Yeah, but that doesn't help if you have the device which decrypts the signal. Then you should be able to get the key (which would probably still be fairly difficult).
A receiver/decrypter only contains the public key, which can't be used to encrypt a new message. Perhaps you aren't familiar with asymmetric / public key cryptography?
And that is one reason the US military spends so much money and effort on anti-tamper[1] design and technology.