Looking at this page: http://technet.microsoft.com/en-us/library/cc179125.aspx
When Office uses CryptoAPI, the
encryption algorithms depend on those
that are availablein a CSP (Crypto
Service Provider), which is part of
the Windows operating system. The
following registry key contains a list
of CSPs that are installed on a
computer:
HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Cryptography/Defaults/Provider
The following CNG encryption
algorithms, or any other CNG cipher
extension installed on the system, can
be used with Office 2010 or the 2007
Office system SP2:
AES, DES, DESX, 3DES, 3DES_112, and
RC2
The following CNG hashing algorithms,
or any other CNG cipher extension that
is installed on the system, can be
used with Office 2010 or the 2007
Office system SP2:
MD2, MD4, MD5, RIPEMD-128, RIPEMD-160,
SHA-1, SHA256, SHA384, and SHA512
Although there are Office 2010
settings to change how encryption is
performed, when you encrypt Open XML
Format files (.docx, .xslx, .pptx, and
so on) the default values — AES
(Advanced Encryption Standard),
128-bit key length, SHA1, and CBC
(cipher block chaining) — provide
strong encryption and should be fine
for most organizations. AES encryption
is the strongest industry-standard
algorithm that is available and was
selected by the National Security
Agency (NSA) to be used as the
standard for the United States
Government. AES encryption is
supported on Windows XP SP2, Windows
Vista, Windows 7, Windows Server 2003,
and Windows Server 2008.
I would think maybe it uses AES 128 bit as the default.
