Cryptography and Network Security
Module 3
Very short, Short & Long Type Questions Solution Model
1. What is meant by the Diffie-Hellman key exchange?
An element g is called a generator of a group G if every element in G can be expressed as the
product of finitely many powers of g. If p≥1 is an integer, then the numbers coprime to p, taken
modulo p, form a group with multiplication as its operation. It is written as (Z/pZ)×or Zp*.
2. How does Diffie-Hellman key exchange achieve security?
Diffie–Hellman key exchange is a specific method of exchanging cryptographic keys. It is one
of the earliest practical examples of key exchange implemented within the field of
cryptography. The Diffie–Hellman key exchange method allows two parties that have no
prior knowledge of each other to jointly establish a shared secret key over an
insecure communications channel. This key can then be used to encrypt subsequent
communications using a symmetric key cipher.
3. What is weak collision resistance? What is the use of it?
For any given block x, It is computationally infeasible to find Y ≠X with H(Y) ≠H(X). It
guarantees than an alternative message hashing to the same value as a given message cannot
found. This prevents forgery when as encrypted hash code is used.
4. What is meant by EIGamal cryptosystem?
The ElGamal system is a public-key cryptosystem based on the discrete logarithm
problem. It consists of both encryption and signature algorithms. The encryption algorithm is
similar in nature to the Diffie-Hellman key agreement protocol.
5. What is meant by one-way property in hash function?
For any given code h, it is computationally infeasible to find X such that H(x) = h. A hash
function, by itself, does not provide message authentication. A secret key must be used in some
fashion with the hash function to produce authentication. A MAC, by definition, uses a secret
key to calculate a code used for authentication.
6. List out the requirements of kerberos.
The requirements of Kerberos are as follows:
(1) Secure (2) Reliable (3) Transparent (4) Scalable
7. What is meant by life cycle of a key?
Keys have limited lifetimes for a number of reasons. The most important reason is
protection against cryptanalysis. Each time the key is used, it generates a number of ciphertexts.
Using a key repetitively allows an attacker to build up a store of ciphertext (and
possibly plaintexts) which may prove sufficient for a successful cryptanalysis of the key value.
If you suspect that an attacker may have obtained your key, then your key is considered
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