] A piece of debris classified "top secret" is somewhere
] among the thousands of shards of the space shuttle
] Columbia spread across Texas.
]
] The communications device handles encrypted messages
] between the shuttle and the ground. According to its
] serial number, it is in a class of equipment labeled
] "TSEC" -- telecommunications security -- that must be
] handled with strict chain-of-command documentation.
]
] Experts differed on the importance to national security
] of recovering the device, which was "keyed," or prepared
] for use with mathematical algorithms.

There are many good reasons for them not wanting anyone to know the details of the equipment they use for secure communications, but I think its unlikely that the discovery of the equipment would not give someone the ability to decrypt any communications using similar equipment. The importance of the keys on this device are likely minimal. Its a big deal if you can get a private key, and that key is in use.. But this key is no longer in use. And its safe to assume it was specific to that particular shuttle. At least, it would be a major oversight if it wasn't specific to that shuttle.

HoustonChronicle.com - Top secret part among the shuttle debris