Table of Contents

Windows Server 2012

Website running on Windows Server 2012 R2.

When testing the website on the SSLLabs https://www.ssllabs.com/ssltest you may receive the message below:

"This server does not support Authenticated encryption (AEAD) cipher suites. Grade capped to B."

SSL Labs

From SSL Labs website:

"Grade will be capped to B, if AEAD suites are not supported. As with forward secrecy, we will not penalize sites if they continue to use non-AEAD suites provided AEAD suites are negotiated with clients that support them."

From <https://blog.qualys.com/ssllabs/2018/02/02/forward-secrecy-authenticated-encryption-and-robot-grading-update>

 

Ciphers available on Windows Server 2012 R2

https://docs.microsoft.com/en-us/windows/desktop/secauthn/tls-cipher-suites-in-windows-8-1

 

The following cipher suites supports AEAD encryption on Windows Server 2012 R2:

TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384_P384

TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256_P256

TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256_P384 

TLS_DHE_RSA_WITH_AES_128_GCM_SHA256

TLS_DHE_RSA_WITH_AES_256_GCM_SHA384

 

The first 3 ciphers listed above are ECDSA ciphers and need an ECDSA certificate with an ECC public key. If you are using a RSA certificate, those ciphers are not used.

 

The other 2 ciphers use DHE. If you enable those 2 ciphers on your webserver, the SSLLabs test will cap your grade to B because the default DHE on Windows uses a 1024 bytes key and it will be marked as "weak".

SSLLabs message: This server supports weak Diffie-Hellman (DH) key exchange parameters. Grade capped to B.

 

To fix it, you can increase the DHE key size to 2048 adding the registry key below:

 

https://docs.microsoft.com/en-us/security-updates/securityadvisories/2016/3174644

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\KeyExchangeAlgorithms\Diffie-Hellman]
"ServerMinKeyBitLength"=dword:00000800

 

Using this setting you will have a AEAD cipher that is not classified as "weak" and SSLLabs will give you an A Grade.

 

 

Windows 2016

Windows Server 2016 has other cipher suites that support AEAD and don't use DHE.

 

Ciphers available on Windows Server 2016

https://docs.microsoft.com/en-us/windows/desktop/secauthn/tls-cipher-suites-in-windows-10-v1607

 

IISCrypto

 

IISCrypto is a good tool to set TLS parameters and modify the ciphers used on Windows

https://www.nartac.com/Products/IISCrypto/Download