Topic Last Modified: 2013-11-11

Microsoft Lync Server 2013 uses TLS and MTLS to encrypt instant messages. All server-to-server traffic requires MTLS, regardless of whether the traffic is confined to the internal network or crosses the internal network perimeter. TLS is optional but strongly recommended between the Mediation Server and media gateway. If TLS is configured on this link, MTLS is required. Therefore, the gateway must be configured with a certificate from a CA that is trusted by the Mediation Server.

Requirements for client-to-client traffic depend on whether that traffic crosses the internal corporate firewall. Strictly internal traffic can use either TLS, in which case the instant message is encrypted, or TCP, in which case it is not.

The following table summarizes the protocol requirements for each type of traffic.

Traffic Protection

Traffic type Protected by





Instant messaging and presence

TLS (if configured for TLS)

Audio and video and desktop sharing of media


Desktop sharing (signaling)


Web conferencing


Meeting content download, address book download, distribution group expansion


Media Encryption

Media traffic is encrypted using Secure RTP (SRTP), a profile of Real-Time Transport Protocol (RTP) that provides confidentiality, authentication, and replay attack protection to RTP traffic. SRTP uses a session key generated by the media relay authentication service in response to a successful authentication of the server request (on behalf of the media participants). The session key is secured by the negotiated username and password presented to the media relay authentication service by the Front End Servers, and sent to the participants over the TLS-secured SIP channel. Decrypting the secured session key with the username and password that the media relay service used, and provided in a secured manner by means of the participant’s TLS certificate and the secured SIP channel, allows the participants to decrypt the SRTP stream. In addition, media flowing in both directions between the Mediation Server and its internal next hop is also encrypted using SRTP. Media flowing in both directions between the Mediation Server and a media gateway is not encrypted. The Mediation Server can support encryption to the media gateway, but the gateway must support MTLS and storage of a certificate.

Audio/Video (A/V) is supported with the new version of Windows Live Messenger. If you are implementing A/V federation with Windows Live Messenger, you must also modify the Lync Server encryption level. By default, the encryption level is Required. You must change this setting to Supported by using the Lync Server Management Shell. For more information, see Deploying External User Access in the Deployment documentation.

Audio and video media traffic is not encrypted between Microsoft Lync 2013 and Windows Live clients.


Lync Server 2013 and Microsoft Exchange Server 2013 operate with support for Federal Information Processing Standard (FIPS) 140-2 algorithms if the Windows Server operating systems are configured to use the FIPS 140-2 algorithms for system cryptography. To implement FIPS support, you must configure each server running Lync Server 2013 to support it. For details about the use of FIPS-compliant algorithms and how to implement FIPS support, see Microsoft Knowledge Base article 811833, The effects of enabling the “System cryptography: Use FIPS compliant algorithms for encryption, hashing, and signing" security setting in Windows XP and in later versions of Windows at For details about FIPS 140-2 support and limitations in Exchange 2010, see Exchange 2010 SP1 and Support for FIPS Compliant Algorithms at