Syskey

The SAM Lock Tool, better known as Syskey (the name of its executable file), is a discontinued component of Windows NT that encrypts the Security Account Manager (SAM) database using a 128-bit RC4 encryption key.

[1] Introduced in the Q143475 hotfix for Windows NT 4.0 SP3, the tool was removed in Windows 10's Fall Creators Update in 2017 because its method of cryptography is considered insecure by modern standards and the fact that the tool has been widely employed in scams as a form of ransomware.

[2][3] Introduced in the Q143475 hotfix included in Windows NT 4.0 SP3,[4] Syskey was intended to protect against offline password cracking attacks by preventing the possessor of an unauthorized copy of the SAM file from extracting useful information from it.

[4] Syskey can optionally be configured to require the user to enter the key during boot (as a startup password) or to load the key onto removable storage media (e.g., a floppy disk or USB flash drive).

[4] Syskey is commonly abused by technical support scammers to lock victims out of their own computers in order to coerce them into paying a ransom.