Authentication

A trusted user is one whose credentials have been authenticated by a trusted server in Skype for Business Server 2015. This server is one of the Front-End servers in The Voice of O365 enterprise Skype for Business environement. Skype for Business Server relies on Active Directory Domain Services as the single, trusted back-end repository of user credentials. The authentication request reaches the Skype for Business frontend server through a web proxy server in the DMZ network (see Architecture for more information).

Authentication is the provision of user credentials to a trusted server. The Voice of O365 uses the following authentication protocols, depending on the status and location of the user.

• MIT Kerberos version 5 security protocol for internal users with Active Directory credentials. Kerberos requires client connectivity to Active Directory Domain Services, which is why it cannot be used for authenticating clients outside the corporate firewall.
  
  
  

• NTLM protocol for users with Active Directory credentials who are connecting from an endpoint outside the corporate firewall. The Access Edge service passes logon requests to a Front End Server for authentication. The Access Edge service itself performs no authentication.
  
  
  

• Digest protocol for so-called anonymous users. Anonymous users are outside users who do not have recognized Active Directory credentials but who have been invited to an on-premises conference and possess a valid conference key. Digest authentication is not used for other client interactions.

Skype for Business Server 2015 authentication consists of two phases:

1. A security association is established between the client and the server.

2. The client and server use the existing security association to sign messages that they send and to verify the messages they receive. Unauthenticated messages from a client are not accepted when authentication is enabled on the server.

User trust is attached to each message that originates from a user, not to the user identity itself. The server checks each message for valid user credentials. If the user credentials are valid, the message is unchallenged not only by the first server to receive it but by all other servers in the trusted server cloud.

Users with valid credentials issued by a federated partner are trusted but optionally prevented by additional constraints from enjoying the full range of privileges accorded to internal users.