Key Management - Everyday Cryptography - page 357

enough for protection of the user's personal files. Likewise, any keys stored
under the limited protection of a portable point-of-sale terminal are probably
still secure from anything other than attacks from experts.
Zone 2 . The security offered by Zone 1 devices is increased substantially when
they are moved into a controlled environment. In the extreme, a key stored in
the clear in software on a general PC provides excellent security if the PC is
not networked and is kept in a physically secure room with an armed guard
at the door! More realistically, encrypted keys stored on PCs that are located
in an office with strong physical security (such as smart card access control to
the rooms) and good network security controls should have better protection
than those on a PC located in a public library or an internet cafe.
Zone 3 . Specialised devices sometimes have to be located in insecure environ-
ments because of the nature of their application. A good example is provided by
Automated Teller Machines (ATMs), which need to be 'customer facing'. Such
devices are thus exposed to a range of potentially serious attacks that are made
possible by their environment, such as an attacker attempting to physically
remove them with the intention of extracting keys back in a laboratory.
Zone 4 . The highest-security zone is provided when a specialist device is kept in a
controlled environment. This is not just themost secure, but themost expensive
zone within which to provide solutions. This level of security is nonetheless
appropriate for important keys relating to high-security applications such as
data processing centres, financial institutions, and certification authorities.
Note that this conceptual 'model' could easily be extended. For example, we have
not considered the different ways in which keys stored on the devices are activated
(see Section 10.6.3).
10.5.5 Key backup, archival and recovery
We have spent most of our discussion about cryptography assuming that the use
of cryptography brings security benefits. However, there are situations where
use of cryptography can potentially have damaging consequences. One such
situation arises if a cryptographic key becomes 'lost'. For example:
1. data stored in encrypted formwill itself be lost if the corresponding decryption
key is lost, since nobody can recover the data from the ciphertext;
2. a digital signature on a message becomes ineffective if the corresponding
verification key is lost, since nobody has the ability to verify it.
The first scenario illustrates the potential need for key backup of critical secret
keys. The second scenario more broadly illustrates the potential need for key
archival , which is the long-term storage of keys beyond the time of their expiry.
Note that because key archival tends to apply to keys after they have expired, it
appears in the key lifecycle of Figure 10.1 as a process occurring after key usage.
However, we have included it in this section as it is closely related to key backup.