Pre-boot authentication

Pre-Boot Authentication (PBA) or Power-On Authentication (POA) serves as an extension of the BIOS or boot firmware and guarantees a secure, tamper-proof environment external to the operating system as a trusted authentication layer. The PBA prevents anything being read from the hard disk such as the operating system until the user has confirmed they have the correct password or other credentials.

A PBA environment serves as an extension of the BIOS or boot firmware and guarantees a secure, tamper-proof environment external to the operating system as a trusted authentication layer. The PBA prevents Windows or any other operating system from loading until the user has confirmed he/she has the correct password to unlock the computer. That trusted layer eliminates the possibility that one of the millions of lines of OS code can compromise the privacy of personal or company data.

Pre-Boot Authentication is generally provided by a variety of full disk encryption vendors, but can be installed separately. Legacy FDE systems tended to rely upon PBA as their primary control. These systems have been replaced with hardware based dual factor systems like TPM chips. However, without some form of authentication, encryption provides little protection. Generally this authentication comes from Active Directory authentication at the GINA step of Windows.

PBA is easily defeated with Evil Maid style of attacks. However, with modern hardware (including TPM) most FDE solutions are able to ensure removal of hardware for brute force attacks are no longer possible.

The standard complement of authentication methods exist for Pre-Boot Authentication including:

...
Wikipedia