Android external storage encryption

I want to know how to encrypt the external storage data (.asec). Which algorithm is used, AES or another method? And then what is the encryption key? If a method to generate encryption key is open source, I think that it is going to cause a开发者_如何学Python problem. I would be grateful for any information about this.

The Android Open Source Project provides a document describing their crypto implementation.

Basically, they use AES in the Linux DM-CRYPT layer. See the document for full details, including how it interacts with vold.