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

HTTPS relies on a family of lower level security protocol implementations called transport level security (TLS), each using different cryptographic algorithms. TLS standards keep developing and improving. At the moment TLS 1.2 is a latest encryption standard powering SSL and TLS 1.3 is in works. In general, anything that is using TLS standard below TLS 1.2 is considered to be non secure because these older encryption algorithms are known to be cracked.

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.

.NET support for TLS

1) .NET 4.6 and above. You don’t need to do any additional work to support TLS 1.2, it’s supported by default.

2) .NET 4.5. TLS 1.2 is supported, but it’s not a default protocol. You need to opt-in to use it. The following code will make TLS 1.2 default, make sure to execute it before making a connection to secured resource:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12

3) .NET 4.0. TLS 1.2 is not supported, but if you have .NET 4.5 (or above) installed on the system then you still can opt in for TLS 1.2 even if your application framework doesn’t support it. The only problem is that SecurityProtocolType in .NET 4.0 doesn’t have an entry for TLS1.2, so we’d have to use a numerical representation of this enum value:
ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;

4) .NET 3.5 or below. TLS 1.2 is not supported (*) and there is no workaround. Upgrade your application to more recent version of the framework.

Another solution: AppContext switches usage in web config file:

    <add key="AppContext.SetSwitch:Switch.System.Net.DontEnableSchUseStrongCrypto" value="false" />
    <add key="AppContext.SetSwitch:Switch.System.Net.DontEnableSystemDefaultTlsVersions" value="false" />

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