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@ -30,7 +30,7 @@ sections:
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For more information about the different authentication flows used by Windows Hello for Business, see [Windows Hello for Business and Authentication](hello-how-it-works-authentication.md).
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- question: What happens after a user registers a PIN during the Windows Hello for Business enrollment process?
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Windows Hello generates a new public-private key pair on the device. The TPM generates and protects this private key; if the device doesn't have a TPM, the private key is encrypted and stored in software. This initial key is referred to as the *protector key*. It's associated only with a single gesture; in other words, if a user registers a PIN, a fingerprint, and a face on the same device, each of those gestures will have a unique protector key. **Each unique gesture generates a unique protector key**. The protector key securely wraps the *authentication key*. The container has only one authentication key, but there can be multiple copies of that key wrapped with different unique protector keys. Windows Hello also generates an administrative key that the user or administrator can use to reset credentials, when necessary (for example, wehn using the PIN reset service). In addition to the protector key, TPM-enabled devices generate a block of data that contains attestations from the TPM.
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Windows Hello generates a new public-private key pair on the device. The TPM generates and protects this private key; if the device doesn't have a TPM, the private key is encrypted and stored in software. This initial key is referred to as the *protector key*. It's associated only with a single gesture; in other words, if a user registers a PIN, a fingerprint, and a face on the same device, each of those gestures will have a unique protector key. **Each unique gesture generates a unique protector key**. The protector key securely wraps the *authentication key*. The container has only one authentication key, but there can be multiple copies of that key wrapped with different unique protector keys. Windows Hello also generates an administrative key that the user or administrator can use to reset credentials, when necessary (for example, when using the PIN reset service). In addition to the protector key, TPM-enabled devices generate a block of data that contains attestations from the TPM.
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At this point, the user has a PIN gesture defined on the device and an associated protector key for that PIN gesture. That means the user is able to securely sign in to the device with the PIN and thus be able to establish a trusted session with the device to add support for a biometric gesture as an alternative for the PIN. When you add a biometric gesture, it follows the same basic sequence: the user authenticates to the system by using the PIN, and then registers the new biometric, after which Windows generates a unique key pair and stores it securely. Future sign-ins can then use either the PIN or the registered biometric gestures.
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- question: What's a container?
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- The IDP can generate the IDP key pair directly, which allows quick, lower-overhead deployment of Windows Hello in environments that don't have or need a PKI.
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- question: How are keys protected?
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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 meansthe 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.
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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.
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- question: How does PIN caching work with Windows Hello for Business?
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Windows Hello for Business provides a PIN caching user experience by using a ticketing system. Rather than caching a PIN, processes cache a ticket they can use to request private key operations. Azure AD and Active Directory sign-in keys are cached under lock. This means the keys remain available for use without prompting, as long as the user is interactively signed-in. Microsoft Account sign-in keys are transactional keys, which means the user is always prompted when accessing the key.
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