot of comments from people that go like "it must be a joke because it says ctrl-alt-delete to logon". Obviously, those people don't use domains otherwise they would have known that it is a security measure that has always been and will always be a part of windows NT.

But why that key combination? Actually, that's very simple to answer, ctrl-alt-delete is the only key combination that is actually a hardware interupt, way before the win 9x line made it famous to reboot a computer it could already be used to send an interupt which the kernel could afterwards process. This means that software isn't able to send that key combination and fire the interupt. And that is why microsoft uses it before the logon screen in corporate environments as it essentially makes it impossible for trojans to try an automated login.

And of course, this is completely wrong. There is no special interrupt for ctrl-alt-del. In real-mode, or using the legacy 8259, there are 16 IRQ lines (15 usable because there are two "chips" and one line cascades to the other "chip"). One of which (IRQ 1) is the keyboard. The only special handling is that the combination is handled by the BIOS. Of course, if you intercept the interrupt yourself by modifying the IVT (or IDT in protected mode remembering to remap the IRQs), you can do what you like, just like any other key combination.

EDIT: And just to reinforce that there's nothing special about ctrl-alt-del, different OSs can use different key combinations to accomplish the same thing. Linux, for example, uses alt-sysrq-k as its "secure attention key".

[+Nik Louch Subscriber²]

ctrl-alt-delete is the only key combination that is actually a hardware interupt

Show me your source, because this is not even remotely true!

[Eric Veteran]

All keypresses generate hardware interrupts, CTRL-ALT-DELETE happens to generate a specific one. It was chosen because it was difficult to hit the keys accidentally and restart the computer.

NT used it because it because it was a unique interrupt that normally restarted the computer. That way if you got a login prompt even after hitting CAD you could be reasonably sure it was the OS's prompt. :)

[BeerFan]

Because it explains the screenshot of the Windows 8 lock screen posted on the home page yesterday. Problem?

No problem, just curious, because i didn't see the connection. Problem?

I'm a little surprised that anyone with a Windows background would be unfamiliar with the good ol' 3-fingered salute.

[KoL Veteran]

I don't know if it works this way but I guess you got that screen with CTRL+ALT+DEL. You have to press those keys to go to another window and enter your password to login. And if there are more users you have to click one of the user then you have to type your password? If we talk about UX that is backwards to me. You should design to simplified the experience. Less steps the better IMO. That's how I design things but maybe I'm wrong.

[BajiRav]

No problem, just curious, because i didn't see the connection. Problem?

I'm a little surprised that anyone with a Windows background would be unfamiliar with the good ol' 3-fingered salute.

I think confusion was that most users who never saw the Ctrl + Alt + Del to login dialog thought the Win 8 screenshot was fake because it said so.

fun fact: if you do C+A+D twice on XP style login screen, you get classic login screen (doesn't work on Vista/7 where classic login was removed).

[Subject Delta]

So in the supposedly april fools windows logon screen news I see a lot of comments from people that go like "it must be a joke because it says ctrl-alt-delete to logon". Obviously, those people don't use domains otherwise they would have known that it is a security measure that has always been and will always be a part of windows NT.

Since NT4, however, it has only been enabled by default on server versions of Windows.

[XerXis]

Since NT4, however, it has only been enabled by default on server versions of Windows.

or when you join a domain with that group policy

[PGHammer]

No problem, just curious, because i didn't see the connection. Problem?

I'm a little surprised that anyone with a Windows background would be unfamiliar with the good ol' 3-fingered salute.

Mostly because most folks weren't familiar with NT until 2000 Professional (if not XP) if they didn't use computers in a business/government setting.

I was involved with NT almost from the beginning (shortly after NT 3.1 went RTM, in fact) out of sheer curiousity (for one thing, if you didn't run DOS, you had three choices at that time - NT, OS/2, or UNIX); NT had the advantage of all those Windows 3.x applications, but the disadvantage of taller system requirements, OS/2 ran DOS applications better than NT, but had issues with Windows applications. UNIX? By and large taller system requirements than even NT, yet handled DOS applications worse than not just OS/2, but NT as well. Windows applications? No way.

Shortly after I built my first exclusively-Windows (no DOS) box (P166 MMX with 32 MB of RAM, and running 95 SR2), I converted the old 95 beta box (AMD386-40 with 16 MB of RAM and a SIIG ISA Master+I/O add-in card) to an NT box - and I did it to settle a bet. (I was told that NT 3.51 Workstation would not run on a 386, which I rather emphatically disagreed with - and proved it with the AMD NT box.)

[PGHammer]

or when you join a domain with that group policy

Or when installing NT4WS with the default settings (which mirrored those of NT 3.51 Workstation, which it would replace).

Actually, if you tell *any* NT-based Windows (until XP) that a password is required to log on, the three-finger salute is a requirement (that includes both NT 4 Workstation and Windows 2000 Professional). It would take XP to banish the three-finger salute (instead, it brings up Task Manager).

[Mr. Dee]

You do not need to be on a Domain to use secure log on. CTRL-ALT-DEL helps prevent malicious applications from stealing account information and rude persons from trying to guess your password and log into your account.

For instance, if you are running Windows 7 Professional, Enterprise or Ultimate, you can open gpedit.msc and set a threshold for how many wrong tries until you successfully remember your password to log in. So you could set a thresh hold of 2 for the amount of mistakes before secure log on kicks in. If you make a mistake after 2 tries, it prevents log on for a certain period. So, if you go to use the bathroom and come back in time, you can catch the person you thought was your best friend trying log into your computer without your permission, then you can argue and say, I thought you were my best friend, I trusted you, how could do this to me? Leave, NOW!

[PGHammer]

You do not need to be on a Domain to use secure log on. CTRL-ALT-DEL helps prevent malicious applications from stealing account information and rude persons from trying to guess your password and log into your account.

For instance, if you are running Windows 7 Professional, Enterprise or Ultimate, you can open gpedit.msc and set a threshold for how many wrong tries until you successfully remember your password to log in. So you could set a thresh hold of 2 for the amount of mistakes before secure log on kicks in. If you make a mistake after 2 tries, it prevents log on for a certain period. So, if you go to use the bathroom and come back in time, you can catch the person you thought was your best friend trying log into your computer without your permission, then you can argue and say, I thought you were my best friend, I trusted you, how could do this to me? Leave, NOW!

You can do that in any version of Windows (XP or later) via a custom install, or via modification of the Users settings in Control Panel post-install.