Cryptography Reference
In-Depth Information
well beyond the scope of this topic. Indeed, the skills that this requires have wider
application than just to cryptosystems and require a broad information security
education to perform.
As a last observation, even after having formulated a notion of practical
security, in a real environment it may not be possible to provide the determined
degree of practical security. A more realistic security target might simply be
'inadequate security, but the best that we can afford'. However, the formulation
of a notion of practical security at least allows such a decision to be placed in
appropriate context.
3.3 Summary
In this chapter we have investigated the difference between providing security in
theory and providing security in practice. We introduced the concept of perfect
secrecy, which is in some sense the best security that a cryptosystem can ever
provide. We described various versions of the only cryptosystem that has perfect
secrecy and then explained why it cannot normally be used in real applications. We
then explored the notion of practical security. We looked at ways of measuring the
strength of a cryptosystem in terms of the difficulty of performing attacks against it.
Finally, we commented on the difficulty of proposing a precise notion of practical
security.
A number of important points arose during this discussion:
• It is impossible to guarantee the security of a cryptosystem. Even if it is
theoretically secure, it may be insecure in practice.
• It is quite acceptable in practice (indeed, necessary) to use cryptosystems that
are theoretically breakable.
• Every attempt should be made to formulate a notion of practical security
for a given environment. This will inevitably involve tradeoffs, estimates and
evaluations of what levels of risk to accept. Formulating this notion will be
difficult.
3.4 Further reading
Alternative explanations of a one-time pad can be found in almost any topic on
cryptography. The foundations of information theory and perfect secrecy were first
set out by Claude Shannon in two important papers [173, 174]. Shannon is regarded
as a father figure in this area and much about his life and works (including copies
of these papers) can be found on the web. Historical uses of one-time pads are
discussed in Kahn [104]. The story of the Venona project [32] includes a fascinating
example of the dangers involved in implementing a one-time pad in practice.
 
 
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