.Net Machinekey.Protect - what algorithm is used?

I'm using Machinekey.Protect and Machinekey.Unprotect to encrypt values in my application. A potential client is performing a security audit and has asked about the encryption algorithms used by this method and I am struggling to find an answer using google.

Any help with verifiable links would be greatly appreciated!

Thanks, John

Is Machinekey protect secure?

The Protect method performs the appropriate operation and securely protects the data. Ciphertext data produced by this method can only be deciphered by the Unprotect method.

What is Machinekey used for?

Use the Machine Key feature page to configure hashing and encryption settings used for application services, such as view state, Forms authentication, membership and roles, and anonymous identification. Machine keys are also used to verify out-of-process session state identification.