Notes

Q1. What are the essential ingredients of a symmetric cipher?
Ans: Plaintext, encryption algorithm, secret key, cipher text, decryption algorithm.
Q2. What are the two basic functions used in encryption algorithms
Ans: Permutation and substitution.
Q3. How many keys are required for two people to communicate via a symmetric cipher? How many keys are required for n people to communicate with each other securely?
Ans: One secret key. n(n-1)/2 keys
Q4. What is the difference between a block cipher and a stream cipher?
a) Why is it not desirable to reuse a stream cipher key?
b) Why do some block cipher modes of operation only use encryption whileothers use bothencryption and decryption?
Ans: A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. A block cipher is one in which a block of plaintext is treated as awhile and used to produce a cipher text block of equal length.a.If two plaintexts are encrypted with the same key using a stream cipher, then cryptanalysis is often quite simple. If the two cipher text streams are XORed together, the result is the XOR
What are the two general approaches to attacking a cipher
The general two approaches for attacking a cipher
1. Cryptanalysis: Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of the general characteristics of the plaintext or even some samples plaintext-cipher text pairs. This type of attack exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used. If the attack succeeds in deducing the key, the effect is catastrophic: All future and past messages encrypted with the key are compromised.
2. Brute-force attack: The attacker tries every possible key on a piece of cipher text until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success.