What if the government obtained the keys? That is possible, right? I imagine the goverment is constantly trying to decipher foreign communications.
Keys should be changed frequently (and generally they are). This is not because the key could be cracked - it’s because the key could be leaked, lost, stolen, or what have you. While I imagine governments do indeed try to obtain encryption keys, I can’t see how they would be valuable for very long. If you were to tap some sort of communications link and store all the ciphertext you can for some period of time (not an easy task), you might be able to get a key for that data in the future (perhaps it would be less protected). That data could be of no use - perhaps it was time-sensitive.