64
Hardware Acceleration
Some processors (CPUs) support hardware-accelerated AES encryption,
*
which is typically 4-8
times faster than encryption performed by the purely software implementation on the same
processors.
By default, TrueCrypt uses hardware-accelerated AES on computers that have a processor where
the Intel AES-NI instructions are available. Specifically, TrueCrypt uses the AES-NI instructions
that perform so-called AES rounds (i.e. the main portions of the AES algorithm).
†
TrueCrypt does
not use any of the AES-NI instructions that perform key generation.
Note: By default, TrueCrypt uses hardware-accelerated AES also when an encrypted Windows
system is booting or resuming from hibernation (provided that the processor supports the Intel
AES-NI instructions).
To find out whether TrueCrypt can use hardware-accelerated AES on your computer, select
Settings
>
Performance
and check the field labeled ‘
Processor (CPU) in this computer supports
hardware acceleration for AES
’.
To find out whether a processor you want to purchase supports the Intel AES-NI instructions (also
called "AES New Instructions"), which TrueCrypt uses for hardware-accelerated AES, please
check the documentation for the processor or contact the vendor/manufacturer. However, note that
some Intel processors, which the Intel website lists as AES-NI-supporting,
actually support the
AES-NI instructions only with a Processor Configuration update. In such cases, you should contact
the manufacturer of the motherboard/computer for a BIOS update that includes the latest
Processor Configuration update for the processor.
If you want to disable hardware acceleration of AES (e.g. because you want TrueCrypt to use only
a fully open-source implementation of AES),
you can do so by selecting
Settings
>
Performance
and disabling the option ‘
Accelerate AES encryption/decryption by using the AES instructions of
the processor
’. Note that when this setting is changed, the operating system needs to be restarted
to ensure that all TrueCrypt components internally perform the requested change of mode. Also
note that when you
create a TrueCrypt Rescue Disk, the state of this option is written to the
Rescue Disk and used whenever you boot from it (affecting the pre-boot and initial boot phase).
To create a new TrueCrypt Rescue Disk, select
System
>
Create Rescue Disk
.
Note: Support for hardware acceleration was introduced in TrueCrypt 7.0.
*
In
this chapter, the word 'encryption' also refers to decryption.
†
Those instructions are
AESENC
,
AESENCLAST
,
AESDEC
, and
AESDECLAST
and they perform the following AES
transformations:
ShiftRows
,
SubBytes
,
MixColumns
,
InvShiftRows
,
InvSubBytes
,
InvMixColumns
, and
AddRoundKey
(for
more details about these transformations, see [3]).