Two and a half years after the release of Super Smash Bros. 4, everyone's favorite sugary-sweet mascot brawler is proving stubbornly ironclad against modding.

The culprit? A pesky proprietary file type known as .mscsb.

As the modder Austyn Bellinger, a.k.a. Karmic Backlash, made known in a Reddit post recently, these mysterious MSC files are stymieing the best efforts of Nintendo fans who want to hack the game.

"We are unable to add anything to the base game," Bellinger told Motherboard. He is heading up a major Smash 4 mod called USM-eM.

Typically, lifecycle for a new Smash Bros. game is very predictable: Nintendo releases the next game in the series. Fans bitch about it. A team of EULA-defying modders goes to work fixing things.

At least that was the path to Project M, a mod of Smash Bros. Brawl for Wii that was downloaded over 100,000 times. With Super Smash Bros. for Nintendo 3DS and Wii U, also known as Smash Bros. 4 or Smash 4, the next entry in the series, six or more different teams of modders have stepped up to the plate. Not one has figured out how to crack the game yet.

The trouble stems from the part of the code that contains the barebones basic fighting moves. It seems the MSC system files contain very important stuff, like the instructions that tell Mario how to jump like Mario, or Kirby to do that annoying flying ground-pound. As of now, no one seems to be able to make heads or tails of the commands. To make matters worse, these confounding files are encrypted on the 3DS, further impeding progress.

This setback is especially frustrating to Smash Bros. players who want new Smash Bros. games to play more like old Smash Bros. games.

"Putting in classic moves like wave-dashing and L-canceling isn't possible on either version," Bellinger said, referencing advance moves and techniques that expert players pine for.

The restriction doesn't merely affect Smash purists who make us passive fans cry. Until modders learn how to decipher the puzzling code, they can't do other things the Smash community craves, like reinstate retired legacy characters to the game, including Pokémon Trainer, Ice Climbers, and Solid Snake.

While the USM-eM mod will allow the player to select new characters, like Knuckles from Sonic the Hedgehog, these alterations are mostly cosmetic. The team can change how a character looks, color it red and give it furry skin, but they can't change how Knuckles behaves at the muscular level, at least for now.

Bellinger is certain that Nintendo's byzantine file system will fall. However, he's not predicting breakthroughs any time soon.