If you are experiencing the following error with your Remote Desktop Connection,
“An authentication error has occurred. The function requested is not supported
This could be due to CredSSP encryption oracle remediation.”
Cause of the error
The “CredSSP encryption oracle remediation” error relates to a change Microsoft rolled out in their May 2018 Windows Updates. This requires that both the client machine (the machine you are connecting from) and your server – to have the latest updates.
You will need to update your server to the latest Windows updates to resolve this. The steps below will help you connect to your server in order to update it.
Best Workaround:
If you are unable to RDP to your server due to the above error, the quickest solution if possible would be to connect from another machine at your side temporarily (another PC or laptop) that doesn’t yet have the May 2018 Windows Updates yet. That should allow you to RDP into your server and then you can run Windows updates.
Alternate Workaround:
If you don’t have access to another machine at your end, then there is a temporary workaround to change the settings on your local computer to allow it to connect in a less-secure manner (you can revert this change later). Once you make the changes and reboot, you should be able to RDP to your server to run updates.
Below is a step-by-step guide to making the changes referenced in Microsoft’s article above.
You will be making a change in Group Policy and in your Registry.
GROUP POLICY CHANGE
IMPORTANT: Please make sure you are logged into your local computer as an Administrator user.
Step 1:
In the search window type “gpedit.msc”
In the search window type “gpedit.msc”
Step 2: Select “edit group policy”
Step 3: Goto the following path
Computer Configuration > Administrative Templates > System > Credentials Delegation
Select: “Encryption Oracle Remediation”
Click: “policy settings”
Make the following selections in “Encryption Oracle Remediation”
1- Enabled
2- Vulnerable
3- Apply
REGISTRY CHANGE
Step 1:
In the search window type “regedit”
Step 2: Select “regedit”
Step 3: Goto the following path
Registry Path: HKLMSoftwareMicrosoftWindowsCurrentVersionPoliciesSystemCredSSPParameters
Update key: AllowEncryptionOracle
Date type: DWORD
Value data: 2
Reboot your computer so the changes take effect, and you should then be able to RDP to your server.
Content retrieved from: https://support.appliedi.net/kb/a1361/credssp-encryption-oracle-remediation-authentication-error.aspx.