Plaintext: This is the original message or data that is fed into the algorithm as
input
2.
Encryption
algorithm:
The
encryption
algorithm
performs
various
substitutions and transformations on the plaintext.
3.
Secret key: The secret key is input to the encryption algorithm.The exact
substitutions and transformations performed by the algorithm depend on the
key.
4.
Ciphertext: This is the scrambled message produced as output. It depends on
the plaintext and the secret key, For a given message. Two different keys will
produce two different ciphertexts.
5.
Decrytion algorithm: This is essentially the encryption algorithm run in
reverse. I t takes the ciphertext and the secret key and produces the original
plaintext.
There are two requirements for secure use of conventional encryption:
1
We need a strong encryption algorithm. At a minimum, we would like the
algorithm to one such that an opponent who knows the algorithm and has
access to one or more cipher texts would be unable to decipher the cipher text
or figure out the key.
2
Sender and receiver must have obtained copies of the secret key in a secure
fashion and must keep the key secure.
There are two general approaches to attacking a conventional encryption scheme. The
first attack is known as
cryptanalysis
. Cryptanalysis attacks rely on the nature of the
algorithm plus some knowledge of general characteristics of the plaintext or even
some sanmple plaintext ciphertext pairs.
The second method, known as the
brute-force attack
, is to try every possible
key on a piece of ciphertext until an intelligible translation into plaintext is obtained.
On average, half of all possible keys must be tried to achieve success.
Figure 18.2 Simplified Model of Conventional Encryption
15.Specify the Message Authentication code.(MAC)
(10 marks)
Solution
Message Authentication Code
One authentication technique involves the use of secret key to generate a small
block
of data, known as a message authentication code. This technique assumes that
two communicating parties, say A and B, share a common secret key KAB . When A
has a message to send to B, it calculates the message authentication code as a function
of the message and the key: MACM=F(KAB,M).The message plus code are
transmitted to the intended recipient. The recipient performs the same calculation on
the received message, using the same secret key, to generate a new message
authentication code. The received code is compared to the calculated code.
Figure 18.7 Message Authentication Using a Message Authentication Code
(MAC)

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