SNC is the best solution for this, using an SNC library that has Kerberos support so you can use the credentials of the user at the .net application server (delegated). This is actually very easy, very secure and common. I have helped at least 20 or 30 companies implement exactly this, and never had any issues with it. Also, there is a bonus since the session between the .net server and back-end SAP system can be encrypted, after the authentication is complete.
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