Columbia subject headings:
Communication in politics--Europe--History--Congresses.
Cryptography--Europe--History--Congresses.
Cryptography--Political aspects--Europe--History--Congresses.
Diplomatic documents--History--Congresses.
Europe--Foreign relations--Sources--Congresses.
Harvard subject headings:
Communication in politics--Europe--History--Congresses.
Cryptography--Europe--History--Congresses.
Cryptography--Political aspects--Europe--History--Congresses.
Diplomatic documents--History--Congresses.
Europe--Foreign relations--Sources--Congresses.
Princeton subject headings: