[baobabKoodaa]

This story is eerily reminiscient of the s-box in DES, except in that case there was no backdoor, the researchers had simply discovered a novel attack method, crafted their s-box to protect against that method, and then kept the attack unpublished for decades:

> The eight S-boxes of DES were the subject of intense study for many years out of a concern that a backdoor (a vulnerability known only to its designers) might have been planted in the cipher. The S-box design criteria were eventually published (in Coppersmith 1994) after the public rediscovery of differential cryptanalysis, showing that they had been carefully tuned to increase resistance against this specific attack. Biham and Shamir found that even small modifications to an S-box could significantly weaken DES.

https://en.wikipedia.org/wiki/S-box