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Logging on with a Smart Card

In standard logons, a user initially proves his or her identity to the KDC by demonstrating knowledge of a secret known only to the user and to the KDC. This shared secret is the cryptographic key derived from the user's password. The same key is used for both encryption and decryption. For this reason, shared secret keys are said to be symmetric. The cryptographic key derived from the user's password is used only during the AS Exchange for the following:

  • When the client encrypts preauthentication data.

  • When the KDC decrypts preauthentication data.

  • When the KDC encrypts the logon session key.

  • When the client decrypts the logon session key.

To support logging on with a smart card, Windows 2000 implements a public key extension to the Kerberos protocol's initial AS Exchange. In contrast to shared secret key cryptography, public key cryptography is asymmetric ; that is, two different keys are needed — one to encrypt, another to decrypt. Together, the keys needed to perform both operations make up a private/public key pair.

When a smart card is used in place of a password, a private/public key pair stored on the user's smart card is substituted for the shared secret key derived from the user's password. The private key is known only to the owner of the pair and is never shared. The public key can be made available to anyone with whom the owner wishes to exchange confidential information.

In the public key extension to the Kerberos protocol, the initial AS Exchange is modified so that the KDC encrypts the user's logon session key with the public half of the user's key pair. The client decrypts the logon session key with the private half of the pair.

The logon process begins when the user inserts a smart card into a card reader attached to the computer. When a computer that is running Windows 2000 is configured for smart card logon, the insertion of the card signals the SAS, just as the key combination CTRL+ALT+DEL signals the SAS on computers configured for password logon. In response, Winlogon dispatches to MSGINA, which displays a logon dialog box. But in this case, the user types just one item of information, a personal identification number (PIN).

MSGINA sends the user's logon information to the LSA by calling LsaLogonUser, just as it does with a password logon. The LSA uses the PIN for access to the smart card, which contains the user's private key along with an X509 v3 certificate that contains the public half of the key pair. All cryptographic operations that use these keys take place on the smart card.

The Kerberos SSP on the client computer sends the user's public key certificate to the KDC as preauthentication data in its initial authentication request, the KRB_AS_REQ message. The KDC validates the certificate, extracts the public key, and then uses the public key to encrypt a logon session key. It returns the encrypted logon session key and a TGT in its KRB_AS_REP reply message to the client. If the client is in possession of the private half of the key pair, it can use the private key to decrypt the logon session key. Both the client and the KDC then use this logon session key in all further communications with one another. The remainder of the authentication process is the same as for standard logon.

For information about the types of smart cards and smart card readers supported by Windows 2000, see the Microsoft Windows Hardware Compatibility List link on the Web Resources page at http://windows.microsoft.com/windows2000/reskit/webresources .

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