Introduction to Cryptography and Data Security - Understanding Cryptography - page 4

can be viewed as building blocks with which applications such as secure Inter-
net communication can be realized. The Transport Layer Security (TLS) scheme,
which is used in every Web browser, is an example of a cryptographic protocol.

Strictly speaking, hash functions, which will be introduced in Chap. 11, form
a third class of algorithms but at the same time they share some properties with
symmetric ciphers.
In the majority of cryptographic applications in practical systems, symmetric and
asymmetric algorithms (and often also hash functions) are all used together. This is
sometimes referred to as hybrid schemes . The reason for using both families of
algorithms is that each has specific strengths and weaknesses.
The main focus of this topic is on symmetric and asymmetric algorithms, as
well as hash functions. However, we will also introduce basic security protocols. In
particular, we will introduce several key establishment protocols and what can be
achieved with crypto protocols: confidentiality of data, integrity of data, authentica-
tion of data, user identification, etc.

1.2 Symmetric Cryptography

This section deals with the concepts of symmetric ciphers and it introduces the
historic substitution cipher. Using the substitution cipher as an example, we will
learn the difference between brute-force and analytical attacks.

1.2.1 Basics

Symmetric cryptographic schemes are also referred to as symmetric-key , secret-key ,
and single-key schemes or algorithms. Symmetric cryptography is best introduced
with an easy to understand problem: There are two users, Alice and Bob, who want
to communicate over an insecure channel (Fig. 1.4). The term channel might sound
a bit abstract but it is just a general term for the communication link: This can be the
Internet, a stretch of air in the case of mobile phones or wireless LAN communica-
tion, or any other communication media you can think of. The actual problem starts
with the bad guy, Oscar 1 , who has access to the channel, for instance, by hacking
into an Internet router or by listening to the radio signals of a Wi-Fi communica-
tion. This type of unauthorized listening is called eavesdropping . Obviously, there
are many situations in which Alice and Bob would prefer to communicate without
Oscar listening. For instance, if Alice and Bob represent two offices of a car man-
ufacturer, and they are transmitting documents containing the business strategy for
the introduction of new car models in the next few years, these documents should

1

The name Oscar was chosen to remind us of the word opponent.