BitLocker Drive Encryption supports 128-bit
encryption keys. Longer encryption keys provide a more enhanced level of security and are less likely to be successfully attacked by the use of brute-force methods. However, longer keys can cause slower encryption and decryption of data. On some computers, using longer keys might result in noticeable performance degradation. In addition, BitLocker supports a Diffuser algorithm
to help protect the system against ciphertext manipulation attacks, a class of attacks in which changes are made to the encrypted data in an attempt to discover patterns or weaknesses.
By default, Windows 7 BitLocker Drive Encryption uses AES encryption with 128-bit encryption keys and Diffuser