Windows 7 / Networking

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Weak Cryptography Removal from PP TP/L2TP

Support for weak or nonstandard cryptographic algorithms has been removed beginning with Windows Vista. This initiative was based on a desire by Microsoft to move customers toward stronger crypto algorithms to increase VPN security, based on recommendations by the NIST and the Internet Engineering Task Force (IETF) as well as mandates toward stronger crypto algorithms from different industry standards bodies and regulators.

The following crypto algorithms are no longer supported on Windows Vista or later versions:

• 40- and 56-bit RC4 encryption, formerly used by the Microsoft Point-to-Point Encryption (MPPE) Protocol for PPTP-based VPN connections
• DES encryption, formerly used by IPsec policy within L2TP/IPsec-based VPN connections
• MD5 integrity checking, formerly used by IPsec policy within L2TP/IPsec-based VPN connections

The removal of support from the default configuration for 40- and 56-bit RC4 encryption means that PPTP-based VPN connections now support only 128-bit RC4 for data encryption and integrity checking. This means the encryption strength remains the same as 128-bit RC4-that is, independent of the encryption settings (Optional Encryption, Require Encryption, or Maximum Strength Encryption) specified by the Advanced Security Settings properties of the VPN connections. This also means that if your existing VPN server does not support 128-bit encryption and supports only incoming PPTP-based VPN connections, clients will not be able to connect. If you are unable to upgrade your existing VPN servers to support 128-bit encryption for PPTP or if 128-bit encryption is unavailable to you because of export restrictions, you can enable weak crypto for PPTP by editing the following registry value:

HKLM\System\CurrentControlSet\Services\Rasman\Parameters\AllowPPTPWeakCrypto

The default value of this DWORD registry value is 0, and by changing it to 1, you can enable 40- and 56-bit RC4 encryption on the computer for both outgoing and incoming PPTP-based VPN connections. You must restart the computer for this registry change to take effect. As an alternative to restarting the computer, you can restart the Remote Access Connection Manager service by opening a command prompt and typing net stop rasman followed by net start rasman.

The removal of support for DES encryption and MD5 integrity checking for L2TP/IPsecbased VPN connections means that L2TP/IPsec-based VPN connections now support the following data encryption and data integrity algorithms by default:

• 128-bit AES, 256-bit AES, and 3DES for data encryption using IPsec
• Secure Hash Algorithm (SHA1) for data integrity using IPsec

The removal of support for DES and MD5 from the default configuration means that L2TP/ IPsec-based VPN connections will not work if your existing VPN server supports only DES for data encryption and/or MD5 for data integrity checking. If you are unable to upgrade your existing VPN servers to support AES or 3DES for data encryption and/or SHA1 for integrity checking or if these crypto algorithms are unavailable to you because of export restrictions, you can disable weak crypto for L2TP by editing the following registry value:

HKLM\System\CurrentControlSet\Services\Rasman\Parameters\AllowL2TPWeakCrypto

The default value of this DWORD registry value is 0, and by changing it to 1, you can enable DES encryption and MD5 integrity checking on the computer for both outgoing and incoming L2TP/IPsec-based VPN connections. You must restart the computer for this registry change to take effect. As an alternative to restarting the computer, you can restart the Remote Access Connection Manager service by opening a command prompt and typing net stop rasman followed by net start rasman.

Note Microsoft recommends that you upgrade your VPN server to support 128-bit RC4 for PP TP and/or AES and SHA1 for L2TP instead of disabling weak crypto support on your VPN clients.

Table summarizes the differences between Windows 7, Windows Vista, and Windows XP with regard to crypto support for data integrity and encryption for VPN connections.

Data Integrity and Encryption Support for VPN Connections in Windows 7, Windows Vista, and Windows XP

Crypto Algorithm	Use	Windows 7	Windows Vista	Windows XP
40-bit RC4	Data encryption and integrity checking for PPTP only			✔;
56-bit RC4	Data encryption and integrity checking for PPTP only			✔
128-bit RC4	Data encryption and integrity checking for PPTP only	✔	✔	✔
DES	Data encryption			✔
3DES	Data encryption	✔	✔	✔
128-bit AES	Data encryption	✔	*
196-bit AES	Data encryption	✔	*
256-bit AES	Data encryption	✔	*
MD5	Integrity checking			✔
SHA1	Integrity checking	✔	✔	✔
256-bit SHA	Integrity checking	✔	*
384-bit SHA	Integrity checking	✔	*

An asterisk (*) means that configuration is possible, but only by using the Netsh command.