https://learn.microsoft.com/ja-jp/dotnet/api/system.net.security.sslstream.beginauthenticateasclient?view=netframework-4.8.1&viewFallbackFrom=net-10.0
SslStream.BeginAuthenticateAsClient Method (System.Net.Security) | Microsoft Learn
Begins an operation to authenticate the client side of a client-server connection.
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