1.Let A and B two m,essages and H() be a hash function that is defined on their

Question

1.Let A and B two m,essages and H() be a hash function that is defined on their message space. If A=B tyhen H(A)=H(B).(this property is a direct consequence of the definition of function). If H(A)=H(B) is it true that A=B?
a)true

b)false

2. is it possible for two different passwords to ‘unlock, the same user account? Explain your answer.

3. What is the value in using an initilialization vector(IV) in conjunction with a block cipher? What is the name of the attack that is mitigated by using an IV. (This attack allowed alan turing shorten world war II and save millions of lives)

4. your www browser contains a cache of self signed CA certificates. If a malefactor wants to learn your banking login and password combination by directing you to a bogus secure www site, how could they use this cache to facilitate this attack.

5. How can a malefic for detect which tcp ports are open at a given IP address? What Is a common name for this surveillance activity? Explain your answer in terms of the part of the TCP protocol that is used to open connections.

6. If one strips down the English alphabet to only lower case letters.(eliminate capitalization, White space, and punctuation), then there are 26 different symbols ,aka letters. We know from combinatorics that there are 26 fractional possible distinct permutations of these 26 symbols, which, is approximately 3*1025 which is about 50 moles, which is approximately the number of water molecules in a glass of water. In other words, there are as many substitution ciphers is HUGE, and this would lead a naïve person to believe they are secure. Please try to explain why substitution ciphers are not secure.(And, Please be concise)

7. Explain how public key cryptography can obviate the need for distributing encryption keys. (Be concise)

8. Explain how Diffie Hielman key exchange, encryption and authentication can be used to mitigate man-in –the middleattacks. In what order would these operations be performed?

Explanation / Answer

There are lot of sub parts in this questions. can you please post one more question. I am answering few parts.

5.

Malefacter send ping and try to send packets to the ports. If a port is open the attackers will get reply from that port and will come to know about it.

This is known as Port Scanning. This is known as the established state in the TCP protocol.

The first stage is the Listen stage where serverrepresents waiting for a connection request from any remote TCP and port.

Second stage:

SYN-SENT

client represents waiting for a matching connection request after having sent a connection request.

Third Stage:

SYN-RECEIVED

server represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request.

Fourth Stage:

ESTABLISHED

both server and client represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.

1.

It is not always true that if H(A) = H(B) then A=B.

Considering an example let us assume that a Hash function is something like x2 -5x +6

Now let us assume two values a and b as 2,3

If we subsititute 2,3 in this equation we get

H(2) = 4-10+6=0

H(3)=9-15+6=0

Now here H(2)=H(3) but 2 is not equal to 3.

2.

yes it is possible. It all depends on the hashing technique used to store the password. If there can be same answer for two solutions in the hash function then it can be unlocked.

Just a sample example let us take a design where user has to choose password as a number from 0 to 1000

Now the hash function is something like x2 -5x +6

now if we calculate value of the functions for x = 2 and x=3 it will be equal to zero.

8.

Diffie Hielman is a key exchange protocol which can be used to mitigate man in the middle attacks.

Authentication and private key plays an important role in this protocol as it does not allow the attack to happen. Without the use of private key on sender and reciever side and along with proper authentication system it is prone to man in the middle attack.

To explain the concepts given by RSA through an example

The Diffie-Hellman key exchange is vulnerable to a man-in-the-middle attack. In this attack, an opponent Carol intercepts Alice's public value and sends her own public value to Bob. When Bob transmits his public value, Carol substitutes it with her own and sends it to Alice. Carol and Alice thus agree on one shared key and Carol and Bob agree on another shared key. After this exchange, Carol simply decrypts any messages sent out by Alice or Bob, and then reads and possibly modifies them before re-encrypting with the appropriate key and transmitting them to the other party. This vulnerability is present because Diffie-Hellman key exchange does not authenticate the participants. Possible solutions include the use of digital signatures and other protocol variants

Now if we follow authentication then it becomes a station to station protocol and then it is not susceptible to these attacks. The idea is explained below:

Prior to execution of the protocol, the two parties Alice and Bob each obtain a public/private key pair and a certificate for the public key. During the protocol, Alice computes a signature on certain messages, covering the public value ga mod p. Bob proceeds in a similar way. Even though Carol is still able to intercept messages between Alice and Bob, she cannot forge signatures without Alice's private key and Bob's private key. Hence, the enhanced protocol defeats the man-in-the-middle attack

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