Cryptography Reference
In-Depth Information
10.5.3
Key storage in hardware
The safest medium in which to store a cryptographic key is hardware. There
are, of course, different
types of hardware device, with varying levels of
security.
HARDWARE SECURITY MODULES
The securest hardware storage media for cryptographic keys are
hardware security
modules
(HSMs). These dedicated hardware devices that provide keymanagement
functionality are sometimes known as
tamper-resistant devices
. Many HSMs can
also perform bulk cryptographic operations, often at high speed. An HSM can
either be peripheral or can be incorporated into a more general purpose device
such as a point-of-sale terminal.
While we have chosen to introduce HSMs as mechanisms for the secure storage
of cryptographic keys, it is important to appreciate that HSMs are often used to
enforce other phases of the key lifecycle.
Keys stored on HSMs are physically protected by the hardware. If anyone
attempts to penetrate an HSM, for example, to extract a key from the device,
tamper-resistant circuitry is triggered and the key is normally deleted from the
HSM's memory. There are various techniques that can be used to provide tamper
resistance. These include:
Micro-switches
. A simple mechanism that releases a switch if an HSM is opened.
This is not particularly effective, since a clever attacker can always drill a hole
and use glue to force the switch off.
Electronic mesh
. A fine-gauge electronic mesh that can be attached to the inside
of an HSM case. This mesh surrounds the sensitive components. If broken, it
activates the tamper-detection circuitry. This mechanism is designed to protect
against penetrative attacks, such as drilling.
Resin
. A hard substance, such as epoxy resin, that can be used to encase sensitive
components. Sometimes electronic mesh is also embedded in resin. Any
attempt to drill through the resin, or dissolve the resin using chemicals,
will generally damage the components and trigger the tamper-detection
circuitry.
Temperature detectors
. Sensors that are designed to detect variations in
temperature outside the normal operating range. Abnormal temperatures may
be an indication of an attack. For example, one type of attack involves, literally,
freezing the device memory.
Light-sensitive diodes
. Sensors that can be used to detect penetration or opening
of an HSM casing.
Movement or tilt detectors
. Sensors that can detect if somebody is trying to
physically remove an HSM. One approach is to use mercury tilt switches,
which interrupt the flow of electrical current if the physical alignment of an
HSM changes.
