6. Public Key Infrastructure &. Message
Authentication
January-2004 [16]
1.
f) What are the three key properties of
hash functions? [4]
4.
b) ‘A wants to
send ‘B’ a signed message. ‘A’ and ‘B’ have certificates for their public keys,
signed by a mutually trusted CA. Explain how ‘A’ generates the signature which
is appended to the message, assuming RSA, and MD5 can be used. [6]
c) Does the
certification Authority need a private key? What for? What happens if this is
compromised? [6]
July-2004 [13]
1.
d) What are session keys? How are they
distributed using PKI? [4]
4.
a) What is hashing? How does it help in
checking integrity of a transmitted text? [4]
b) Given a
message, describe the steps involved in arriving at a digital signature for the
message. [5]
January-2005 [22]
1.
f) We consider
the random cipher model with random variables M, C and K
for plaintext, ciphertext and key, respectively. Give an interpretation in
cryptographic terms of the equation
H(M,C) = H(M)+H(C). [4]
6.
a) We consider
the use of RSA encryption with a 1024 bits modulus to transmit a 56 bit DES key
to be used as session key. One can develop a meet-in-the-middle attack on this
practice, based on the fact that a random 56 bit number m can with
significant probability be factored as m = m1 m2,
where both m1 and m2 are 28 bit numbers.
So, assume that the DES key m has such a factorization and that the
ciphertext c = me mod N has been intercepted by
an adversary. Describe the attack in detail and give estimates of how much
computation and storage that is needed for the attack. [12]
b) Nikita and
Michael decide to agree on a secret encryption key using the Diffie-Hellman key
exchange protocol. You observe the following:
(i) Nikita chooses p=13 for the modulus and
g=2 as generator.
(ii) Nikita sends 6 to Michael.
(iii) Michael sends 11 to Nikita. [6]
July-2005 [36]
1.
c) What are the three key properties of
hash functions? [4]
g) What is non-repudiation? How does
Asymmetric key encryption ensure non-repudiation? [4]
2.
a) What is a digital signature? Which
algorithms are used to digital signatures? [4]
6.
a) How are
Digital Certificates used to provide third party trust? [5]
b) What are the
components of X.506 v3 format for digital signatures? [6]
c) What is CRL?
How is it used to validate digital certificates? [7]
7. Write short notes on the following:
a) Public Key
infrastructures (PKI) [6]
January-2006 [32]
1.
a). What is digital signature? Which algorithms are used for
digital signatures? [4]
c) How
does message digest help in checking the integrity of a transmitted text? [4]
2.
a) Suppose you are doing RSA encryption
with the prime numbers p=13 and q=7. Also, assume that encryption
exponent e=5. Find the least positive decryption exponent d. Next,
encrypt the message m=7. Now decrypt the cipher c=2.
[9]
3.
b) How does RSA based digital signature
help in "non-repudiation"? Explain with a concrete example scenario
between a sender and a receiver. [6]
c) Describe the Digital Signature (DS)
Algorithm based on DS standard of NIST. How are signing and verifying done in
DS standard? [9]
July-2006 [22]
1.
d) What basic
arithmetical and logical functions are used in MD5 and SHA-1? [4]
4.
a) Differentiate
between both the MD5 and SHA-1 algorithms. [6]
b) Suppose that
A has a data file namely “d” that B needs. A and B want to ensure a secure
transmission of file. They do not want that anyone should know the content of
file even if it is intercepted during transmission. B also wants to know
whether or not whatever is transmitted from A has not been corrupted or altered
in transit and that the file was sent by A . It is assumed that A and B share a
secret symmetric key that no one else knows and there is a public key
infrastructure available.
Describe
the steps that A takes to send the data file “d” meeting the requirements give
as above. Your solution should only use as few a number of symmetric and/or
public key as necessary while meeting the above requirements. [12]
January-2007 [20]
3.
a) In
RSA Encryption method if the prime number p and q are 3 and 7
respectively, the encryption exponent e is 11, find the following:
i) the least positive decryption exponent
d
ii) public and
private key
iii) cipher text when the plain text P is
encrypted using the public key ? [10]
5.
a) Alice sends some message M
to Bob using RSA public-Key encryption Algorithm where public key is (5,119)
and private key is (77,119). The Cipher text is 66. Find the message M sent to
Bob. [5]
6.
b) How
does Asymmetric key encryption ensure “Non-Repudiation”? Explain with an
example? [5]
July-2007 [22]
1.
c) A
hash function takes a long string (or ‘message’) of any length as input and
produces a fixed length string as output, termed a message digest or a digital
fingerprint. Briefly explain message authentication code (MAC) in message
digest. [4]
4.
c) Why
is MD5 (Message-Digest algorithm 5) widely used in cryptographic hash function
with a 128-bit hash value? [6]
6.
a) How
does Asymmetric key encryption ensure “Non-Repudiation”? Explain with an
example. [6]
7.
d) RSA
involves a public and private key. The public key can be known to everyone and
is used for encrypting messages. How are the keys for the RSA algorithm generated?
Write steps. [6]
January-2008 [40]
1.
f) What
do you understand by authentication? Explain authentication using public key
cryptography. [4]
4.
a) What
are the various classes of Digital certificates? List three primary functions
of CERT. [9]
5.
a) What
is encryption? Why is it required? Explain the RSA algorithm of encryption with
example. [9]
b) What
are the major differences between MD4 and MD5? [6]
6.
b) How
does Asymmetric Key Encryption ensures “Non-Repudiation and Privacy”? Explain
with an example. [6]
7.
b) What
is CRL? How is it used to validate digital certificates? [6]
July-2008 [28]
1.
e) A hash
function takes a long string (or 'message') of any length as input and produces
a fixed length string as output, termed a message digest or a digital
fingerprint. Explain message authentication code (MAC) in message digest in
brief. [4]
2.
b) RSA
involves a public and private key. How are these keys, for the RSA algorithm,
generated? Write steps. [6]
c) Explain
attack in Diffie-Hellman Key Exchange algorithm. [6]
3.
b) In cryptography, MD5
(Message-Digest algorithm 5) is widely used cryptographic hash
function with a 128-bit hash value. Explain the
algorithm. [6]
5.
c) How does Assymetric key encryption
ensure “Non-Repudiation”? Explain with an
example. [6]
January-2009 [25]
4.
c) How does
RSA based digital signature help in “non-repudiation”? Explain with a suitable
example scenario between a sender and a receiver. [6]
6.
b) What are the various classes of
Digital certificates? What are the advantages of
International
format Standard X.509v3? [9]
c) What is
Snooping? Is Diffie-Hellman algorithm susceptible to this attack? [4]
7. Write short notes on any three of
the following:
a) Message Digest [6]
July-2009 [13]
1.
f) What
are the three key properties of hash functions? [4]
6.
a) The RSA algorithm involves three steps,
key generation, encryption, and decryption. Explain each step. [9]
January-2010 [7]
1.
f) Compare the digital signature and
conventional signature with respect to the following four parameters:
Inclusion, Verification, Relation and Duplicity. [4]
2.
c) What are the three broad categories
of applications of public key cryptosystems? [3]
July-2010 [21]
2.
a) What are the various classes of Digital
Certificates? List three primary functions of CERT. [6]
4.
a) List four public key cryptography
algorithms. Explain one of the algorithms where public key cryptosystems is
used. [9]
7. Write short notes on any three of
the following:
b) MD [6]
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