Microsoft KB Archive/325158

= Default Encryption Settings for the Microsoft L2TP/IPSec Virtual Private Network Client =

Article ID: 325158

Article Last Modified on 3/1/2007

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APPLIES TO


 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows Millennium Edition
 * Microsoft Windows 98 Second Edition
 * Microsoft Windows 98 Standard Edition

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This article was previously published under Q325158



SUMMARY
The following list contains the default encryption settings for the Microsoft L2TP/IPSec virtual private network (VPN) client for earlier version clients:
 * Data Encryption Standard
 * Secure Hash Algorithm
 * Diffie-hellman Medium
 * Transport Mode
 * Encapsulating Security Payload

The client does not support the following settings:
 * Tunnel mode
 * AH (Authentication Header)

These values are hard-coded in the client and you cannot change them.



Data Encryption Standard
Data Encryption Standard (3DES) provides confidentiality. 3DES is the most secure of the DES combinations, and has a bit slower performance. 3DES processes each block three times, using a unique key each time.

Secure Hash Algorithm
Secure Hash Algorithm 1(SHA1), with a 160-bit key, provides data integrity.

Diffie-Hellman Medium
Diffie-Hellman groups determine the length of the base prime numbers that are used during the key exchange. The strength of any key derived depends in part on the strength of the Diffie-Hellman group on which the prime numbers are based.

Group 2 (medium) is stronger than Group 1 (low). Group 1 provides 768 bits of keying material, and Group 2 provides 1,024 bits. If mismatched groups are specified on each peer, negotiation does not succeed. You cannot switch the group during the negotiation.

A larger group results in more entropy and therefore a key that is harder to break.

Transport Mode
There are two modes of operation for IPSec:


 * Transport mode - In transport mode, only the payload of the message is encrypted.
 * Tunnel mode (not supported) - In tunnel mode, the payload, the header, and the routing information are all encrypted.

Encapsulating Security Payload
Encapsulating Security Payload (ESP) provides confidentiality, authentication, integrity, and anti-replay. ESP does not ordinarily sign the whole packet unless the packet is being tunneled. Ordinarily, only the data is protected, not the IP header. ESP does not provide integrity for the IP header (addressing).

Authentication Header (Not Supported)
Authentication Header (AH) provides authentication, integrity, and anti-replay for the whole packet (both the IP header and the data carried in the packet). AH signs the whole packet. It does not encrypt the data, so it does not provide confidentiality. You can read the data, but you cannot modify it. AH uses HMAC algorithms to sign the packet.

