The Client and Server Cannot Communicate, Because They Do Not Possess a Common Algorithm - ASP.NET C# Iis Tls 1.0/1.1/1.2 - Win32Exception

The client and server cannot communicate, because they do not possess a common algorithm - ASP.NET C# IIS TLS 1.0 / 1.1 / 1.2 - Win32Exception

There are several other posts about this now and they all point to enabling TLS 1.2. Anything less is unsafe.

You can do this in .NET 3.5 with a patch.

You can do this in .NET 4.0 and 4.5 with a single line of code

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; // .NET 4.5
ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072; // .NET 4.0

In .NET 4.6, it automatically uses TLS 1.2.

See here for more details:
.NET support for TLS.

SoapHttpClientProtocol and TLS 1.2 - The client and server cannot communicate, because they do not possess a common algorithm

If your client application was compiled against .NET Framework 4.5.2 or lower, then by default ServicePointManager.SecurityProtocol is initialized to SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls (SSL 3.0 and TLS 1.0 only), so it won't be able to connect to a remote server that requires TLS 1.2.

There are several ways to allow your client application to use TLS 1.2:

• Recompile your client application against .NET Framework 4.6 or later. (In Visual Studio, open your project's property pages, go to the Application tab, and change the Target Framework.)
• On the client machine, run RegEdit.exe, go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ .NETFramework\v4.0.30319, add a DWORD (32-bit) value named SchUseStrongCrypto, and set it to 1. (This flag causes ServicePointManager.SecurityProtocol to be initialized to Tls | Tls11 | Tls12.)
• When your client application starts up, turn on TLS 1.2: ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls12;

There's no need to regenerate your proxy class because it's not responsible for negotiating the TLS protocol or cipher.

The client and server cannot communicate, because they do not possess a common algorithm on Windows Server 2008 Web

The problem was the Operating system. We were using Windows Server 2008 and we didn't realize the application need OS's protocol to communicate with other server. Since we have .NET Framework 4.5 installed and we were also using the code ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; to force application to use Tls1.2 (according to the requirement), hence believed that everything should work fine, but obviously this wasn't going to happen.

tl;dr; We installed Windows Server 2012 on the machine and the application is running fine now (as it should)

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