The person answering doesn't really comment on deep space missions, but I think the conclusion you draw (no encrypted link) is almost certainly right for Spirit

The person answering doesn't really comment on deep space missions, but I think the conclusion you draw (no encrypted link) is almost certainly right for Spirit and Opportunity, which were launched in 2003. The thinking used to be that a 70m radio dish (and all the accompanying deep knowledge about pointing, relative velocity, channel codes, etc.) would be enough of an obstacle. This thinking has definitively changed in the meantime.