TLS and MTLS for Skype for Business Server

Transport Layer Security (TLS) and Mutual Transport Layer Security (MTLS) protocols provide encrypted communications and endpoint authentication on the Internet. Skype for Business Server uses these two protocols to create the network of trusted servers and to ensure that all communications over that network are encrypted. All SIP communications between servers occur over MTLS. SIP communications from client to server occur over TLS.

TLS enables users, through their client software, to authenticate the Skype for Business Server servers to which they connect. On a TLS connection, the client requests a valid certificate from the server. To be valid, the certificate must have been issued by a CA that is also trusted by the client and the DNS name of the server must match the DNS name on the certificate. If the certificate is valid, the client uses the public key in the certificate to encrypt the symmetric encryption keys to be used for the communication, so only the original owner of the certificate can use its private key to decrypt the contents of the communication. The resulting connection is trusted and from that point is not challenged by other trusted servers or clients. Within this context, Secure Sockets Layer (SSL) as used with Web services can be associated as TLS-based.

Server-to-server connections rely on MTLS for mutual authentication. On an MTLS connection, the server originating a message and the server receiving it exchange certificates from a mutually trusted CA. The certificates prove the identity of each server to the other. In Skype for Business Server deployments, certificates issued by the enterprise CA that are during their validity period and not revoked by the issuing CA are automatically considered valid by all internal clients and servers because all members of an Active Directory domain trust the Enterprise CA in that domain. In federated scenarios, the issuing CA must be trusted by both federated partners. Each partner can use a different CA, if desired, so long as that CA is also trusted by the other partner. This trust is most easily accomplished by the Edge Servers having the partner's root CA certificate in their trusted root CAs, or by use of a third-party CA that is trusted by both parties.

TLS and MTLS help prevent both eavesdropping and man-in-the middle attacks. In a man-in-the-middle attack, the attacker reroutes communications between two network entities through the attacker's computer without the knowledge of either party. TLS and Skype for Business Server specification of trusted servers (only those specified in Topology Builder) mitigate the risk of a man-in-the middle attack partially on the application layer by using end-to-end encryption coordinated using the Public Key cryptography between the two endpoints, and an attacker would have to have a valid and trusted certificate with the corresponding private key and issued to the name of the service to which the client is communicating to decrypt the communication. Ultimately, however, you must follow best security practices with your networking infrastructure (in this case corporate DNS). Skype for Business Server assumes that the DNS server is trusted in the same way that domain controllers and global catalogs are trusted, but DNS does provide a level of safeguard against DNS hijack attacks by preventing an attacker's server from responding successfully to a request to the spoofed name.