Strong authentication is essential for remote access to on-premises resources over the public Internet. Using the Protected Extensible Authentication Protocol (PEAP) in combination with user certificates issued by the organization’s internal certification authority (CA) provides high assurance for remote user authentication. It includes the added benefit of making the Always On VPN connection completely seamless for the user, as their certificate is presented to the authentication server transparently during VPN connection establishment. Using PEAP with user certificates is the recommended authentication method for Always On VPN deployments.
Reason Code 16
When configuring Always On VPN to use PEAP with client authentication certificates, administrators may encounter a scenario in which a user has a valid certificate. Yet, their authentication request is rejected by the Network Policy Server (NPS) server when attempting to connect remotely. Looking at the Security event log on the NPS server, administrators will find a corresponding event ID 6273 in the Network Policy Server task category from the Microsoft Windows security auditing event source. In the Authentication Details section, you’ll find that the reason code for the failed request is Reason Code 16, with the following reason specified.
“Authentication failed due to a user credentials mismatch. Either the user name provided does not map to an existing user account or the password was incorrect”.
Password Incorrect?
The reason code indicates the user may have entered an incorrect password. However, the user does not enter their password when using PEAP with client authentication certificates, so there’s no chance the password was entered incorrectly.
TPM
I have increasingly encountered this scenario with many customers deploying Always On VPN over the last year or so. This error is often caused by a known issue with older TPM models. Specifically, those with a TPM specification sub-version of 1.16 and earlier. You can view these TPM details by opening the Windows Settings app and entering ‘security processor’ in the search field.
Workaround
These older TPM models seem to have an issue with RSA-PSS signature algorithms, as described here. If possible, administrators should upgrade devices with older TPM versions to ensure the highest level of security and assurance for their remote users. However, in cases where that is not feasible, administrators can remove RSA-PSS signature algorithms from the registry, which forces the use of a different signature algorithm and seems to restore functionality.
To do this, open the registry editor (regedit.exe) and navigate to the following registry key.
HKLM\SYSTEM\CurrentControlSet\Control\Cryptography\
Configuration\Local\SSL\00010003\
Double-click the Functions entry and remove the following algorithms from the Value data section.
- RSAE-PSS/SHA256
- RSAE-PSS/SHA384
- RSAE-PSS/SHA512
Once complete, reboot the device and test authentication once again.
Intune Proactive Remediation
Administrators using Intune Proactive Remediation will find detection and remediation scripts to make these changes published on GitHub.
Additional Information
Windows TPM 2.0 Client Authentication in TLS 1.2 with RSA PSS
Always On VPN NPS Auditing and Logging
The IKEv2 protocol is a popular choice when designing an Always On VPN solution. When 
