The engine module provides tools to perform cryptographic operations on Nintendo 3DS data, including emulating keyslots and the key scrambler.
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The 3DS uses keyslots in an attempt to obscure the encryption keys used. Each slot consists of X, Y, and normal keys.
Often, one slot contains a fixed key (often X), while the other is a key unique to something such as a game or console (often Y). When key Y is set, both keys are put into a key scrambler, and the result is stored as a normal key. The AES engine would then use the result for encryption. A normal key can also be set directly.
The AES engine only has keyslots 0x0 to 0x3F (0 to 63). Keyslots 0x0 to 0x3 are for DSi-mode software, and use the DSi key scrambler. This module uses keyslots above 0x3F for internal use.