Salesforce Authentication Failing

Question

I am trying to use OAuth authentication to get the Salesforce Authentication Token, so I referred wiki docs, but after getting authorization code, when I make a Post request

Answer 1

Salesforce is requiring an upgrade to TLS 1.1 or higher by July 22, 2017 in order to align with industry best practices for security and data integrity: from help.salesforce.com.

try to add this code:

System.Net.ServicePointManager.SecurityProtocol = 
SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

Another option is to edit your registry:

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001

Check this link for more detailed answers: Default SecurityProtocol in .NET 4.5