The anonymous key exchange offers encryption without any indication of the peer’s identity. This kind of authentication is vulnerable to a man in the middle attack, but can be used even if there is no prior communication or shared trusted parties with the peer. It is useful to establish a session over which certificate authentication will occur in order to hide the indentities of the participants from passive eavesdroppers.
Unless in the above case, it is not recommended to use anonymous authentication. In the cases where there is no prior communication with the peers, an alternative with better properties, such as key continuity, is trust on first use (see Verifying a certificate using trust on first use authentication).
The available key exchange algorithms for anonymous authentication are shown below, but note that few public servers support them, and they have to be explicitly enabled.
This algorithm exchanges Diffie-Hellman parameters.
This algorithm exchanges elliptic curve Diffie-Hellman parameters. It is more efficient than ANON_DH on equivalent security levels.