Answer die following questions with short answers Explain th

Answer die following questions with short answers: Explain the difference between stream ciphers and block cipher. Explain the difference between passive and active attacks. Describe the difference between authentication and non-repudiation Explain the difference between a MAC and message digest. Give one advantage and one disadvantage of public-key cryptography over symmetric-key cryptogra Why is CBC mode considered preferable to ECB mode?

Solution

a)
stream cipher and block cipher differ in whitch the plain text is encoded and decoded.
the basic difference is how the logic used for the encryption and decryption. the idea of block
cipher is partition the plain text into relatively larger blocks and further each block will be encoded.
and same key is used for both encryption and decryption.example DES,IDEA
the idea of stream cipher is plain text is encrypted and transfered bit by bit.but key property of
stream cipher is different key is generated for each block or bit.example -ISAAC,FISH
b)
passive attack is indirect attack.the attacked host is completely unaware about the attack.
hence it is called passive attack like some body listens the packet transformation or observing the host in network.
active attack is direct attack.its nothing but the attacked one gets aware of the attack.
like somebody listens the id and password and stop the services(denial of service) or logic bomb.
both are harmful.
c)
authentication and non repudiation are two different things.
authentication verifies the who you are(user-id).where the non-repudiation verifies the what you did(messages).
authentication is done through a central instance where non repudiation can done by anyone
d)
message digest is a hash of a message.its output of a cryptographic hash function appilied to input data it refered to as message.
mac is a piece of information that proves the integrity of a message.
message digest is take only single input and produces a message digest(aka hash) if the message change it produce different hash code.
mac (message authentication code) is take input and security code to check the integrity of message. HMAC is one of this kind.
e)
advantage of public key cryptography over the symmetric key cryptography is increased security and convenience.
and no need of key distribution(transmitted to anyone).and it provide digital signatures that cannot be repudiated.
the disadvantage of public key cryptography over symmetric cryptography is speed.they are many symmetic key methods runs faster than the public key cryptography.
f)
the reason why cbc is considered better than ecb has nothing to do with situations involving
an attacker with partial cipher text.
problem with ECB is that it leaks the information like when two blocks have same plain text then the corresponding cipher blocks also be same where in case
of the CBC it randomly independent of plain text to produce the cipher text.