Computer Networks Test 7

Let C be ciphertext then Encrypted message (C)= E_k(P)

Decrypted message(P) = D_k(C)

∴ P = D_k(E_k(P))

SHA-256 and MD5 both generate the message digest.

1. SHA-256 is novel hash functions computed with 32-bit words.
2. MD5 –MD5 is a widely used cryptographic hash function that produces a 128-bit hash value.

Therefore option 1 is correct.

Important Point:

RSA and DES both are encryption algorithms.

• RSA is also used to decrypt the encrypted message by the same RSA algorithm.
• Data encryption standard (DES) is a block cipher that encrypts data in blocks of size 64 bits each. That is 64 bits of plain text goes input to the DES which produces 64 bit of ciphertext. The key length is 56 bits.
• RSA and SHA- 256 do not allow generating message digests.

Masquerading:

Masquerade means disguise. In terms of communication security, it is a type of attack where attacker pretends to be someone else (or we can say that pretends to be an authorized user of a system ) in order to gain access to the system.  Most common is OS masquerading. Masquerading will be done by using stolen ID and passwords.

Phishing:

In phishing, attacker set up fake websites, which look like real websites. It uses email to mislead a user into entering confidential information. In this, attacker decides to create her own website.  The attacker exploits the email system to suggest that the sender of the email is some authorized person. When customer clicks on the URL specified in email, he/she is taken to the attacker’s site.

Hijacking:

In this, attacker somehow manages to impersonate a legitimate user. For this to be possible, the attacker taps into electronic i.e. computer based conversation between two genuine users , servers or their combinations. Then attacker poses as one of the users and tries to fool the other side.

Concept:

Euler's Totient function counts the positive integers up to a given integer n that are relatively prime to n. It is written using the Greek letter phi as φ(n) or ϕ(n) and may also be called Euler's phi function.

Explanation:

ϕ(n) = ϕ(23552)

= ϕ (1024 × 23)

= ϕ(1024) × ϕ(23)

= ϕ(210) × ϕ(23)

Since 23 is prime

ϕ(17) = 23 – 1 = 22

Since 2 is prime

ϕ(24) = 1024 × (1 –  ½ ) = 512

ϕ(272) = 512 × 22 = 11264

Concept

Packet filtering firewalls:

• As the most “basic” and oldest type of firewall architecture, packet-filtering firewalls create a checkpoint at a traffic router or switch.
• These work at the Network layer of the OSI networking model.
• The firewall performs a simple check of the data packets coming through the router—inspecting information such as the destination and origination IP address, packet type, port number, and other surface-level information without opening up the packet to inspect its contents.

Circuit-Level Gateways

• This simplistic firewall type is meant to quickly and easily approve or deny traffic without consuming significant computing resources, circuit-level gateways work by verifying the transmission control protocol (TCP) handshake.
• These work at the session layer of the OSI model. These are also assumed to be working as a "shim-layer" between the application layer and the transport layer of the TCP/IP stack.
• This TCP handshake check is designed to make sure that the session the packet is from is legitimate.

Concept:

RSA is one of the first public-key cryptosystems and is widely used for secure data transmission.

Data:

Public Key = (n,e) Private Key = (n,d)

e = 19, d = ? 

ϕ(n) = ϕ(1247)  

Calculation:

ϕ(1247) = ϕ(29 × 43) = 28 × 42 = 1176

e × d ≡ 1 mod (ϕ)

11 × d ≡  1 mod (1176)

(11 × 107) mod 1176 ≡ 1

∴ d = 107 

A digital signature needs a public-key system. The signer signs with his/her private key and the verifier verifies with the signer’s public key.

In cryptosystem the sender uses the public key of the receiver to encrypt; the receiver uses his own private key to decrypt.

Asymmetric-key cryptosystems are very inefficient when dealing with long messages. The solution is to sign a digest of the message, which is much shorter than the message.

3 mod 7 = 3

9 mod 7 = 2

27 mod 7 = 6

81 mod 7 = 4

243 mod 7 = 5

729 mod 7 = 1

2187 mod 7 = 3

6561 mod 7 = 2 and so on

3⁵³ mod 7 = (3^{6 × 8 +5} ) mod 7 ≅ 3⁵ mod 7 = 5

The value of (353 mod 7)  is 5

Concept:

In symmetric key cryptographic system, every pair of nodes need a separate key.

Hence, there are nC2 pairs are required for n system.

Formula:

 nC2  = \(\frac{n(n-1)}{2}\)

Calculation:

51C2  = \(\frac{51(51-1)}{2} = 1275\)