Cryptography Reference
In-Depth Information
So, given an application that requires encryption using a block cipher, which
mode of operation should be used? We have attempted to provide some basic
information in this chapter but, as with all aspects of cryptography, it is advisable to
consult best-practice guidelines and standards before selecting and implementing
any mode of operation.
In this chapter we have concentratedon symmetric encryption algorithms.We looked
at the twomajor classes of symmetric encryption algorithm, streamciphers and block
ciphers. Simple stream ciphers are fast and do not propagate errors, making them
suitable for poor quality channels and for applications where errors are intolerable.
Block ciphers do propagate errors (to a limited extent), but are quite flexible and can
be used in different ways in order to provide different security properties, in some
cases to achieve some of the benefits of stream ciphers. In particular we looked at
two influential block ciphers, DES and its 'successor' AES. We then discussed different
modes of operation of a block cipher and the different properties that they provide.
Perhaps the most important two messages to take away from this chapter are
the following:
• Stream ciphers and block ciphers are different types of symmetric encryption
algorithm. They have slightly different properties and are therefore suitable for
different applications.
• The properties of cryptographic algorithms are not only affected by algorithm
design, but also by the ways in which the algorithms are used. Different modes
of operation can significantly change the properties of a block cipher.
There is an abundance of cryptographic literature for those seekingmore information
about symmetric encryption algorithms. All the symmetric encryption algorithms
that we discussed in this chapter are well known and details can be found in
most recommended cryptography topics. A good overview of symmetric encryption
standards, including modes of operation, is Dent and Mitchell [55]. The ISO standard
series relating to encryption mechanisms is ISO/IEC 18033 [12].
Until relatively recently there were few publicly known stream ciphers. Even well-
used stream ciphers such as RC4 were originally proprietary designs, making the
Wikipedia web portal [203] probably the best source of further information on it.
A major development was the European eSTREAM project [164], which resulted in
the development of a number of stream ciphers with a particular emphasis on those
that would either run fast in software or be resource-efficient in hardware.
