The DVDs are encrypted to avoid analysis, and signed to avoid modifications.
The encryption is a symmetric crypto, 128 bit AES. Symmetric means that the same key is used for both encryption and decryption.
The Wii DVD contains of several partitions. Each partition has its own AES key. This key is stored on the disc, in the partition information, but it is encrypted with the master AES key. So, with the master AES key you can decrypt the partition keys, and with the partition keys you can decrypt the partitions. Lucky for us, the master AES key was extracted by the Tweezer hack.
For more details, see [[Wiidisc#Partition_Data|Partition Data info on the Wiidisc page]].
=== Savegames on SD cards ===