There are stories floating around of hackers connecting to old satellites (for fun), so it's possible. NASA could have gotten away with sending a password in p

There are stories floating around of hackers connecting to old satellites (for fun), so it's possible. NASA could have gotten away with sending a password in plaintext (can anyone intercept a focused radio signal?), or DES, but it's possible they didn't use anything. Where it is now, you'd need some serious equipment to transmit to Voyager, and network security is all about cost-benefit analysis; it's not worth the while for any adversary.