How to configure cipher suites for STARTTLS?

Question

I configured the available cipher suites for an Exchange 2013 server as https://learn.microsoft.com/en-us/exchange/plan-and-deploy/post-installation-tasks/security-best-practices/exchange-tls-configuration?view=exchserver-2019#windows-server-2012-and-windows-server-2012-r2 by putting them into the appropriate registry key:

HKLM:\SYSTEM\CurrentControlSet\Control\Cryptography\Configuration\Local\SSL\00010002

But when looking at a packet capture of an outgoing SMTP session of that server which used STARTTLS, I observed that it was offering a completely different set of ciphers. Is that registry key not being used for STARTTLS by Exchange Server and if so, where do the ciphers for that have to be configured instead?

andres-bohren · Answer

Hi&nbsp;SAMFS&nbsp;I guess that these depended from what the OS is configured. Have a look herehttps://learn.microsoft.com/en-us/exchange/plan-and-deploy/post-installation-tasks/security-best-practices/exchange-tls-configuration?view=exchserver-2019&nbsp;Kind RegardsAndres

samfs · Answer

Thanks for you quick reply! That is the document I referred to in my post. So, what's in there either does not answer my question or I got it wrong 😅

andres-bohren · Answer

How to you check the Ciphers offered after STARTTLS?

samfs · Answer

By capturing the traffic to and from the server using Wireshark. The Exchange server contacted another SMTP host and after sending the STARTTLS command it sent the following cipher suites as part of the handshake:TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256TLS_ECDHE_RSA_WITH_AES_256_CBC_SHATLS_ECDHE_RSA_WITH_AES_128_CBC_SHATLS_RSA_WITH_AES_256_CBC_SHATLS_RSA_WITH_3DES_EDE_CBC_SHATLS_RSA_WITH_AES_128_CBC_SHATLS_RSA_WITH_RC4_128_SHATLS_RSA_WITH_RC4_128_MD5While the ciphers in the above registry key are as follows:TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P384TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P256TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P384TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384_P384TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384_P256TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256_P384TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256_P256TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384_P384TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384_P256TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256_P384TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256_P256TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P384TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P256TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA_P384TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA_P256TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA_P384TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA_P256TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA_P384TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA_P256TLS_RSA_WITH_AES_256_GCM_SHA384TLS_RSA_WITH_AES_128_GCM_SHA256Which matches the list from the best practice document.

andres-bohren · Answer

Hi SAMFS&nbsp;I've captured the Traffic from an openssl STARTTLS Testopenssl s_client -starttls smtp -connect mail.icewolf.ch:25After STARTTLS and the Server Responding 220The client sends a List of Cipher Suites it supports&nbsp;Then the Server picks oneSimilar to HTTPS&nbsp;Kind RegardsAndres&nbsp;&nbsp;&nbsp;

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How to configure cipher suites for STARTTLS?

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