Bitlocker uses the XTS-AES (Advanced Encryption Standard) algorithm to encrypt the data. It was built on Windows 10 (build 1511).
By default the key is 128 bit. It is possible to set to 256 bit in the Windows registry as per instructions below:
HKLM SOFTWARE Policies Microsoft FVE
Operating System drives: Encryptionmethodwithxtsos
Fixed Data drives: Encryptionmethodwithxtsfdv
Removable Data drives: Encryptionmethodwithxtsrdv
Possible values:
- AES-CBC 128bit - value 3
- AES-CBC 128bit - value 4
- AES-XTS 128bit - value 6
- AES-XTS 256bit - value 7
More details here.