Microsoft Security Advisory 3174644
Updated Support for Diffie-Hellman Key Exchange
Published: September 13, 2016
Microsoft is providing updated support to enable administrators to configure longer Diffie-Hellman ephemeral (DHE) key shares for TLS servers. The updated support allows administrators to increase the size of a DH modulus from the current default of 1024 to either 2048, 3072, or 4096.
Note: All versions of Windows 10 support the new DH modulus settings and use 2048 as the DH modulus default setting.
What is the scope of the advisory?
The purpose of this advisory is to inform customers that Microsoft is providing updated support to enable administrators to configure longer Diffie-Hellman ephemeral (DHE) key shares for TLS servers.
What does the updated support for DHE key shares provide?
The current size modulus in the DHE key exchange implementation is 1024 bit. This updated support enables administrators to configure a modulus size of 2048, 3072, or 4096.
Is this a security vulnerability that requires Microsoft to issue a security update?
No. However, enabling administrators to configure longer DHE key shares can increase security for the TLS servers that they manage.
Why is Microsoft enabling administrators to configure longer DHE key shares for TLS servers?
Enabling administrators to configure longer DHE key shares for TLS servers will facilitate implementing groups 14, 15 and 16 (RFC3526) that correspond to 2048, 3072 and 4096 as the default minimum security standard on TLS servers.
Administrators can change the size of the modulus by adding the registry key value in the following procedure. If the key value is absent, the default of the modulus remains 1024 bit. The following example sets the modulus size to 2048 bit. Valid key values are decimal: 1024, 2048, 3072 and 4096.
To change the default size of the modulus:
Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
- Open Registry Editor.
Access the following registry location:
Update the following DWORD value to:
Additional Suggested Actions
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- V1.0 (September 13, 2016): Advisory published.
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