Azure Active Directory - Service to
Service calls using Client Credentials
The Trusted Application APIs only accept an
AAD oauth token issued to an application
identity. In order that your application
gets such a token from AAD, you need to either upload your application's
credentials ( a certificate ) to AAD, and use that
certificate whenever you need an oauth token, or use
a client secret.
More
info here:
https://msdn.microsoft.com/en-us/library/azure/dn645543.aspx
https://azure.microsoft.com/en-us/documentation/articles/active-directory-authentication-scenarios
https://github.com/Azure-Samples/active-directory-dotnet-daemon-certificate-credential
https://msdn.microsoft.com/en-us/office/office365/howto/building-service-apps-in-office-365