Cryptography Reference
In-Depth Information
Theoretical versus
Practical Security
3
In this chapter we consider the relationship between theoretical and practical
security of a cryptosystem. This is a subject that we touched upon briefly in
Chapter 2. The concept of perfect secrecy is introduced and a cryptosystem
is exhibited that has this property. However, as always, nothing comes for
free. The compromises that real systems adopt for reasons of practicality are
investigated with the intention of working towards a notion of practical security
of a cryptosystem.
Note that, once again, the focus of this chapter is on cryptosystems that are
being used to provide confidentiality. However, the main lessons learnt from this
chapter have wider implications for provision of other types of cryptographic
service.
At the end of this chapter you should be able to:
• Explain the concept of perfect secrecy.
• Recognise that, in theory, there exist 'unbreakable' cryptosystems.
• Appreciate the limitations of theoretical security.
• Identify some of the issues involved in assessing practical security.
• Recognise that there are (at least) two significantly different levels of
computational complexity.
• Appreciate that selection of a cryptographic primitive should be considered
as part of a wider cryptographic process.
• Formulate a notion of practical security.
In Section 1.6 we described a cryptosystem as being
broken
if a method of
determining the plaintext from the ciphertext was found that did not involve
being legitimately given the decryption key. We also discussed one method that
can, at least in theory, be used to break any cryptosystem: an exhaustive key search.
