Regarding Windows Vista Disk Encryption Algorithm.

[Sarad AV]

Hello,

I ran across this paper titled "AES-CBC + Elephant
diffuser A Disk Encryption Algorithm for Windows
Vista". 
Paper downloadable at:
http://download.microsoft.com/download/0/2/3/0238acaf-d3bf-4a6d-b3d6-0a0be4bbb36e/BitLocker
Cipher200608.pdf
or
http://blogs.msdn.com/si_team/archive/2006/09/15/756622.aspx

There are a few questions I would like to know about.

pg 3:[ Bitlocker makes use of a tamper resistant chip
security chip mounted on the motherboard. Bitlocker
makes use of the TPM security chip that will be
incorprated in most PC's.]

How do we know if future PC's make use of this chip on
their motherboards in future and how can we trust this
chip?

pg 3.[The seal/unseal functions of the TPM allow
selective access to cryptographic keys based on PCR
values. The seal function is used to encrypt a key
into a string which can only be decrypted by that same
TPM. Furthermore, the TPM will decrypt the string if
and 
only if the selected PCRs have the value that was
specified during the seal operation. In other words:
we can store a key in an encrypted string so that it
can only be accessed when selected PCRs have a
particular value.

During the boot process the PCRs are used to keep
track of the code that runs. The key used to encrypt
the disk is sealed against a particular set of PCR
values. During a normal boot the PCRs reach the same
values, and the key can be unsealed by the TPM. If an
attacker boots into any other operating system, the
machine will be fully functional but the PCR values
will be different and the TPM will not unseal the key.
Thus, other operating systems cannot read the data on
the disk, or find out how to modify the disk to reset
the
Administrator password.]

It prevents the protected operating system from being
operational as the key cannot unlocked. The encrypted
disk can still be removed from the machine with the
security chip and re-inserted elsewhere and its
contents dumped for further analysis. Doesn't a data 
recovery expert usually work under the assumption that
the encryption keys are unavailable? 
   Isn't this the case, if we are looking at stolen
laptop(S)?
    If the disk can be removed and dumped, what
advantage does the TPM security chip provide over
software encryption?


pg 6.
[BitLocker also allows users to use a PIN that the TPM
checks, or a USB key that contains a cryptographic
key.Without the right PIN or USB key the laptop
doesn't have the right information to even find the
disk decryption key, so the information is safe unless
the PIN is written on a post-it stuck to the machine,
or the USB key is left in the laptop bag. In practice,
we expect that many laptops will be used in the
TPM-only mode and that scenario is the main driver for
the disk cipher design.]

But if we work with the assumption that the attacker/
recovery expert will not be able to find the key on
the disk ,is there any need to implement the TMP
security chip? Is the assumption reasonable?


pg 7.
[A software implementation of AES runs in around 20-25
cycles per byte on a P4 class CPU.(Synthetic
benchmarks can achieve somewhat higher speeds, but
they exclude various overheads encountered in real
system implementations.) Other overhead adds around 5
cycles per byte for a total of 25-30 cycles per byte.
Based on this data, our performance analysis concluded
that a single pass of AES, for example using AES in
CBC mode, would have acceptable performance. An
algorithm
twice as slow as AES (45-55 cycles/byte) would be on
the edge of being unacceptable, and a high-risk choice
given the many uncertainties in the analysis. Anything
slower than that would be unacceptable.]

2.6 BitLocker encryption algorithm requirements
pg 7 & 8
[We get the following major requirements for our
BitLocker encryption algorithm:
...
It is fast enough that the slow-down of the laptop is
acceptable to most users. Our best estimate is that a
speed of 40 cycles/byte or faster will be acceptable.]

It looked like that the AES-CBC implementation was
part of the hardware security chip to speeden it up.
>From the above, judging by the clock speed, it however
appears that AES-CBC is software implemented by
BitLocker?


pg 9.
3.2 AES-CBC
[Any time you want to encrypt data, AES-CBC is a
leading candidate. In this case it is not suitable,
due to the lack of diffusion in the CBC decryption
operation. If the attacker
introduces a change 'delta' in ciphertext block i,
then plaintext block i is randomized, but plaintext
block i + 1 is changed by 'delta'. In other words, the
attacker can flip arbitrary bits in one block at the
cost of randomizing the previous block. This can be
used 
to attack executables. You can change the instructions
at the start of a function at the cost of damaging
whatever data is stored just before the function. With
thousands of functions in the code, it should be
relatively easy to mount an attack.]

This appears to be why the diffusers are being used.
The overview of the AES-CBC + diffusors if given in
Figure 1, pg 13.

pg 16.
[Our AES implementation uses about 20 cycles/byte for
AES-CBC on a a Pentium 4. The diffuser takes about 10
cycles/byte.The overall cipher speed is just over 30
cycles per byte, including various overhead.]

Appendix A on pg 18 gives a sketch of a proof on why
AES-CBC+Diffusers are atleast as secure as AES-CBC.
The diffusers consumes about 1/3 rd of the cycles per
byte. Given this overhead is it useful to implement
the diffusers unless the implementation can be shown
to be more secure than AES-CBC?

Thankyou,
Sarad.




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