An error has occurred during a TLS handshake.

[NVentimiglia]

WebSocketSharp.WebSocketException: An error has occurred during a TLS handshake.
---> System.IO.IOException: The authentication or decryption has failed.
---> Mono.Security.Protocol.Tls.TlsException: The authentication or decryption has failed.
  at Mono.Security.Protocol.Tls.RecordProtocol.ProcessAlert (AlertLevel alertLevel, AlertDescription alertDesc) [0x00000] in <filename unknown>:0
  at Mono.Security.Protocol.Tls.RecordProtocol.InternalReceiveRecordCallback (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 \r\n  --- End of inner exception stack trace ---\r\n  at Mono.Security.Protocol.Tls.SslStreamBase.AsyncHandshakeCallback (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 \r\n  --- End of inner exception stack trace ---\r\n  at WebSocketSharp.WebSocket.setClientStream () [0x00000] in <filename unknown>:0 \r\n  at WebSocketSharp.WebSocket.doHandshake () [0x00000] in <filename unknown>:0 \r\n  at WebSocketSharp.WebSocket.connect () [0x00000] in <filename unknown>:0
TlsHandshakeFailure

I am receiving this exception when connecting to a TLS server using the Unity platform. Connection works fine if I disable TLS or connect via another platform.

[hultqvist]

These are all speculations.
It sounds like an issue with mono and/or certificates.

If you are running on linux, try mozroots --import --sync, you will find mozroots in the package mono-devel.

If the server you are trying to connect to is only using TLS 1.1 or newer your are out of luck since mono does not yet have them implemented.