Hush-hush and Iron curtain

Hush-hush adjective - Not known or meant to be known by the general populace.

Iron curtain and hush-hush are semantically related In some cases you can use "Iron curtain" instead a noun phrase "Hush-hush".

Iron curtain noun - An impenetrable barrier to communication or information especially as imposed by rigid censorship and secrecy; used by Winston Churchill in 1946 to describe the demarcation between democratic and communist countries.

Hush-hush and iron curtain are semantically related in speech topic. Sometimes you can use "Hush-hush" instead a noun phrase "Iron curtain", if it concerns topics such as silence.