CS 445 Midterm
three most important goals in computer security - CORRECT
ANSWER-C-onfidentialty — keeping info secret unless allowed to see it
I-ntegrity — protecting info from changes like when in transit,
forgery, from correct sourse
A-vailability — keep sys/data accessible/running/operating, no
denial of service attacks
Which security principle is characterized by the use of multiple, different defense
mechanisms with a goal of improving the defensive response to an attack? -
CORRECT ANSWER-defense in depth
Frequency analysis can be used best for breaking which algorithm? - CORRECT
ANSWER-Caesar's cipher
Consider a system using a shared key between all pairs of users. When there are 10
users in the system, how many keys are needed in total? - CORRECT ANSWER-45
n(n-1)/2
10(9)/2
Which one of the following is considered perfect cipher? - CORRECT ANSWER-one
time pad
Encrypt the plaintext message "1011 0010" using one time pad with the key stream
"0110 1001". - CORRECT ANSWER-XOR that hoe
1101 1011
What is the block size and key size in DES? - CORRECT ANSWER-64 bit block, 56
bit key
The effective key length of Double DES is not twice of the key size of the single
DES. Why is that? - CORRECT ANSWER-Due to Meet-in-the-middle attack
Choose an incorrect description on AES. - CORRECT ANSWER-simulates one time
pad
Among the following encryption modes, which mode allows random access to any
block while producing different ciphertext blocks for the same plaintext blocks? -
CORRECT ANSWER-CTR (Counter)
After launching a brute-force attack on the AES, how did the Cryptool software figure
out the right key? - CORRECT ANSWER-By looking for the lowest entropy value
,Consider a perfect one-way hash algorithm that generates 256-bit hash value. To
create a hash collision with 50% chance, how many messages should be tried?
(Hint: This is called the level of safety.) - CORRECT ANSWER-2^128
In RSA algorithm, the parameters are given as following: p = 11, q = 3. Then n = 33.
In this case, what is the value of the Totient function $ (n)? - CORRECT
ANSWER-20
(p-1)(q-1)
(10)(2)
In the above question, if you choose the public key (e) = 3, what is the value of d?
(Hint: • (n) +1 should be evenly divisible by e) - CORRECT ANSWER-7
20 + 1 = 21
21/3
Why is the RSA algorithm considered secure? - CORRECT ANSWER-Because it is
difficult to factor to two prime numbers
Diffie-Hellman key exchange algorithm is considered secure because? - CORRECT
ANSWER-It is hard to compute discrete logarithm
What is the advantage of ECC algorithm compared with RSA? - CORRECT
ANSWER-It requires a smaller key for the same level of security
What is the disadvantage of asymmetric key cryptography (or public key) compared
with symmetric key cryptography? - CORRECT ANSWER-Slower
RSA algorithm has the following feature:
D(E(M, Kpub), Kpriv) = M = D(E(M, Kpriv), Kpub)
In other words, you can encrypt plaintext with a private key. This does not give any
confidentiality, but serves a different purpose. Where can you best use this feature? -
CORRECT ANSWER-Digital signature
Upon receiving a signed document, which key is used to verify the digital signature?
- CORRECT ANSWER-Sender's public key
Since a public key can be faked by attackers, there is a certification infrastructure
called PKI. Find a correct description on it. - CORRECT ANSWER-PKI forms a
hierarchy
Find a correct description about MAC address - CORRECT ANSWER-The first 24-bit
is vendor code
, Using a packet sniffer, you observe that the value in the "Length or Type" field is
"Ox0800". What can you infer from that? - CORRECT ANSWER-The frame is an
Ethernet II frame = If this value is greater than 1536(0x600), it is Ethernet II frame
A switch is in fact a generalized multi-port bridge. Find a correct explanation about
switch. - CORRECT ANSWER-It offers a full-duplex communication
Which device connects two segments of the same LAN and extends the collision
domain? - CORRECT ANSWER-Repeater
Some ranges of IP addresses are assigned for private network. Which one of the
following is such a private address range? - CORRECT ANSWER-192.168.0.0 -
192.168.255.255
CID (Classless Interdomain Routing) was created to mitigate the wastage of IP
address space. In the notation 205.16.37.32/28, how many IP addresses are
available for hosts? - CORRECT ANSWER-16
Which protocol is used for determining the MAC address of the destination device
when the source device knows only the
IP address of the destination? - CORRECT ANSWER-ARP
Traceroute algorithm sends a series of packets to the destination with varying TTL
values. When the TTL value becomes 0 in a router, what happens? - CORRECT
ANSWER-The router drops the packet and sends an ICMP packet to the sender
An IP packet size is 4200 bytes. If it is going be transported over Ethernet, how
many fragmented packets will be generated? - CORRECT ANSWER-3
Choose a correct description on UDP - CORRECT ANSWER-It is suitable for
real-time traffic
The following tasks except one are performed by TCP. Find the one that is not the
function of TCP. - CORRECT ANSWER-Multicasting
TCP connection is established by 3-way handshaking process. What are the TCP
flags that are involved in the second step?
(Hint: There are 6 types of flags, URG, ACK, PSH, SYN, FIN, RST). - CORRECT
ANSWER-SYN-ACK
When the DNS query answer comes from the local DNS's cache, what is it called? -
CORRECT ANSWER-Non-authoritative answer
three most important goals in computer security - CORRECT
ANSWER-C-onfidentialty — keeping info secret unless allowed to see it
I-ntegrity — protecting info from changes like when in transit,
forgery, from correct sourse
A-vailability — keep sys/data accessible/running/operating, no
denial of service attacks
Which security principle is characterized by the use of multiple, different defense
mechanisms with a goal of improving the defensive response to an attack? -
CORRECT ANSWER-defense in depth
Frequency analysis can be used best for breaking which algorithm? - CORRECT
ANSWER-Caesar's cipher
Consider a system using a shared key between all pairs of users. When there are 10
users in the system, how many keys are needed in total? - CORRECT ANSWER-45
n(n-1)/2
10(9)/2
Which one of the following is considered perfect cipher? - CORRECT ANSWER-one
time pad
Encrypt the plaintext message "1011 0010" using one time pad with the key stream
"0110 1001". - CORRECT ANSWER-XOR that hoe
1101 1011
What is the block size and key size in DES? - CORRECT ANSWER-64 bit block, 56
bit key
The effective key length of Double DES is not twice of the key size of the single
DES. Why is that? - CORRECT ANSWER-Due to Meet-in-the-middle attack
Choose an incorrect description on AES. - CORRECT ANSWER-simulates one time
pad
Among the following encryption modes, which mode allows random access to any
block while producing different ciphertext blocks for the same plaintext blocks? -
CORRECT ANSWER-CTR (Counter)
After launching a brute-force attack on the AES, how did the Cryptool software figure
out the right key? - CORRECT ANSWER-By looking for the lowest entropy value
,Consider a perfect one-way hash algorithm that generates 256-bit hash value. To
create a hash collision with 50% chance, how many messages should be tried?
(Hint: This is called the level of safety.) - CORRECT ANSWER-2^128
In RSA algorithm, the parameters are given as following: p = 11, q = 3. Then n = 33.
In this case, what is the value of the Totient function $ (n)? - CORRECT
ANSWER-20
(p-1)(q-1)
(10)(2)
In the above question, if you choose the public key (e) = 3, what is the value of d?
(Hint: • (n) +1 should be evenly divisible by e) - CORRECT ANSWER-7
20 + 1 = 21
21/3
Why is the RSA algorithm considered secure? - CORRECT ANSWER-Because it is
difficult to factor to two prime numbers
Diffie-Hellman key exchange algorithm is considered secure because? - CORRECT
ANSWER-It is hard to compute discrete logarithm
What is the advantage of ECC algorithm compared with RSA? - CORRECT
ANSWER-It requires a smaller key for the same level of security
What is the disadvantage of asymmetric key cryptography (or public key) compared
with symmetric key cryptography? - CORRECT ANSWER-Slower
RSA algorithm has the following feature:
D(E(M, Kpub), Kpriv) = M = D(E(M, Kpriv), Kpub)
In other words, you can encrypt plaintext with a private key. This does not give any
confidentiality, but serves a different purpose. Where can you best use this feature? -
CORRECT ANSWER-Digital signature
Upon receiving a signed document, which key is used to verify the digital signature?
- CORRECT ANSWER-Sender's public key
Since a public key can be faked by attackers, there is a certification infrastructure
called PKI. Find a correct description on it. - CORRECT ANSWER-PKI forms a
hierarchy
Find a correct description about MAC address - CORRECT ANSWER-The first 24-bit
is vendor code
, Using a packet sniffer, you observe that the value in the "Length or Type" field is
"Ox0800". What can you infer from that? - CORRECT ANSWER-The frame is an
Ethernet II frame = If this value is greater than 1536(0x600), it is Ethernet II frame
A switch is in fact a generalized multi-port bridge. Find a correct explanation about
switch. - CORRECT ANSWER-It offers a full-duplex communication
Which device connects two segments of the same LAN and extends the collision
domain? - CORRECT ANSWER-Repeater
Some ranges of IP addresses are assigned for private network. Which one of the
following is such a private address range? - CORRECT ANSWER-192.168.0.0 -
192.168.255.255
CID (Classless Interdomain Routing) was created to mitigate the wastage of IP
address space. In the notation 205.16.37.32/28, how many IP addresses are
available for hosts? - CORRECT ANSWER-16
Which protocol is used for determining the MAC address of the destination device
when the source device knows only the
IP address of the destination? - CORRECT ANSWER-ARP
Traceroute algorithm sends a series of packets to the destination with varying TTL
values. When the TTL value becomes 0 in a router, what happens? - CORRECT
ANSWER-The router drops the packet and sends an ICMP packet to the sender
An IP packet size is 4200 bytes. If it is going be transported over Ethernet, how
many fragmented packets will be generated? - CORRECT ANSWER-3
Choose a correct description on UDP - CORRECT ANSWER-It is suitable for
real-time traffic
The following tasks except one are performed by TCP. Find the one that is not the
function of TCP. - CORRECT ANSWER-Multicasting
TCP connection is established by 3-way handshaking process. What are the TCP
flags that are involved in the second step?
(Hint: There are 6 types of flags, URG, ACK, PSH, SYN, FIN, RST). - CORRECT
ANSWER-SYN-ACK
When the DNS query answer comes from the local DNS's cache, what is it called? -
CORRECT ANSWER-Non-authoritative answer
