In that case, why not brag about cracking some code that's just too hard to crack? Make the enemy question the value of a difficult code, and put them through the expense of replacing code books, cypher machines, or at least changing out all public keys.
You'd have to come up with something you could leak to prove that the code was broken, I suppose. But what about bragging about a code you've broken pretty thoroughly, and then giving really shoddy evidence that only arguably proves you've broken it? Sort of the opposite of Cryptonomicon's Unit 2702. You might make the enemy snort in derision, and believe that the cypher in question was still secure.
Do that a few times, and then suddenly reveal a lot of stuff that could only have come from one of the bragged-about-in-a-shoddy-fashion cyphers. Now all cyphers, especially the one's not bragged about come under suspicion.
You'd have to come up with something you could leak to prove that the code was broken, I suppose. But what about bragging about a code you've broken pretty thoroughly, and then giving really shoddy evidence that only arguably proves you've broken it? Sort of the opposite of Cryptonomicon's Unit 2702. You might make the enemy snort in derision, and believe that the cypher in question was still secure.
Do that a few times, and then suddenly reveal a lot of stuff that could only have come from one of the bragged-about-in-a-shoddy-fashion cyphers. Now all cyphers, especially the one's not bragged about come under suspicion.
Wow, this is a fun game!