> It's surprising that we didn't invent any kind of physical padlock that accepts two keys, with the lock mechanism such that, when locked with one of the two k

> It's surprising that we didn't invent any kind of physical padlock that accepts two keys, with the lock mechanism such that, when locked with one of the two keys, can only be unlocked by the other key. Key exchange blocks are basically this, except one of the two keys is perpetually locked inside the keyblock, and you must use the other to retrieve it (whereupon the key you just used is now locked in the block). So not quite the same thing, but the closest example I know of.