> I'm not a crypto geek, but that isn't my understanding. I think that the challenge is key distribution. The balloon will have been loaded with a database of k

> I'm not a crypto geek, but that isn't my understanding. I think that the challenge is key distribution. The balloon will have been loaded with a database of keys. Perhaps they will have relatively few keys and will just rotate them, or even just use one key per mission. Better would be one key to be used for each message, or one to be used for each hour of the mission - but each key that is used will be discarded/deleted properly in the hardware of the balloon so that even if the USA gets the hardware they won't be able to find the keys for old messages. If asymmetric encryption is used, it doesn't matter much in this context if the USA gets the keys; they can only use them to encrypt new messages, not to decrypt old messages. This applies whether the keys are used directly to encrypt messages, or if the keys are used to encrypt ephemeral symmetric keys that are themselves used to encrypt messages and then discarded.