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 unsecure by modern standards and the fact that the tool has been widely employed in scams as a form of ransomware. Microsoft officially recommended use of BitLocker disk encryption as an alternative.[2][3]