An extreme example for this kind of approach might be found in the history of the Enigma cipher and the British/Allied successful attempts in decryping German communications during the war. If it would have been obvious that information about an impeding attack could only have been gathered from intercepted and decrypted communications, the British would rather let boats be sunken and people die than give away the fact that the code had been broken.
Because if the fact would be known, the adversary will immediately cease to use this method of encryption, rendering the advantage of breaking the cipher void.
As a small aside, for anyone interested in this, The Imitation game was in my opinion fantastic, though I assume anyone here would already have seen it, or have plans to.
Because if the fact would be known, the adversary will immediately cease to use this method of encryption, rendering the advantage of breaking the cipher void.