TLS enables users, through their client software, to authenticate the Lync Server 2010 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.

How many bits has got this key ??   In SRTP, I think Lync AES-128...

Lync use 128-BIT AES encryption

I supposed that TLS/MTLS use AES-128 too, not ??