Sadly I have some coworkers (most are not that brainy) that do NOT believe that I have such a contract with the Navy. I tried to inform that the real reason for the length it is not so much WHAT I know but HOW I learned the data. Sadly too many of our politicians, and the public too, don't understand is that if a enemy or potential enemy don't know about how information is gathered the better and longer a chance for information TO be gotten.
During the Crimean War, the popular 'secret message cypher' was considered to be uncrackable, and was in use by nearly everyone. Charles Babbage, on a challenge, worked out how to crack it, but did not publish either that he had broken it, or his method - and so, lost the glory for breaking the unbreakable cypher. But if he had 'leaked', the enemy would have changed their cypher, and the British would have lost the ability to gain intelligence information from reading the other guy's mail. A German later published the method, and got the glory, but Babbage said nothing - it was only revealed after his death that he had enabled the Brits to read the uncrackable cypher.
The really amazing thing about personnel security is that sometimes, it actually works. Nobody seems to have blabbed about Bletchley Park's activities for thirty years after WWII, and there were over 7,000 people working there at one time or another! When books and newspaper articles started being released in 1972, one husband, who knew only that his wife had worked at Bletchley, is reported to have said to his wife, (thirty years after the end of the war,) "So THAT's what you did in the war!"