Disable ECDH public server param reuse in Windows Server IIS

Windows Server IIS is known for reusing DH key values, but there is a way to disable ECDH public server param reuse in Windows Server IIS and here is how.

Learn how to disable ECDH public server param reuse in IIS and Windows Server by setting the registry value for EphemKeyReuseTime to 0.

ECDH public server param reuse is when a server uses the same DH (Diffie-Hellman) key value for multiple handshakes, instead of generating a new one for every handshake. They should be “ephemeral” though which is why it is called “DHE” or “ECDHE”, and this means the key is single-use and should never be reused. Windows Server IIS is known for reusing DH key values, but there is a way to disable ECDH public server param reuse in Windows Server IIS.

Generating a new NIST P-256 ECDH or X25519 key is cheap, so there is no need to reuse it for performance reasons, but IIS does this for some reason. Disabling ECDH public server param reuse makes sure a key is not cached a new key is generated for every handshake. Reusing keys might get exploited.

ECDH public server param still reused in Windows Server IIS

Disable ECDH public server param reuse in the Windows registry

A less documented registry key EphemKeyReuseTime can be set to 0 which forces a new key to be generated for every connection.

Follow these steps:

open the Registry Editor (regedit
navigate to HKLM\System\CurrentControlSet\Control\SecurityProviders\Schannel\KeyExchangeAlgorithms\ECDH
- if the registry key ECDH doesn’t exist you can create it.
in ECDH click New > DWORD (32-bit) value
- Value name: EphemKeyReuseTime
- Value data: 0

Using PowerShell:

if(!(Test-Path "hklm:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\KeyExchangeAlgorithms\ECDH")) {
  New-Item "hklm:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\KeyExchangeAlgorithms" -name "ECDH"
}

New-ItemProperty "hklm:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\KeyExchangeAlgorithms\ECDH" -Name EphemKeyReuseTime -Value 0 -PropertyType DWord

You can add your own checks and validation in the PowerShell code to make it more robust.

Sources:

Disabled ECDH public server param reuse in Windows Server IIS

Frequently Asked Questions

What is ECDH public server param reuse?

“ECDH public server param reuse” is when a server uses the same DH key value for multiple handshakes, instead of generating a new one for every handshake. The DH should be “ephemeral”, that is why it’s called “DHE” or “ECDHE”, and this means the key is single-use and should never be reused.

Generating a new NIST P-256 ECDH or X25519 key is cheap, so there is no need to reuse it for performance reasons, but some SSL accelerator appliances do that. This is bad and they should be patched/configured to not do that.

(source @ StackExchange)

What is ECDH?

ECDH (Elliptic Curve Diffie-Hellman) is a key agreement protocol that enables two parties to establish a shared secret key over an insecure channel. This shared secret can be used directly as a key or to derive another symmetric key for secure communication. It’s important to note that ECDH itself doesn’t provide authentication; it only establishes a shared secret.

How to disable ECDH key-reuse on IIS

You can disable this via Registry Editor. Just go to HKLM\System\CurrentControlSet\Control\SecurityProviders\Schannel\KeyExchangeAlgorithms\ECDH create a value (DWORD (32-bit)) named EphemKeyReuseTime and set it to `0`.

Conclusion

In this post you learned how to disable ECDH public server param reuse in Windows Server IIS by adding setting a registry value name EphemKeyReuseTime with value 0. Afterwards you can use Qualys SSL Labs to verify whether or not public server param reuse is properly turned off.