How important is secure communication in space?
Science-related research is often made public, but does that mean all communications must be unprotected?
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This question is not actually related to space, except insofar as comms in space could be picked up from anywhere on Earth (as opposed to surface comms which may be limited to line of sight or short range)
The real question to ask is: what type of comms do you need to protect, either through encryption to keep them private, or through authentication to ensure the right comms are being acted upon.
For control of craft, strong security is useful, to ensure the craft is not hijacked in any way, and for satellites with military applications, this is also true. But it costs, in terms of bandwidth and processing power, so especially for low bandwidth comms you are right that for some craft (e.g. Cassini), sending their signal home in the clear makes a lot of sense, as you don't need to protect that data.
Have a read of this question on what you would need to hijack a satellite over on Security Stack Exchange.
It isn't as important as you would think, unless you are doing military type stuff. Here's a list of things that you would need to know to uplink to the satellite:
And even if you did manage to do all of this, you would then have to access the satellite at a time when it wasn't otherwise in communication with Earth, or else the odds that you were successful would be quite low.
Next comes the question of receiving a signal and decoding it. That is a fair bit easier to do. You need most of the same information, but you don't have to broadcast any signals, and you can get away with a smaller antenna. In fact, there have been some NASA missions which amateurs have received and decoded some of the signals. A few instances of spacecraft which have had their signals decoded are Curiosity, Rosetta, and a host of others seen at this article.
Bottom line, it would be rather difficult to make this work overall. No doubt a large power (IE, government) could make it happen if required, but they would only consider doing this if the mission was of particular importance to them. NASA usually doesn't encrypt it's signals, as it is more hassle then it is worth. If one wanted to hack, say, Curiosity, it could be done, if one is devoted enough to do so. But it would be quite expensive to do so.