To all the people commenting about how you can't prove it: in America, proof means "beyond a reasonable doubt". If you really did have random radio telescope data, the prosecution would have to prove beyond a reasonable doubt that your random data is actually encrypted. The proof for the defense would be called an alibi, wherein you would just say where you downloaded it from and what you used it for.

You don't need to prove that it could never be decrypted, you need to provide an alibi and then the prosecution needs to prove beyond a reasonable doubt that your alibi doesn't hold true. Your alibi is your proof.