Enhancing SSL Security

The default configuration of most operating systems allow any set of supported ciphers and hashes to be used by applications when acting as SSL client or server. While this ensures full compatibility with other client and server applications, it does no longer match the expectation in SSL encrypted communication in regards to privacy and trust due to supporting insecure protocols, cipher suites and hash algorithms.

Therefore enhancing the security of SSL mainly consists of disabling these insecure protocols, ciphers and hashes as well as prioritize cipher suites that allow the usage of Perfect Forward Secrecy.

As MailStore Server relies on Windows' security support provider (SSP) called Secure Channel (also known as Schannel), a number of registry keys have to be created or modified in order to disable insecure protocols, ciphers and hashes. Although Microsoft's Technet article How to restrict the use of certain cryptographic algorithms and protocols in Schannel.dll describes in detail which registry keys affect the security provider settings, it is not recommended to manually change these keys. A safer way to adjust the Schannel settings for server applications is Nartac Software's IIS Crypto tool.

Important note: Modifying the configuration of the security support provider (SSP) in Windows may affect general operating system functions such as authentication services and remote desktop and management capabilities or other third party applications that rely on SSP. Thus careful testing of all services is required after applying the changes.

Recommended Settings

Highest level of security can be achieved with the following settings in IIS Crypto on the MailStore Server computer.

Protocols Enabled	TLS 1.1 TLS 1.2
Ciphers Enabled	AES 128/128 AES 256/256
Hashes Enabled	SHA SHA256 SHA384 SHA512
Key Exchange Enabled	Diffie-Hellman PKCS ECDH
SSL Cipher Suite Order	TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA