It is alleged that the RQ-170 was expressly designed for risky operations (where loosing it eventually is likely, and retrieval/destruction may not be possible) and so was created without sensitive technology.

The crypto for US military GPS uses the same keys worldwide, and rotated ~ weekly. That means if you capture any device which has the crypto keys, then you can spoof military GPS for the rest of the week, allowing you to do lots more damage.

Therefore, the military often doesn't even use the keys in their own devices.

How do you know this?

It's in the published specs of the satellites. They have a whole key schedule bit.

IIRC there was speculation Russia helped to decrypt it?