Cryptography Reference
In-Depth Information
These contradictory perspectives are not surprising. Risk is subjective, and risks
differ between applications. Indeed the assessment and management of risk is a
major information security topic in its own right and one that many organisations
devote entire departments to studying. It is prudent to think about questions such
as those identified in Section 1.2.2, but whether we act on them and introduce
security controls to address them is another issue altogether.
In fact there are at least three different issues to consider when contemplating
the use of any security mechanism, including a cryptographic control:
Appropriateness . Is it the right tool for the job? It is important to understand the
precise properties that a cryptographic mechanism will provide. One aim of
this topic is to explain how the various tools of cryptography can (and in some
cases cannot) be used to provide different notions of security.
Strength . Why put in an expensive burglar alarm in situations where awarning sign
would suffice? Different information security mechanisms provide different
levels of protection for data, just as different security mechanisms in the
physical world provide a range of strengths of physical protection.
Cost . Do the security gains justify the costs? The cost of a security mechanism is of
fundamental importance. By 'cost' we do not necessarilymeanmonetary value.
Cost can be measured in terms of ease of use and efficiency of operation, as well
as directly in terms of financial worth. As we will see throughout Chapter 12,
in many real applications it is cost considerations that determine the security
mechanism adopted, rather than the strength of security that the mechanism
provides. In the past, some military and government sectors may have opted
for strong security, whatever the cost, but in most modern environments this
is not appropriate. A sensible commercial question might be: what strength of
security is appropriate given the value of our assets? A more commonly asked
question of modern security managers however is: what strength of security can
we obtain within our budgetary constraints? One of the challenges of managing
security in such environments is tomake a case for having information security
controls. This case can often be built on the argument that good security may
succeed in reducing other costs.
Returning to our email example, an appropriate tool for preventing emails from
being read by unauthorised parties is encryption. The strength of the encryption
used is dependent on the cryptographic algorithm and the number of decryption
keys (which we will discuss later). The cost is that it is necessary to buy and
install suitable software, manage the relevant keys, configure the email client
appropriately and incur some small time and communication costs every time the
software is used.
So, is it worth encrypting an email? There is of course no general answer,
since this very much depends on the value of the information in the email
and the perceived risks. However, an overall aim of this topic is to advise how
cryptography can help in this type of situation and what the related issues
are. We will focus on explaining the appropriateness and strength of various
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