Not positive proof -- merely suggestive. Which is true of a lot of things in the secret world of the intelligence services. The more you use a secret cipher,

Not positive proof -- merely suggestive. Which is true of a lot of things in the secret world of the intelligence services. The more you use a secret cipher, the easier it is to break. It is simply good operational practice to use a different cipher for a small fraction of communications -- namely, the most secret ones. Just like certain antibiotics are reserved for drug-resistant organisms. You don't want it to lose its effectiveness through overuse. First, security-by-obscurity does indeed buy you some additional time. Because the cipher is secret, your opponent has to figure out the algorithm as well as the attack. Second, this reduces the amount of traffic that the opponent can analyze. For example, suppose that only 1% of messages use Suite A, and 99% of messages use Suite B. With fewer messages to analyze, the job of breaking the cipher becomes much harder. Third, the reduced volume also makes known-plaintext attacks more difficult. Especially if you avoid committing the cardinal sin of repeating the same message using two different ciphers.