Introduction to Cryptography and Data Security - Understanding Cryptography - page 6

LANs. Later in this topic we will learn methods for establishing keys over insecure
channels. In any case, the key has only to be transmitted once between Alice and
Bob and can then be used for securing many subsequent communications.
One important and also counterintuitive fact in this situation is that both the en-
cryption and the decryption algorithms are publicly known. It seems that keeping
the encryption algorithm secret should make the whole system harder to break.
However, secret algorithms also mean untested algorithms: The only way to find
out whether an encryption method is strong, i.e., cannot be broken by a determined
attacker, is to make it public and have it analyzed by other cryptographers. Please
see Sect. 1.3 for more discussion on this topic. The only thing that should be kept
secret in a sound cryptosystem is the key.
Remarks:
1. Of course, if Oscar gets hold of the key, he can easily decrypt the message since
the algorithm is publicly known. Hence it is crucial to note that the problem of
transmitting a message securely is reduced to the problems of transmitting a key
secretly and of storing the key in a secure fashion.
2. In this scenario we only consider the problem of confidentiality, that is, of hiding
the contents of the message from an eavesdropper. We will see later in this topic
that there are many other things we can do with cryptography, such as preventing
Oscar from making unnoticed changes to the message (message integrity) or
assuring that a message really comes from Alice (sender authentication).
1.2.2 Simple Symmetric Encryption: The Substitution Cipher
We will now learn one of the simplest methods for encrypting text, the substitution
(= replacement) cipher . Historically this type of cipher has been used many times,
and it is a good illustration of basic cryptography. We will use the substitution cipher
for learning some important facts about key lengths and about different ways of
attacking ciphers.
The goal of the substitution cipher is the encryption of text (as opposed to bits
in modern digital systems). The idea is very simple: We substitute each letter of the
alphabet with another one.
Example 1.1.
A → k
B → d
C → w
···
For instance, the pop group ABBA would be encrypted as kddk .