Anytime key material is generated, it must be protected against attack. The most robust way to do this is through specialized hardware. There's a long history of using hardware security modules (HSMs) to generate, store, and process keys for security-critical applications. Smart cards are a special type of HSM, as are devices that are compliant with the Trusted Computing Group TPM standard. Wherever possible, the Windows Hello for Business implementation takes advantage of onboard TPM hardware to generate and protect keys. Administrators can choose to allow key operations in software, but it's recommended the use of TPM hardware. The TPM protects against a variety of known and potential attacks, including PIN brute-force attacks. The TPM provides an additional layer of protection after an account lockout, too. When the TPM has locked the key material, the user will have to reset the PIN (which means the user will have to use MFA to reauthenticate to the IDP before the IDP allows re-registration). Resetting the PIN means that all keys and certificates encrypted with the old key material will be removed.