Diplomacy
A diplomatic bag is a shipping container having diplomatic immunity from search or seizure. It need not be an actual bag.
Cryptography
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. Like Alice and Bob, it is an example of a metasyntactic variable when used this way.
In actual 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 '20s when a cypher machine being shipped to the German Warsaw Embassy — 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 much apology (and with still more protest), on the following Monday, but had been thoroughly inspected by Polish cryptography personnel over the weekend.
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