Cryptography Reference
In-Depth Information
The decryption function is the same finite state machine that gen-
erates the same keystream, with which the plaintext is extracted
from the ciphertext.
A block cipher typically consists of a complex Substitution-Permutation
Network (SPN) [81] or a Fiestel structure [99] and involves complicated
mixing of message and key bits. On the other hand, a stream cipher is
typically defined on the binary alphabet {0, 1} and a plaintext is bitwise
XOR-ed with the keystream to generate the ciphertext, i.e.,
E
k
r
(m
r
) = m
r
⊕k
r
.
During the decryption operation, the ciphertext is bitwise XOR-ed with
the same keystream to get back the plaintext.
2. Public Key Cryptosystem or Asymmetric Key Cryptosystem. There are
two major disadvantages of a private key cryptosystem. First, the com-
mon key has to be agreed between Alice and Bob before the commu-
nication begins. Second, n communicating parties require
n
2
keys to
communicate with each other. The latter leads to high key storage re-
quirement. The public key cryptosystem takes care of both these issues.
Bob (or any receiver) fixes two distinct keys, one for encryption and one
for decryption. He publishes the encryption key k
e
, called the public
key, that can be used by anybody who likes to send a message to Bob.
He keeps the decryption key e
d
, called the private key secret and uses
it to decipher any message which has been encrypted by the public key.
Thus, for n communicating parties, only 2n keys are required, a (private
key, public key) pair for each person.
In 1973, National Bureau of Standards (now the National Institute of
Standards and Technology, or NIST) of the United States published a set of
requirements for a cryptosystem. This led to the development of the famous
block cipher, called Data Encryption Standard (DES) [45]. In 1999, it was re-
placed by an improved block cipher called the Advanced Encryption Standard
(AES) [34,46]. An overview of stream ciphers is discussed in the next chapter.
The idea of a public key cryptosystem was first proposed by Di
e and Hell-
man [37, 38] in 1976. Since then many public key cryptosystems [120] have
been invented, the most important and famous one being the RSA Cryptosys-
tem [144] due to Rivest, Shamir and Adleman in 1977.
How the secret key can be established between the sender and the receiver
who are geographically far apart is a pertinent problem in the private key
cryptosystem. There are two broad categories of key establishment proto-
cols [23,25], namely, key distribution and key agreement. In the first category,
a trusted third party (TTP) acts as a central server that generates the re-
quested key and distributes it among the individual communicating parties.
In the second category, there is no need for a central server to generate the
