Cryptography Reference
In-Depth Information
}
∗
. More often than not, the key space is set to
ciphertext, and key spaces to
{
0
,
1
}
∗
) for a reasonably sized key length
l
.
Additive, multiplicative, and affine ciphers are the the simplest examples of
monoalphabetic substitution ciphers
. In a monoalphabetic substitution cipher, each
letter of the plaintext alphabet is replaced by another letter of the ciphertext alphabet.
The replacement is fixed, meaning that a plaintext letter is always replaced by the
same ciphertext letter. Consequently, monoalphabetic substitution ciphers can easily
be attacked using frequency analysis. An early attempt to increase the difficulty of
frequency analysis attacks on substitution ciphers was to disguise plaintext letter
frequencies by homophony. In a
homophonic substitution cipher
, plaintext letters
can be replaced by more than one ciphertext letter. Usually, the highest frequency
plaintext letters are given more equivalents than lower frequency letters. In this way,
the frequency distribution is flattened, making analysis more difficult. Alternatively,
polyalphabetic substitution ciphers
flatten the frequency distribution of ciphertext
letters by using multiple ciphertext alphabets. All of these substitution ciphers are
overviewed and discussed in the literature. Most of them, including, for example,
the
Vigen ere cipher
,
4
are quite easy to break. You may refer to any topic about
(classical) cryptography if you want to get into these historically relevant ciphers
and the cryptanalysis thereof (some topics are mentioned in the Preface and Chapter
1). For the purpose of this topic, we don't look into these ciphers. Instead, we focus
on ciphers that are considered to be secure and hence are practically relevant. We
begin with a classification of such symmetric encryption systems.
l
(instead of
{
0
,
1
}
{
0
,
1
10.1.2
Classes of Symmetric Encryption Systems
Every practically relevant symmetric encryption system processes plaintext mes-
sages unit by unit. A unit, in turn, may be either a bit or a block of bits (e.g., one or
several bytes). Furthermore, the symmetric encryption system may be implemented
as an FSM, meaning that the
i
th
ciphertext unit depends on the
i
th
plaintext unit, the
secret key, and some internal state. Depending on the existence and use of internal
state, block ciphers and stream ciphers are usually distinguished.
Block ciphers:
In a
block cipher
, the encrypting and decrypting devices have no
internal state (i.e., the
i
th
ciphertext unit only depends on the
i
th
plaintext
unit and the secret key). There is no memory involved, except for the internal
memory that is used by the implementation of the cipher. Block ciphers are
further addressed in Section 10.2.
4
The Vigenere cipher is a polyalphabetic substitution cipher that was published in 1585 (and
considered unbreakable until 1863) and was widely deployed in previous centuries.
