Diplomatic communications

A diplomatic bag, also known as a diplomatic pouch, is a container with certain legal protections used for carrying official correspondence or other items between a diplomatic mission and its home government or other diplomatic, consular, or otherwise official entities. The physical concept of a "diplomatic bag" is flexible and therefore can take many forms (e.g., a cardboard box, briefcase, duffel bag, large suitcase, crate or even a shipping container). Additionally, a diplomatic bag usually has some form of lock and/or tamper-evident seal attached to it in order to deter interference by unauthorized third parties. The most important point is that as long as it is externally marked to show its status, the "bag" has diplomatic immunity from search or seizure, as codified in article 27 of the 1961 Vienna Convention on Diplomatic Relations. It may only contain articles intended for official use. It is often escorted by a diplomatic courier, who is similarly immune from arrest and detention.

In discussions of cryptography, the diplomatic bag is conventionally used as an example of the ultimate secure channel used to exchange keys, codebooks, and other necessarily secret materials.

In contemporary practice, diplomatic bags are indeed used for exactly this purpose. An illustration is the strenuous protest made by German diplomats in Poland in the late 1920s when a cypher machine being shipped to the German Embassy in Warsaw – a commercial version of the famous Enigma machine – was mistakenly not marked as protected baggage and was opened, under protest, by Polish Customs. It was released to them, supposedly without many apologies (and with still more protest), but had perhaps been subjected to inspection by Polish cryptography personnel.

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