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Preshared key authentication

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Preshared key authentication

IPSec can use preshared keys for authentication. Preshared means that the parties agree on a shared, secret key that is used for authentication in an IPSec policy.

During security negotiation, information is encrypted before transmission by using a session key. The session key is created by using a Diffie-Hellman calculation and the shared, secret key. Information is decrypted on the receiving end using the same key. One IPSec peer authenticates the other peer's packet by decryption and verification of the hash inside the packet (the hash inside the packet is a hash of the preshared key). If authentication fails, the packet is discarded.

The use of preshared key authentication is not recommended because it is a relatively weak authentication method. Preshared key authentication creates a master key that is less secure (that might produce a weaker form of encryption) than certificates or the Kerberos V5 protocol. In addition, preshared keys are stored in plaintext in the registry. In Active Directory, preshared keys are stored in readable hexadecimal format.

Preshared key authentication is provided for interoperability purposes and to adhere to IPSec standards. It is recommended that you use preshared keys only for testing and that you use certificates or Kerberos V5 instead in a production environment. For more information about IPSec authentication methods, see Authentication methods and Public key certificate-based authentication.

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