Crash gets keys in the Cortexbonus rounds. Once Crash reaches the end, Cortex drops a key and Crash can use it to get into a locked level. There's two of these bonus rounds and each are found in Sunset Vista and Jaws Of Darkness. These keys unlock Whole Hog, which is on the second island, and Fumbling In The Dark, which is on the third. This is the only time up to Crash: Mind Over Mutant that keys have appeared in the main series. This game has only two keys.
Crash Bandicoot - Getting The Keys (Neo Cortex Bonus Stages)
Keys serve a different purpose than in Crash 1: they now play a role in the story. The player has to get the keys from Ripper Roo, Papu Papu, Komodo Joe, and Pinstripe Potoroo by racing each of them and winning their keys in order to race the alien Nitros Oxide. Then, once the player beats them, they give the player the key to the next door which leads to the next world. With the exception of Ripper Roo, they almost always give a warning that Oxide won't be as easy (although in Ripper Roo's unused sound clips in which he talks in normal English, he does warn the player about Oxide).
Crash Nitro Kart
The keys in Crash Nitro Kart have the same purpose as Crash Team Racing had. They still play a role in the story and the player needs to race against each of Velo's galactic champions, Krunk, Nash, Norm, and Geary, for each of their keys to become the galactic champion. Keys also open gates which lead to different worlds. Like Crash Team Racing, this game has four keys.
Crash: Mind Over Mutant
Keys also appear Crash: Mind Over Mutant where they are used for things such as opening doors to new areas in the Ratcicle Kingdom and the Wasteland.
Crash Bandicoot N. Sane Trilogy
Keys reappear in the Crash Bandicoot N. Sane Trilogy remaster of the first of the original three Crash games. They have the same purpose as they do in the original version of the first game.
Crash Bandicoot Sunset Vista Walkthrough - Cortex's Crazed Contest Trophy